IL38776A - Process of manufacturing drawn sheet glass including the use of a thermal barrier in the kiln - Google Patents

Description

PROCBSS OP MAHUPACTORIHG PBA¾B SHUT C SS IHCLDPING THE USB CP A IBEHHAL B&BKEER IH TOE cnoa v?i3 ^ΪΌΉΒΟ nrnoo wtat "m* T¾TB nyapa Ka una »nm o¾ontaa This invention relates to a process of manufacturing sheet glass by continuously feeding molten glass into a kiln to establish a continuous forward flow of glass to a drawing zone at which molten glass is continuously upwardly drawn from the surface of the molten glass in the kiln in the form of a ribbon- The invention also relates to apparatus for use in carrying out such process- The invention also relates to sheet glass.

In the performance of a glass drawing process as referred to above, the thermal and flow conditions in the kiln are of critical importance for the quality of the drawn glass. It is in all cases necessary for these conditions to be such that a substantially stable meniscus is established at the surface of the glass at the drawing zone but the formation and maintenance of such a meniscus by no means ensures that the drawn glass will be of good quality. Glass is drawn inwardly into the meniscus from surface regions of the molten glass surrounding the meniscus and the difference in the temperature of the glass which invariably exists between surface regions at different distances from boundary walls of the kiln, combined with the rather complex flow pattern, tend to prevent the formation of a ribbon which is truly flat and of substantially uniform thickness over its width, and also to lead to optical defects due to mixing of currents composed of glass of different viscosities. These tendencies become more marked according as the drawing speed increases.

The problems referred to arise in all drawing processes in which the glass is drawn from the surface of the molten glass in the kiln* as distinct from processes in which the molten glass is extruded into the ribbon from beneath the surface of the molten glass in the kiln as in the classical Pourcault process. In such extrusion processes the flow pattern of the glass is quite different and the problems above referred to do not arise. Broadly speaking, drawing processes with which the present invention is concerned can be divided into two categories according to the depth of the kiln at the drawing zone. On the one hand use can be made of a shallow kiln from which glass is drawn from the full depth of molten glass at the drawing zone. This category of process includes the classical Colburn process in which the glass ribbon drawn upwardly from the kiln is bent over a bending roller and is conveyed through a horizontal annealing lehr. On the other hand, use can be made of a deep kiln in which the forward current of glass flowing to the drawing zone flows over a return current of colder glass coming from the terminal end region of the kiln. This category of process includes the classical Pittsburgh process in which the glass ribbon is drawn upwardly through a vertical drawing tower.

Numerous modifications of the said classical processes are possible within the broad categories referred to. For example the glass ribbon can in any given type of process be drawn from the kiln at an inclination to the vertical and a ribbon drawn from a deep kiln can be bent over a bending roller instead of being drawn through a vertical drawing tower.

The demands for high quality glass and higher rates of production have stimulated a continuous search by manufacturers for ways of creating better thermal and flow conditions in the drawing plant and numerous proposals to this end have been made in recent years.

Thus it has been proposed externally to heat bottom and side wall portions of the kiln to particularly high temperatures in order to reduce flow retardation along the walls. This expedient does not produce conditions which are favourable to the production of high quality sheet glass. In fact there is an increased risk of the drawn glass becoming contaminated by grains of refractory material or to contain gas bubbles. The tendency for the refractory material to be corroded or eroded increases with increase of temperature of the refractory material.

The present invention aims to provide for an improvement in the drawing of sheet glass by influencing the basic flow pattern within the body of molten glass. The invention aims in particular to provide for an increased rate of drawing without the usual increased risk of refractory corrosion and erosion which this has hitherto entailed.

According to the present invention there is provided a process of manufacturing sheet glass by continuously feeding molten glass into a kiln to establish a continous forward flow of glass to a drawing zone at which molten glass is continuously upwardly drawn from the surface of the molten glass in the kiln in the form of a ribbon, characterised in that in at least one place which in plan aspect of the kiln is spaced inwardly from a boundary of the surface of the molten glass in the kiln, the molten QISLSS in the kiln is heated to maintain at that place a thermal barrier formed by an upward low of molten glass which rises to said surface from a position in the vicinity of a wall portion of or in the kiln so that molten glass between said and the said boundary -bo-hind the barrie^is substantially prevented by such wall portion from flowing beneath said barrier the depth of the molten, lass bothjab ye Jhe_said_wall portion being_less than r " " The'process according to the invention affords the "~ ' ^he depth of the said bath in the part adjacent thereto, important advantage , that in at least one region around the meniscus at the root of the drawn glass ribbon, molten glass which is about to feed into the meniscus is given a lower viscosity and therefore a higher flowability without Buch glass flowing against a heated kiln wall near the flux line where corrosion and erosion are most likely to occur. The heated upwardly flowing molten glass rises to the surface of the molten glass against a quantity of molten glass which is excluded by the thermal barrier from access to the drawing zone. That excluded quantity of molten glass is continuously kept in movement due to a rather complex system of convection currents which exists on that side of the thermal barrier. Such currents help to avoid or reduce stagnation of any part of the molten glass such as is liable to produce devitrified grains of glass which might possibly become entrained towards the drawing zone. It is a further important advantage of the process however that molten glass which flows downwardly along a side or end wall of the kiln, behind the thermal barrier, and becomes cooler during such downward flow, is prevented from flowing beneath the thermal barrier and directly exerting a cooling action on the molten glass which in plan aspect of the kiln is in front of the barrier. Such a cooling action could adversely affect the velocity of flow of molten glass into the ribbon.

The invention makes an important contribution to achieving a good stratification of the glass comprising the ribbon so that glass of high quality can be drawn at faster rates. The extent to which this result is promoted depends in part on the place or places at which a thermal barrier as referred to is created and on the extent of such barrier or barriers in the horizontal plane .

In certain embodiments of the invention wherein the process is of a kind in which molten glass at the surface of said forward flow feeds directly into the bottom of the ribbon at its front side whereas molten glass at a lower level of such forward flow rises at a position behind the drawing zone and forms an oppositely directed surface flow which feeds into the ribbon at its rear side, a said thermal barrier is created at a position coinciding with said position where molten glass rises "behind the drawing zone. A thermal barrier at that place has a particularly marked beneficial effect. Over the important main part of the ribbon width, excluding its margins, the rear side of the ribbon is wholly or mainly derived from the said oppositely directed surface flow. If the glass of that oppositely directed surface flow is significantly less fluid than the glass at the surface of said forward flow, the drawing speed has to be kept low if the drawn glass is to be of an acceptable quality. If the drawing speed is raised beyond a certain value, which depends to an appreciable degree on the flow resistance of the glass feeding the rear side of the ribbon, the ribbon becomes deformed or forms sheet glass which is optically very defective. A drawing speed substantially higher than would normally be permissible can however be adopted if a thermal barrier in accordance with the invention is formed along a zone extending transversely of the kiln at a position such that some of the heated glass flowing upwardly at that position serves to form the oppositely directed surface flow of glass into the rear side of the meniscus. In such embodiments, the said thermal barrier may be spaced from the rear end wall of the kiln but this is not essential because , as hereafter exemplified, a part of that wall located beneath the glass surface can be shaped so as to extend inwardly towards or up to a position beneath the drawing zone so that in that case the thermal "barrier can "be located over a submerged part of that rear end wall.

Depending on the extent of such thermal barrier in the widthwise direction of the kiln, it may be possible for currents of glass to move from a position near the central part of the rear end wall of the kiln and to move past the ends of the thermal barrier, towards the end portions of the drawing zone from which the margins of the ribbon are drawn. However any impurities thus drawn inwardly to the drawing zone are therefore directed to such end portions of the drawing zone and do not contaminate the central portion of the drawing zone from which the main usable portion of the ribbon is drawn.

Preferably a said thermal barrier located at a place disposed rearwardly of the drawing zone extends across the full width of the kiln or at least over a portion of tlie kiln width which is at least co-extensive with the width of the ribbon. In that case the whole or substantially the whole of the body of molten glass which is adjacent to the rear end wall of the kiln near the flux line and would normally be pulled inwardly to the drawing zone by the currents caused by the drawing operation, is effectively isolated by the thermal barrier.

A significant improvement in the drawing process can however be realised by forming a thermal barrier in accordance with the invention at a position which in plan aspect of the kiln is situated adjacent a side boundary ! of the surface of the molten glass in the kiln and at a position from which there is surface flow of molten glass towards an edge or margin of the ribbon. By means of a thermal barrier so placed, the flowability of the molten glass feeding the corresponding edge or margin of the ribbon is improved due to the heating of such glass and due to a reduction in frictional restraint. The flow of glass feeding the edge or margin of the ribbon is shielded from currents of more viscous glass which has been cooled by contact with the side wall of the kiln near the flux line. In consequence the width of the marginal portions of the ribbon which have to be discarded when the ribbon is cut is reduced.

In the case that a thermal barrier is located adjacent a side boundary of the glass surface as above referred to it is of course preferable for a similar thermal barrier also to be maintained adjacent the other side boundary of such surface so that similar thermal and flow conditions determine the formation of both side edge or marginal portions of the ribbon.

It is of course very advantageous for thermal barriers according to the invention to be maintained adjacent the two side boundaries of the molte glass surface and also rearwardly of the drawing zone. In that case the glass feeding the rear side of the ribbon and its margins can be kept in a highly flowable condition commensurate with that of the main forward surface flow which directly feeds the front side of the ribbon, and the permissible drawing speeds are at a maximum.

According to certain important embodiments of the invention, there is at least one thermal barrier function-ing as hereinbefore referred to, which is maintained above a threshold completely immersed in the molten glass.

The threshold serves positively to localise the ascending cu currents due to the local heating of the glass. The body of glass which is behind the threshold tends to be kept in stable rotary movement around a horizontal axis, which also helps towards avoiding or reducing any tendency for an accumulation of devitrified grains to be formed in the .said body.of glass. Moreover the threshold also serves as a mechanical barrier against inward displacement beneath the thermal barrier of any such grains of devitrified material or of any refractory grains with which the said body of glass might possibly become contaminated.

Inasmuch as the threshold is submerged and is therefore not in contact with the air above the molten glass, the threshold is not so liable to suffer corrosion by the molten glass currents as is the refractory material of the flux line blocks.

It is advantageous for the upward flow of glass forming a thermal barrier to commence below the level of the top of the threshold if such is provided so that the glass rises against the threshold and continues its upward movement above the level of the top of such threshold. The threshold then "better serves to stabilise the upward flow of glass.

The side faces of the threshold may be vertical or inclined to the vertical or one such face may be vertical and the other inclined. The height and shaping of the threshold influences the direction of the upward flow paths of the glass forming the thermal barrier and thus the flow pattern of the glass currents feeding the ribbone Where a threshold is provided, the threshold may be hollow and the heat required for creating the thermal barrier can be generated in the space within the threshold. The heating means is thus shielded by the threshold from direct contact with the molten glass such as would restrict the choice of heating means. As heating means, use can be made, e.g. of gas or oil burners or electrical resistances.

Use can alternatively be made of a threshold constituted by a single solid wall. In that case the heat for creating the thermal barrier may be generated at the bottom of such wall.

There are also advantages when making use of a submerged threshold in using heating means which is incorporated in or forms part of the or a wall forming the threshold or part of the threshold. For example, such wall may be formed in part by one or more electrically conductive refractory elements through which electric current can be passed to generate heat by Joule effect This method is favourable for creating a strong thermal action which is localised where It is most effective for preventing flow of glass currents over the threshold.

For maintaining the thermal barrier at the location of a threshold it is also possible to use heating means which is actually in contact with the molten glass adjacent the threshold. The use of heating means which is in contact with the molten glass but does not actually form part of the threshold is useful for generating heat at a well defined zone or zones while leaving the threshold free from any direct heating function and thus widening the choice of its design specifications. By way of example , use can be made of heating elements disposed on a side face and/or on the top face of the threshold.

Alternatively or in addition use can be, made of heating elements, e.g. electrical resistance heaters, which are located within the body of molten glass and spaced from the threshold. Such an arrangement is of advantage for reducing any risk of corrosion of the threshold. Where it is required to raise to a certain level the temperature of the glass at a particular region spaced from the threshold, the said arrangement enables that heating to be effected with a lower energy consumption than if the heat were to be generated in or immediately o the threshold.

In a process in which the kiln is provided with a threshold, the latter can "be upwardly extended "by a plate to increase the height of the mechanical harrier preventing inward flow of impurities such as devitrified material or "bubbles; Such plate can be made of metal, e*g. , molybdenum. It is advantageous for the top of the plate to be disposed as close as possible to the free surface of the molten glass in the kiln.

A thermal barrier at one or more places as required by the invention can of course be created by generating heat within the kiln, by electrical resistance heaters or other means, regardless of whether or not a threshold is provided at that place or places. However, the provision of a said threshold is of particular interest because it helps to stabilise the thermal barrier.

A very advantageous way of creating a thermal barrier is to pass an electric current or currents through the molten glass at the region to be heated, between suitably placed electrodes. This type of heating system produces the required calories directly in the molten glass itself and the glass can be kept at a required high temperature while the electrodes are at a lower temperature level which may be low enough substantially to avoid any risk of corrosion of the electrodes by the molten glass.

Electrodes for use in a heating system using the electrical conductivity of the molten glass can be in the form of plates or rods. It is very advantageous however, to use electrodes formed by pools of molten metal or molten metal salt. Molten metal or molten salt electrodes can be of large surface area producing the additional "benefit of a very low frictional restraint on the flow of molten glass in contact with the electrodes* One very satisfactory heating system makes use of electrodes one at least of which is disposed above a threshold completely immersed in the molten glass.

Another possible arrangement of electrodes which is very suitable in certain cases is an arrangement of the electrodes on opposite sides of a said threshold. By using electrodes disposed in that manner, a considerable volume of molten glass covering the threshold can be directly heated for a comparatively low energy consumption. It is convenient in such a system to use electrodes with large surface areas in contact with the glass so that a given heating effect can be realised with a low current density, which is desirable for avoiding bubble formation in the glass» In the case that a metal plate is used to increase the height of the mechanical barrier formed by the threshold and heating is achieved by an electric current or currents passing through the glass between electrodes disposed on opposite sides of the threshold, the plate will define an equipotential surface in the electric field and the plate may be of a shape selected to achieve a required preferential direction of the electric current. In certain embodiments of the invention the molten - Ik - glass in the kiln is locally heated to create a thermal barrier by passing electric current through the molten glass between electrodes one of which is disposed beneath the position at which the ribbon is drawn from the surface of the molten glass in the kiln. In this way a thermal barrier can be maintained very close to the drawing zone-In one very satisfactory arrangement, a draw bar is provided beneath the said drawing position and an electrode in the form of a quantity of molten metal or molten metal salt is held in this draw bar. The presence of a pool of molten metal or molten metal salt at that position is beneficial in regard to the low frictional restraint which it imposes on the molten glass flowing into the meniscus.

Another process feature which is of value comprises the production of a surface current of molten glass over the thermal barrier and in a direction away from the drawing zone , and the withdrawal of surplus glass from a region behind such barrier. The outward surface current supplements the action of the thermal barrier in countering any tendency for impurities to be entrained inwardly towards the drawing zone, A said outward surface current can be produced by withdrawing glass via at least one skim hole in a boundary wall of the kiln at a region behind the or a thermal barrier.

The invention includes apparatus for use in drawing sheet glass, said apparatus comprising a kiln having a gasst an means cn nu y a ng n o upwardly from the surface cf the glass at a drawing aone in the kiln* characterised in that there is Beas for locally heatiag the molten glass in the kiln in at least one place which in plan aspect o£ the kiln is spaced inwardly from a boundary of the said molten glass surface in order to maintain at that place a thermal barrier formed by an upward flow of molten glass hich rises to the said surface from a position in the vicinity of a vail portion of or in the kiln whereby said wall portion serves to prevent molten glass etwee the said barrier and the isatd boundary froo flowing beneath it» tt&e dept of tho molte glass β*& &he said nail portion being less than the depth of the said bath in the part adjacent thereto. the provision of local heating means according to the invention permits the creation of a hot some which renders she glass feeding the ribbon om at least one region around the meniscus more fluid» while forming a thermal barrier preventing cooler glass flowing into the ribbon from the wall portion of the kiln behind the thermal barrier* Consequently glass showing good stratification and which is generally of high quality can be drawn at a aster rate by apparatus according to the invention* In preferred apparatus according to the invention* the da* dcjawing esene is spared om that boundary of the surace of the molten glass in the kiln which is opposite tfce feed end of the kiln and a said local heating means is provided for maintaing a said upward flow of glass at a place which in plan aspect of the kiln is between the drawing zone and the said opposite boundary reached by the surface of the molten glass in the kiln when the apparatus is in use. The advantage of this and other optional apparatus features hereinafter referred to will be appreciated from the statements which have been made hereinbefore as to the advantages of the corresponding process features.

In the most important embodiments of the invention, there is means for maintaining a said upward flow of glass between the drawing zone and the said opposite glass surface boundary, such upward flow occurring across the full width of the kiln or at least over a portion of the kiln width which is substantially co-extensive with the width of the ribbon, this width being determined by the position of the conventional edge rollers between which the margins of the drawn ribbon are drawn» Advantageously however, apparatus according to the invention can comprise means for maintaining a said upward flow of molten glass in at least one place which in plan aspect of the kiln is adjacent a side boundary to which the surface of the molten glass in the kiln extends when the apparatus is in use. It is particularly beneficial for the apparatus to incorporate means for maintaining such an upward flow of glass adjacent each side boundary of the said molten glass surface and in certain apparatus according to the invention, in addition to the heating means for creating the upward flow at those places, there is means as above referred to for creating a hot zone directly rearwardly of the drawing zone. In that case the flow of molten glass into the rear side of the ribbon and into its edges or margins can be promoted to enable very high drawing rates to be achieved- Apparatus according to the invention may be of a type comprising a shallow kiln, the drawing means being adapted to draw glass from the full depth of the molten glass in the kiln; in that case the means for creating a local hot zone in accordance with the invention can be arranged so that it is effective for maintaining an upward flow of molten glass at that zone, over the full depth of the molten glass in the kiln* Apparatus according to the invention may alternatively be of a type comprising a deep kiln, the drawing means being adapted to draw glass from an upper portion of the molten glass in the kiln. In that case the means for creating a local hot zone in accordance with the invention is arranged so that it is effective for maintaining an upward flow of molten glass at that zone, at least in the upper portion of the depth of molten glass in the kiln and the apparatus is arranged so that the upward flow of molten glass at the hot zone takes place from a position in the vicinity of at least one wall portion so that flow of molten glass beneath the thermal barrier is substantially prevented.

Certain apparatus according to the invention incorporate a threshold located so as to be completely immersed in the molten glass in the kiln when the apparatus is in use, and so that the or a said heating means maintains a said upward flow of molten glass above such threshold* Preferably there is heating means for maintaining a said upward flow of molten glass from a position below the level of the top of a said threshold so that the upward flow occurs against the threshold and continues above its topmost level.

Advantageously the portion of the bottom of the kiln disposed behind the or a threshold is higher than the portion in front of the threshold. In that case the depth of unused glass behind the threshold is reduced. In the case of a keep kiln process there is the further advantage that the molten glass at the lower levels of the kiln in front of the threshold can be more effectively cooled, which promotes a more positive downward movement of the glass forming the submerged return current.

Advantageously, the apparatus comprises a said threshold which is hollow and means is provided for generating within the threshold the heat required for creating the local hot zone at that place » In other advantageous embodiments, there is a said threshold constituted by a single solid wall and the means for creating the local hot zone at that place is arranged to generate heat at the bottom of such wall.

For creating a said local hot zone , certain apparatus according to the invention is provided with heating means which is incorporated in or forms part of the or a wall forming a said threshold or part of a threshold.. By way of example, a said wall may be formed in part by one or more electrically conductive refractory elements, such as one or more tin oxide bricks , through which electrical heating current can be passed. Such heating means can have a surface which is flush with or which is set back or projects from an adjacent part of the threshold surface.

Preferably, the apparatus incorporates heating means for maintaining a said local hot zone and a threshold at that zone, such heating means being located within the kiln so as actually to be in contact with the molten glass, adjacent or spaced from such threshold. By way of example, heating elements may be located above the threshold so that the heating of the glass in that region can be more intense or the temperature gradient at that region can be different from that which would be possible if the heating elements were located below or in the threshold. Of course, heating elements can also be provided at a lower level for giving the glass adjacent the threshold an upward impulse.

According to a very satisfactory arrangement, there is a said local heating means for heating molten glass in the kiln to maintain a said upward flow of molten glass , which heating means generates heat directly within the kiln- In the most preferred embodiments of apparatus according to the invention, the heating means for maintaining a said thermal barrier comprises electrodes between which electric current can be passed through molten glass in the kiln. Such electrodes may be in the form of plates or rods but preferably they are formed by pools of molten metal or molten metal salt. In one arrangement, there is at least one electrode disposed above a said threshold, but preferably the heating means comprises electrodes located on opposite sides of the threshold- It is advantageous to place an electrode beneath the position at which the glass ribbon is drawn from the surface of the molten glass in the kiln when the apparatus is in use. By way of example, a draw bar may be provided at that position and such draw bar may incorporate or hold an electrode. An electrode located beneath the drawing position as aforesaid can in any case be incorporated in or held by an element which is integral with or connected to the rear end wall of the kiln. In some cases the absence of a free path behind such electrode, along which molten glass can flow upwardly into the path of the electric heating current from a lower level in the kiln, is beneficial in promoting better thermal and dynamic flow conditions within the kiln.

The use of electrodes for heating the molten glass avoids the necessity to transmit heat energy through refractories with consequent risk of their being heated to such an extent as to involve substantial risk of corrosion of the refractories by the molten glass. More-over there is less risk of inducing turbulent uncontrollable convection currents in the glass than when using heating means which relies entirely on convection currents in the glass for heating the glass in the hot zone.

The use of electrodes disposed on opposite sides of the threshold enables a large volume of glass to be heated at the hot zone, giving rise to a very marked upward current. Electrodes with large surface areas can be conveniently located at those positions, which is desirable for avoiding high current densities and the formation of bubbles. It is advantageous for the top of the threshold to be fairly close to the surface level of the molten glass so as to give a relatively high current density over the threshold and to make the threshold as effective as possible as a barrier against the inward movement of impurities into the drawing zone.

When using electrodes on opposite sides of the threshold and at the bottom of the kiln, it is advantageous for the bottom of the kiln behind the threshold to be at a higher level than in front of the threshold because in that case, in addition to the advantages attributable to such a difference in level as hereinbefore referred to, there is the advantage that the electrodes can be closer together.

In the case that electrodes are used, at least one electrode may "be made of a solid metal or of an electrically conductive refractory material, preference "being given to refractory precious metals, molybdenum, tungsten and Sn02, doping agents being incorporated if required. It has been found that these materials behave satisfactorily in molten glass at elevated temperature even when an electric current passes through the surface of the material in contact with the glass. Moreover the solid electrodes can be of a shape selected so as to achieve a predetermined current density distribution.

Special advantages attach to the use of electrodes composed of molten metal or molten metal salt* If the molten metal or salt is heavier than the molten glass, the electrodes are disposed beneath the molten glass and assist in reducing frictional resistance to flow of molten glass in the kiln. It is possible to use electrodes composed of molten metal or molten metal salt of lower specific gravity than the molten glass. Such electrodes do not in any way impede the surface currents of molten glass. Such electrodes can be readily changed in course of time and their thicknesses can be altered when required; moreover their electrical properties can be modified by changing their chemical compositions, without interrupting the sheet glass production.

Suitable molten metals heavier than glass, are tin and lead. These metals have a high electrical conductivity,, According to one optional but very advantageous feature there is a reservoir for holding an electrode composed of molten metal or metal salt in contact with the molten glass in the kiln, and such reservoir has an extension leading to a cooler region where metal or metal salt occupying such extension can be connected to a conductor cable. In this way, problems of maintaining good electrical connection of the cable to an electrode at a very high temperature zone, e.g. at the draw zone side of a threshold, are solved. By way of example, a molten tin electrode may be held in a reservoir with an extension channel leading to a cooler zone so that the tin in this channel is in a solid or at least a cooler state , and the cable may be connected to the solid or cooler tin. A molten metal salt electrode may likewise make contact with a body of the same or another metal salt in solid or at any rate cooler condition, provided that the cooler salt is sufficiently electrically conductive.

The kiln of an apparatus according to the invention may be provided with at least one skim opening at a position located in a boundary wall thereof opposite a position where means for creating a local hot zone is provided. When the apparatus is in use, a small amount of glass can be withdrawn continuously or intermittenly from the surface of the molten glass, via such skim opening. The withdrawal of glass from the surface in - 2k - that way, induces an outward current through the top of the thermal barrier to serve as an additional check against the inward flow of impurities.

According to a further aspect of the present invention, it relates to sheet glass possessing novel characteristics..

Sheet glass is composed of seams of glass of different refractive indices. The optical quality of the sheet glass depends to a large extent on the relative distribution of the differently refractive seams within the sheet. If there is an appreciable interpenetration of seams of different refractive indices the sheet tends to give a distorted appearance to objects viewed through the glass under various conditions, even if the main faces of the sheet are optically flat and parallel. The seam pattern depends on the spatial distribution of currents of glass of different viscosities feeding into the ribbon during the drawing process. The seam pattern existing in any given sample of drawn glass can be seen in an enlarged photographic image of a cross-section of the sample, normal to the line of draw.

An object of the present invention in its said further aspect is to provide sheet glass composed of seams of glass of different refractive indices distributed in a way such that light rays entering the sheet glass through either of its faces, over a wide range of angles of incidence, are subject to very little if any refraction ¥i/ithin the sheet.

Sheet glass according to the invention is characterised in that the distribution of seams of glass of different refractive indices in a cross-section which extends over the full width of the drawn ribbon and which is normal to the line of draw is discernable in any such cross-section as a pattern of mainly substantially parallel contour lines forming or visually suggesting a pattern of flat ellipses one within another, and in that said distribution is such that there is no abrupt sub-stantial change of refractive index from one seam of glass to another such as to cause a marked break in the continuity of the parallel interference fringes when the sheet is examined by means of an interferential microre-fractometer using a light beam which is projected through the sheet glass parallel with its main faces.

Sheet glass according to the invention in its said further aspect has excellent optical characteristics · The sheet glass gives rise to very little if any apparent distortion of objects viewed through the glass even at shallow or changing angles relative to the plane of the sheetβ When sheet glass according to the invention is examined in cross-section normal to the line of draw to detect the pattern of contour lines formed by the juxtaposition of seams of glass of different refractive indices, the contour pattern is seen to be substantially free of crossing lines. The contour lines are mainly ■ * substantially parallel. The contour lines may substantially all describe figures approximating to flat ellipses but that is not essential. However, in all cases the system of contour lines as a whole at least suggests to the eye a basic pattern of flat ellipses due to the fact that there are numerous contours which form flat ellipses or which at least form major parts of flat ellipses.

Techniques of examining and photographically recording the pattern of distribution of differently refractive seams of glass within a drawn glass sheet are well known in glass technology and have been used extensively in relation to samples of sheet glass hitherto available and drawn by various processes. In general the known sheet glass exhibits under such examination a system of contour lines which cross at one or more locations and/or which do not fall into any kind of elliptical pattern.

The pattern formed by the contour lines in sheet glass according to the invention in its said further aspect is not however the only important characteristic of such glass. A further important characteristic is the absence of abrupt change in refractive index from one seam of glass to another within the thickness of the sheet. The refractive index variations through the thickness of the sheet are so small or gradual that they do not cause a marked break in the continuity of the interference fringes when the sheet is examined by means of an interferential microrefractometer. An interferential s microrefractometer is an instrument in which a beam of light is projected through a sample to be examined, and is divided into unequally retarded parts to give rise to a pattern of interference fringes. When a Sample of sheet 5 glass according to the invention is examined by means -of such an instrument , the sheet being located so that the light beam enters one edge face of the sample and emerges from the opposite edge face (such opposed edge faces being parallel) no marked faults appear in the interference fringes whatever be the orientation of the sheet about the axis of the light beam. A very suitable type of interferential microrefractometer is the one developed by Nomarski. The Nomarski interferential method is, for example, described in the "Techniques de 1 ' Ingenieur" Chapter R3422, year 196l, paragraphs 6,3 to 6,6 (especially paragraph 6,52) under the heading "Objectif interferential a prisme de Wollaston" . This paper is published by "Techniques de 1 ' Ingdnieur" , 21, rue Cassette, Paris VI, France. The Nomarski instrument incorporates a slit light source and a combination of prisms and polarising filters to create an interference pattern in the form of a series of parallel lines or bands. When using such an apparatus for testing a sheet of glass the sheet should be located in a plane which is non-parallel with the interference lines or bands. The sheet can for example be located in a plane which is at k5° to such lines or bands. A sheet of glass according to the invention does not however give rise to a marked break in the interferences lines or bands whatever be the orientation of the sheet about the axis of the light beam so that the sheet can be rotated about such axis during the test and no marked faulting of the interference lines or bands will appear during rotation of the sheet through 36Ο0· Sheet glass according to the invention in its said further aspect as above de ined can be produced by a drawing process according to the invention as hereinbefore defined. By means of such a process, sheet glass according to the invention can be produced with consistent reliability, particularly when a thermal barrier is created directly rearwardly of the drawing zone. It is supposed that one result of establishing and maintaining a thermal barrier or two or more thermal barriers as hereinbefore described is that the flow profile existing in the mass of molten glass in the kiln at a position in front of the drawing zone is substantially maintained over the part of the kiln in which the glass currents change direction to feed into the front and rear sides of the ribbon.

In preferred embadiments of sheet glass according to the invention in its aforesaid further aspect, the sheet glass is characterised by the substantial absence of brush lines.

One feature of much of the known sheet glass is the presence on at least one face of the sheet glass of defects known as "brush lines". Such defects can he observed and recorded by interferonstry , using the known izeau fringes, or by examining a reflected image of the glass face, obtained by causing a light beam to be reflected from the said face onto a light-diffusing screen as will hereafter be more particularly describedo Sheet glass which is according to the invention in its said further aspect and which is further characterised in that brush lines are not present or only negligibly so, can be produced consistently and reliably by a process according to the invention as hereinbefore defined if a thermal barrier is maintained at a position which is directly rear-wardly of the drawing zone. The effect of such a thermal barrier is to protect the current of glass which rises and feeds into the rear side of the glass ribbon from contact with the rear end wall of the kiln and it is presumably this fact which entirely or mainly accounts for the substantial absence of brush lines from the drawn sheet glass.

Various embodiments of the invention, selected by way of example, will now be described with reference to the accompanying diagrammatic drawings which illustrate parts of various glass drawing machines and in which: Fig. 1 is a vertical transverse cross-section of part of a Pittsburgh-type machine; the section being taken on line Ι-Γ of Fig. 2; Pig* 2 is a plan view of the machine part shown in Pig. 1; Pig. 3 is a vertical longitudinal cross-section of part of a Colburn-type machine; Pig. k is a vertical longitudinal cross-section of part of another Colburn-type machine, the section being taken on line IV-IV in Pig. 5; Pig. 5 is a broken plan view of the machine part shown in Pig. i+; Pig. is a vertical longitudinal cross-section of part of another Pittsburgh-type machine; Pig. 7 is a vertical longitudinal cross-section of part of another Pittsburgh-type machine; Pig. 8 is a vertical transverse cross-sectional elevation of part of the machine represented in Pig. 7, the section being taken on the line VIII-VIII in Pig. 7» Pig. 9 is a vertical longitudinal cross-section of part of another Colburn-type machine; Pig. 10 is a vertical longitudinal cross-section of part of another Colburn-type machine; Pig. 11 is a vertical longitudinal cross-section of part of another Colburn-type machine; Pig. 12 to 17 are detail views showing portions of the kilns of six different glass-drawing machines in vertical cross-section parallel with the longitudinal - ' axes of the kilns; Pig, 18 is a vertical longitudinal cross-section of part of another Pittshurgh^type machine; and Figi 19 is a vertical longitudinal cross-section of part of another Pittsburgh—type machine.

Fig. 20 represents the sectional contour pattern appearing on an anamorphosic photograph (called "stria-scope") of a sample of sheet glass according to the invention; Fig. 21 shows the interference fringes formed "by light rays projected through the sample of sheet glass represented "by Fig. 20 during testing of such sample in an interferential microrefractometer according to the known method of Nomarski; Fig. 22 represents the sectional contour pattern such as appears on a "striascope" above referred to, of a sample of sheet glass drawn by a classical Pittsburgh type drawing process; Fig. 23 shows the appearance of the interference fringes formed by light rays projected through a sample of sheet glass drawn by a classical P t sburgh-type drawing process, during testing of such sample in an interferential microrefractometer according to the "Nomarski" method hereinbefore referred to; Fig. 2k is a typical brush line pattern detected by photographic methods on a face of a sample of sheet glass drawn by a classical Libbey-Owens type process; Fig. 25 represents a schematic view of a striascopic apparatus for photographically recording the homogeneity of the glass within the cross-section of a glass sheetj and Pig* 26 represents a schematic view of the optical system of an interferential microrefractometer useful . according to the "Noma ski" method.

The machine represented by Pigs. 1 and 2 comprises a kiln 1 for holding a quantity of molten glass. The lower portion of the kiln comprises a sole 2 , a lower rear end wall portion 3 and lower side wall portions and 5· At its upper portion the plan dimensions of the kiln are extended by horizontal wall portions 6 , 7 and 8 which extend outwardly from the said walls, 39 k and 5 * and support an upper end wall 9 and upper side walls 10 and 11. When the apparatus is in use, molten glass is continuously fed into the kiln at its end (not shown) remote from the end wall 3 » 9 so as to maintain the surface 12 of the molten glass at t he indicated level within the kiln while glass is drawn upwardly from such surface in the form of a continuous ribbon 13 having side edges ¾. ,15· Within the kiln there are three horizontal electrodes 16 disposed one over the other parallel with but inwardly spaced from the upper end wall portion 9 of the kiln. A similar group of three horizontal electrodes 17 is disposed parallel with but inwardly spaced from the upper side wall portion 10 and a third and similar group of three horizontal electrodes 18 is disposed parallel with hut inwardly spaced from the upper side wall portion 11 of the kiln. The electrodes 17 are connected to a source 19 of continuous electric current, the central electrode 17 "being connected to one pole of such source and the top and bottom electrodes 17 being connected to the other pole thereof. The other group of electrodes, i.e., the electrodes 16 and 18 are similarly connected to electric current sources which however are not shown.

The drawing machine incorporates, over the kiln, a drawing chamber and tower section through which the ribbon of galss is drawn by rollers. These and other parts of the machine are in accordance with well known practice and need no explanation or illustration.

The electrical potential applied to the electrodes of each of the groups 16, 17 and 18 is such that a continuous electric current passes through the molten glass in the kiln between the electrodes at different potential in that group. The voltage is such that at no place is the current density in the molten glass in excess of 0. amps/cm , and by virtue of the passage of the electric current through the molten glass the temperature of the glass in the vicintiy of each group of electrodes is about i+0°C higher than it would otherwise be. In consequence a thermal barrier is established and maintained at the site of each group of electrodes.

At this site there is a continuous upward low of molten glass which rises from a position in the vicinity of the underlying wall portion 6, 7 or 8 as the case may "bee Some of the upwardly flowing molten glass flows inwardly ¾long the surface of the molten glass in the kiln towards the meniscus at the "bottom of the ri"b"bon 13» and the remainder of such upwardly flowing molten glass flows outwardly towards the adjacent wall portion 9, 10 or 11 as the case may "be. Behind each thermal "barrier there is a relatively cool zone of molten glass and the shaping of the side and end walls of the kiln to provide the wall portions 6, 7 and 8 underlying the thermal "barriers prevents glass from such relatively cool zones from flowing inwardly, "beneath said "barriers. In consequence there is appreciably less risk of devitri-fied glass grains which may form adjacent the wall portions 6, 7, 8, 9, 10 and 11, and of grains of corroded refractory material which may separate from the flux line "blocks of the wall portions 9, 10 and 11, from passing inwardly into the currents of glass feeding into the meniscus at the "bottom of the glass ri"b"bon 13· The quantities of glass in the said relatively cool zones "behind the electrode groups are circulated about horizontal axes and this in turn reduces the risk of devi-trification and "bubble formation occurring in the molten glass at lower levels of such zones. Moreover the local heating of the glass at the sites of the electrode groups reduces the viscosity of the glass flowing into the rear side and into the side margins of the ribbon so that the width of the marginal portions of the ribbon which exceed the maximum permissible thickness is reduced for any given speed of drawing and glass of a given quality standard can be drawn at a faster rate than in conventional processes. The increase in the usable idth of the drawn ribbon when using a process according to the invention can be for example as much as 10 cm.

In a modification of the process described with reference to Figs. 1 and 2, only the electrodes 16 were used. In that case the maximum drawing speed was somewhat less due to the restraint imposed by the some-what higher viscosity of the molten glass feeding into the marginal portions of the glass ribbon and the width of the marginal edge portions of the ribbon which had to be discarded as waste was greater due to contamination of the glass in those regions by devitrified grains.

The quality of the sheet glass at the operative drawing speed was however considerably better than when operating the plant in the same way but without using the electrodes 16.

According to another modification, only the elec-trode groups 17 and 18 were used. In that case the maximum drawing speed at which a ribbon having a given standard of flatness and uniformity of thickness could be drawn was higher than in a conventional process without a thermal "barrier, the width of the marginal portions which exceeded the maximum permissible thickness being normal. However it was found that there were appreciably more defects in the central part of the drawn ribbon, due to the presence of devitrified grains and bubbles, than when the electrodes 16 were used.

Reference is now made to Fig, 3 representing part of a Colburn-type glass-drawing machine equipped for perform-ing a process according to the invention. The machine comprises a drawing kiln or pot 20 comprising a sole 21, a rear end wall 22 and side walls only one of which, wall 23 , appear in the drawing. The kiln is supported by piers 2I. and 25. Molten glass is fed continuously along the kiln from a glass melting furnace towards the rear end wall of the kiln and a ribbon of glass 26 is drawn continuously from the surface 27 of the glass in the kiln and passes around a bending roller 28 into an annealing lehr. The annealing lehr, the rollers employed for conveying the glass ribbon through the lehr, the tip tiles and other standard parts of the machine are not illustrated and need no explanation as they are in accordance with standard practice.

The kiln 20 is shallow and the glass entering the continuously drawn ribbon 26 is drawn from the full depth of the molten glass in the kiln.

Within the kiln 20, at a position between the drawing zone and the rear end wall 22 , there are two electrical resistance heaters 29,30 extending transversely and over the full width of the kiln, parallel with such drawing zone and rear end wall. During the glass drawing process electric current is passed continuously through the resistance heaters 29 and 30 so as to effect local heating of the molten glass in the corresponding place in the kiln. The heaters increase the local temperature of the glass "by about 50°C. In consequence there is a continuous upward flow of molten glass to the surface 27 in the vicinity of the heaters. Such upward flow of molten glass takes place from a position in the vicinity of the underlying portion of the sole wall 21 of the kiln. The "basic flow pattern of the molten glass in the kiln, in the vertical longitudinal plane, is represented "by arrows in the drawing. It will "be noted that the flow pattern differs from that which takes place in conventional shallow kiln processes in that the flow of glass into the rear side of the ribbon does not take place from the rear end wall of the kiln but from a position which is spaced inwardly from that wall along the surface of the molten glass, that position being determined by the position of the heaters 29 ,30. The molten glass situated behind the vertical transverse plane containing the said heaters is substantially excluded from the flow of glass to the meniscus by the thermal barrier. Flow of molten glass beneath the thermal barrier cannot take place because the upward flow of molten glass commences from the bottom of the kiln* The said molten glass behind the thermal barrier is kept in movement by continuous circulatory convection currents. Any grains of devitrified glass which may form in the vicinity of the rear end wall or and/or any grains of corroded refractory material which may separate from the refractory wall at that region are virtually excluded from access to the drawing zone by such thermal barrier. By virtue of this fact and the low viscosity of the glass feeding into the rear side of the ribbon the maximum drawing speed at which good quality sheet glass can be drawn is increased by about 0 Reference is now made to Fig. U.Here again only such parts of the machine are illustrated as are required for the purposes of describing how the invention is incorporated. The drawing shows part of a kiln 31 into which the molten glass is continuously fed from a glass melting furnace, this kiln being supported on piers 33, 3k» The region 35 above the kiln is in fact enclosed, this being the region within the conventional drawing chamber, but the drawing chamber with its lip tiles, the annealing lehr, the conveying rollers by which the glass ribbon is supported and conveyed through the lehr, and other parts which are standard in this type of machine, have not been shown as they are not relevant to an understanding of the invention. The only part located above the kiln which is illustrated in the drawing is the so called bending roller 36 around which the glass ribbon passes before entering the annealing lehr.

The kiln 31 has a rear end wall 37 and a sole comprising sections 38 ,39 abreast of a threshold 40 which extends transversely across the kiln and is formed by walls 41 ,42 and 43» The threshold divides the lower portion of the kiln into an end compartment 44 and a front compartment 45· A series of electrical resistance heaters 46 extend upwardly through the threshold so as to intrude into the molten glass in the kiln and are connected to a source of electric currents which source is not shown* The lower portions 47 of these electrodes, located externally of the kiln, are sheathed with refrac-tory material. Glass is drawn upwardly from the surface of the molten glass in the kiln, so that a meniscus 48 the front and rear faces of which are designated 49 and 50 is established at the said glass sur ace , leading into the glass ribbon 51 having front and rear faces 52 ,53 respectively.

In an actual embodiment of the process using apparatus as illustrated by Pigs. 4 and 5» the electrodes 46 , which were located at intervals of 25 cm across the kiln, were connected to a voltage source such that the total power output responsible for heating the molten glass over the threshold 40 was 30 KW. This power output maintained the glass in the region over the threshold at a temperature 30°C a¾ove what it would otherwise he. It was observed that this local heating of the molten glass produced a stable upward flow of molten glass along the walls 1+1*2+3 of the threshold and continuing up to the surface of the molten glass in the kiln. The currents of molten glass ascending along the wall 2+1 flowed outwardly towards the rear end wall 37 of the kiln and descended along this wall so that the quantity of glass occupying the compartment 1+1+ was maintained in circulatory movement and was effectively isolated from the surface flow of glass feeding into the rear side of the meniscus 1+8. This surface flow of glass into the rear side of the meniscus derived wholly or mainly from the upward flow of molten glass along the front wall 1+3 of the threshold. Grains of corroded refractory material or of devitrified glass forming in the compartment 2+2+ were entirely or almost entirely excluded from access to the drawing zone, i.e., from the region at the surface of the molten glass in the kiln where the meniscus 1+8 was formed. This' latter result was moreover further promoted by the fact that the rate of feed of molten glass along the kiln from the glass melting furnace was slightly (about 1%) in excess of the rate of withdrawal of molten glass from the kiln in the form of the ribbon 51· The excess glass was continuously discharged through a series of skim holes formed in the end wall 37 at the level of the molten glass surface - 1+1 - and leading into flow channels 55<> A similar result could be achieved by forming the holes 5 in the end wall 37 at a lower level. Whatever "be the level of the skim holes, the discharge of glass through these holes could "be intermittent instead of continuous.

Pig. 6 shows part of a Pittsburgh-type machine. The drawing shows part of the kiln 60 having a sole 6l, a rear end wall 62 and side walls only one of which, wall 63, appears in the drawing. Preferably the end wall 62 is formed from a plurality of refractory elements of different compositions in order that the temperature of the mass of molten glass in the kiln can "be better controlled, but that is not essential. The kiln is kept filled with molten glass up to a level 6I., just sufficient to maintain a slow continuous discharge of molten glass via an overflow 65 at the top of the end wall 62.

The end wall 62 is specially formed to provide a threshold 66 completely submerged in the molten glass in the kiln and extending over the full width of the kiln. The top faces of the portions of the wall 62 which are abreast of the threshold 66 and are likewise submerged in the molten glass, support electrodes 67,68 formed of tungsten.

The kiln 60 is surrounded by the usual drawing chamber which is bounded at the rear and front by rear and front L-blocks 69,70, and through which the molten glass is drawn continuously in the form of a ribbon 71· The drawing chamber is surmounted by a tower section through which the ribbon is drawn upwardly by entraining rollers. These and other parts of the machine, such as the coolers provided within the drawing chamber and the edge rolls between which the edges of the ribbon are gripped a short distance above the surface of the molten glass in the kiln are not shown as they are in accordance with well established practice and are not relevant for the purpose of describing the invention.

The electrodes 67,68, which extend over the full width of the kiln 60 are connected to the poles of an electric current source as diagrammatically represented in the drawing, so that an electric current passes continuously through the molten glass between such electrodes. The electrical potential is such that the current density within the glass is nowhere in excess of 2 0.6 Amps/cm · The glass ribbon is fed by a forward flow of molten glass, Molten glass at the surface region of this forward flow feeds directly into the front side of ribbon as indicated by arrow 72 whereas some of the glass at a lower level in such forward flow continues past the drawing zone and then flows upwardly to the surface of the glass in the vicinity of the threshold 66, as indicated by arrow 73» before flowing back to the meniscus along the surface region of the molten - k3 - glass mass. Some of the molten glass flowing forwardly to the region of the rear end wall 62 descends against this wall and forms the submerged return current 7*4·.

The surface flow of glass into the rear side of the meniscus is maintained at a relatively low viscosity due to the continuous local heating of glass in the vicinity of the threshold. In consequence the stratification of the glass in the ribbon is improved and drawing can proceed at a faster rate than in a conventional process. The increase in the rate of drawing can be as much as 20-30 or even more. The quantity of glass behind the threshold 66 is effectively isolated from the flow of molten glass to the meniscus due to the thermal barrier maintained over the threshold by the electric heating current through the glass. The threshold itself presents glass from flowing beneath such thermal barrier. The heating of the glass at the vicinity of the threshold keeps the glass behind the threshold in circulation as represented by the current line 75 and this helps to reduce the risk of pollution of the glass flow 73 "by impurities such as grains of devitrified glass and of corroded refractory material. The threshold itself, being completely submerged in the molten glass, is subject only to a very slight extent to corrosion.

The restraint on the inward movement of impurities towards the drawing zone is increased by virtue of the outward surface current due to the withdrawal of a quantity of molten glass as an overflow current 76 via the overflow 65· It is to be particularly noted that the spacing of the drawing zone from the rear end wall 62 of the kiln is substantially less than in conventional processes. In fact, by adopting the illustrated system, the mass of glass held in the kiln rearwardly of the vertical plane containing the meniscus can be reduced by about one half as compared with conventional processes of the Pittsburgh type.

Referring to Figs. 7 and 8, the Pittsburgh-type machine there represented comprises a kiln 80 having a rear end wall 81, side walls, one of which, wall 82, appears in the drawings, and a sole 8-+. A ribbon of glass 85 with rear and front faces 86,87, respectively, is drawn from the molten glass in the kiln, the molten glass entering the ribbon via a meniscus 88 having front and rear faces 89,90, formed at the surface 91 of the molten glass in the kiln. The position of the meniscus 88 is stabilised by a draw bar 92.

The conventional superstructure comprising the drawing chamber and its associated parts and the tower section through which the glass ribbon is drawn have not been shown in the drawings as they are in accordance with conventional design.

The side margins of the ribbon 85 are of greater thickness than the main central part of the ribbon - U5 - width in the same way as the side margins of the ribbon shown in plan view in Fig. 2 , "but the greater thickness of the remote side margin of the ribbon shown in Fig. 7 has not been indicated in the figure.

The rear end portion of the kiln is locally widened in its upper portion. Fig, 8 shows the form of one side of the kiln at this rear end portion; the other side of the kiln is of identical form, the local widening of the kiln being symmetrical with respect to its central longitudinal vertical plane* Referring to Fig.3: at the rear end portion of the kiln, the side wall 82 is of reduced height, and terminates at the level 3k At the region of the upper level of this shallower side wall portion there is a horizontal wall portion 95 extending outwardly to the bottom of an upper side wall portion 96. A threshold 97 extends upwardly from the horizontal wall portion 95· This threshold extends over the full distance between the rear end wall 81 of the kiln and a wall 98 which forms the front boundary wall of the lateral extension of the kiln. The horizontal wall portion 95 supports tungsten plates 99 and 100 located on opposite sides of threhold 97. These tungsten plates are connected to the poles of a source 101 of alternating electric current. When the machine is in use, the source 101 produces an alternating electric current which passes through the molten glass, between the plates 99 , 100 and thus over - S - the threshold 97. A hot zone is thereby maintained at this region, involving a continuous upward flow of molten glass along the sides of the threshold and up to the surface of the molten glass in the kiln. As appears from Fig. 7 the electrode plates 99 , 100 do not extend over the full distance "between the walls 81 and 98. The plates could of course extend over the full distance if so required. In fact, the dimensions of the plates is chosen in order to attain a required predetermined electric current density through the molten glass. The electric currents through the molten glass located within the lateral extensions of the kiln heat the currents of glass flowing from the rear end of the kiln into the end portions of the meniscus 88 from which the side margins of the glass ribbon are drawn. In conventional processes the molten glass which flows back to the end regions of the meniscus from the side walls of the kiln near its rear end tend to acquire an appreciably lower temperature and thus a higher viscosity. This is one of the factors which normally limits the maximum permissible drawing speed. By heating such currents of molten glass by means of a thermal barrier, the drawing speed can be increased and/or there is an increase in the width of the usable portion of the ribbon over which its thickness is below a permissible maximum.

By way of modification of the process described - kl - with reference to Pigs. 7 and 8 , the metal plate electrodes could be replaced by pools of molten metal, such as molten tin, or by pools of molten metal salt, with the advantage that the frictional restraint on the flow of the molten glass feeding into the ends of the meniscus at its rear side is still further reduced.

Under favourable conditions, it is possible by adopting the form of kiln and the thermal barrier system described with reference to Pigs. 7 and 8 to attain a drawing speed which is of the order of 1.8 to 2.0 times the maximum drawing speed attainable in conventional processes.

In Pig. 9 there is illustrated a part of a Colburn-type machine, namely a part of the kiln 102 from which molten glass is drawn, and the bending roller 103 over which the drawn ribbon of glass passes preparatory to conveyance through the annealing lehr. The drawing chamber and the annealing lehr and various other parts which are standard in machines of this kind but are not relevant for the purpose of describing the invention have been omitted from the drawing. The kiln 102 comprises a rear end wall 10k and a sole comprising sections 105 ,106 respectively located to the rear and in front of a threshold 107 extending transversely across the kiln. The threshold 107 separates the lower portion of the kiln into a rear compartment 108 and a front compartment 109. The - J+8 - sole sections 105,106 are covered by layers 110 and 111 of molten tin which likewise extend over the full internal width of the kiln. The layer 111 is retained at its front boundary "by a sill 112. The side wall of the kiln incorporates, in place of one of the normal refractory "blocks, a block of tin oxide* The drawing shows the tin oxide block 114 which is incorporated in the side wall 113 which appears in the drawing. The other side wall incorporates an identical tin oxide block at a position directly opposite the block lli+.

The layers 110, 111 of molten tin at the bottom of the kiln are connected to opposite poles of a source of electric current as represented in the drawing. The pole of the electric current source which is connected to the layer 110 is also connected to each of the tin oxide blocks incorporated in the side walls of the kiln. In consequence a flow of electric current is maintained along three paths within the mass of molten glass held in the kiln. One electric current path lies between the layers of molten tin 110,111 at the bottom of the kiln, and consequently over the threshold 107» A second electric current path lies between the layer 111 of molten tin and the tin oxide block 114» A third electric current path lies between that same layer 111 and the opposite tin oxide block in the other side wall of the kiln. The electric current paths between the layer 111 and the tin oxide blocks - k9 - extend through quantities of molten glass immediately adjacent the inner surfaces of the side walls of the kiln» The superficial area of the layers 110, 111 of molten tin is such that the current density along the first of said paths is nowhere in excess of 0.5 2 Amps/cm , while the current density along the second and third of such paths is nowhere in excess of 2 0.2 Amps/cm . At such low current densities there is little risk of bubble formation within the molten glasso The heating of the glass by the electric current flowing along the first said path causes molten glass to flow upwardly along the front and rear faces of the threshold 107 and to continue moving upwardly to the surface of the molten glass. The electric current density along this path is at its greatest in the region above the threshold and the glass is consequently heated to the greatest extent at that position. Any impurities such as grains of devitrified glass or grains of corroded refractory material which becomes entrained in the flow of molten glass at the rear side of the threshold 107 do not become caught up in the flow of glass to the drawing zone but are recirculated back in the compartment 108, towards the rear end wall 10 . In this compartment the molten glass is maintained in stable anticlockwise rotary movement. In the compartment 109, the molten glass flowing along the layer 111 of molten tin flows upwardly along the front side of the threshold 107 , up to the surface region of the molten glass and "back along that surface region into the meniscus through which the glass flows into the ribbo 115 At the same time, the con-tinuous heating of glass in the zones extending along the side walls of the kiln, between the layer 111 of molten tin and the tin oxide "blocks such as 111+ in such side walls, stabilises the currents of glass along such walls while diminishing the frictional retardation of the currents of molten glass along these walls and promoting the free flow of molten glass to the meniscus., Using such a process, a ribbon of glass of substantially constant thickness can be drawn at a speed which is of the order of 1.5 to 1.7 or even more times the maximum drawing speed in conventional processes.

Pig. 10 illustrates a modification of the process and machine described with reference to Fig. 9. The modification resides in the provision of a top extension element on the threshold 107 , this extension element comprising a base portion 116 supporting a curved plate 117 which extends upwardly to a short distance below the surface of the molten glass. This extension element is made of molybdenum and the plate 117 constitutes an equipotent ial plane in the path of the electric current between the layers 110 and 111 of molten tin. The form and dimensions of this plate influence the electric field lines and in consequence the electric current density.

Moreover the plate 117 forms a supplementary "barrier against the movement of polluted glass currents over the threshold from compartment 108 to compartment 109.

Fig. 11 represents part of a shallow kiln machine generally of the Colburn type, in which the molten glass 120 in the kiln floats on quantities of molten metal held in the "bottom of the kiln. Within the lower part of the kiln there are rear and front compartments 121, 122 respectively containing, at the "bottom of the kiln, layers of molten tin 123,12^, the said compartments and the said different quantities of molten tin, "being separated "by a threshold 125» The said quantities of molten tin are respectively connected to the opposite poles of an alternating electric current source 126 so as to create a thermal "barrier at the position of the threshold. Molten glass is drawn from the surface of the molten glass in the kiln, through a meniscus 127, into a rihbon 128.

A special feature of the illustrated machine consists in the construction of the refractory walls of the kiln so as to define channels and recesses 129,130 which are maintained filled with quantities of tin respectively continuous with the molten tin layers 12-4-,123= The quantities of tin 129,130 are in the solid state, the dimensions of the recesses in which these quantities of tin are held being such that the current density through the metal within these recesses is not sufficient to melt the metal. By virtue of the solid state of the metal at these positions the connections to the alternating electric current source can be made very easily, avoiding the multiple problems well known to be involved in the connection of electric cables to metal electrodes maintained at very high temperatures Figs. 12 to 17 represent various forms of threshold and associated heating means for creating a thermal barrier. Any of the threshold and heating systems shown in these figures can be used in a Pittsburgh-type and in a Colburn-type process and any of such systems can thus be used in any of the machines or processes already described with reference to FigSo 1 to 11. When adopting a Pittsburgh or any deep-kiln type process in which the molten glass is not drawn from the full depth of the molten glass in the kiln it is of course necessary for the threshold to reach above the upper level of the submerged return current of glass flowing back along the kiln towards its feed end.

The threshold shown in each of Figs. 12 to 17 is designated 131 · The threshold is hollow and comprises a top wall portion 132 and rear and front walls 133 , 134 respectively.

In the system shown by Fig. 12 , the threshold is heated by flames IkO issuing from a gas burner I l extending along the interior of the threshold, and located so that the flames play against the top wall portion 132 and the upper portions of the rear and front walls 133 ,13 » The elongated form of these walls is favourable to an intense and uniform heat exchange "between the threshold and the molten glass located at the site of this threshold.

The top wall portion 1 2 of the threshold shown in Fig, 13 is constituted "by an electrically conductive plate composed of tin oxide. The "bottom face of this plate is in contact with a non-oxidising atmosphere.

The tin oxide plate is connected to an electric current source (not shown). The plate is traversed by electric current which is of uniform density over the whole area of the plate so that there is an entirely uniform heat distribution to the molten glass in contact with the top of the plate.

Referring to Pig. l > the threshold here shown is formed of ordinary refractory material hut the top wall portion supports a molybdenum plate l 2 having a width somewhat less than the width of the top wall portion 132 of the threshold, taking into account the good electrical conductivity of molybdenum. The molybdenum plate is connected to a source of electric current.

The current density within the plate is uniform over its entire area, which ensures a. uniform heat distribution, this heat being concentrated in the column of molten glass of relatively restricted horizontal - 5k - s cross-section, located a"bove the plate 11+2· It is not necessary to maintain a special atmosphere in contact with the "bottom of the metal plate.

In Fig. 15 the top wall portion 132 of the threshold 5 is formed "by an electrically conductive plate of tin oxide* In the corner angles "between the sole section 11+3 and the rear wall 133 of the threshold on the one hand and "between the sole section 11+1+ and the front wall 13k of the threshold. on the other hand there are fillets 11+5 , 11+6 made of tin oxide and extending along the entire length of the threshold. The fillets 11+5 , 11+6 are connected to opposite poles of an alternating electric current source 11+7. The cross-sectional forms of the fillets are selected in dependence on the required current density distribution within the molten glass, taking into account its Theological properties, in the vicinity of the threshold. In operation, electric current passes through the molten glass, between the electrodes 132 and 11+5 on the one hand and "between electrodes 132 and 11+6 on the other hand, such electric currents creating a hot zone which envelops the threshold. The arrangement illustrated in Pig. 15 affords the advantage of countering any tendency for the temperature of the molten glass along the walls 133 > 131+ of the threshold to become too low or for the molten glass adjacent those walls to stagnate and to give rise to the formation of devitrified grains.

In the system shown in Fig. 16 , there are two electrodes 11+8 .1-+9 disposed along the entire length of the thresholdo The electrode 11+3 which is connected to one pole of an electric current source 150 is a plate of platinum disposed against the rear wall 133 of the thresholdo The other electrode 1 , which is connected to the other pole of the electric current source, comprises a molybdenum crucible containing a quantity of molten tin* This arrangement is suitable in cases in which there is naturally a very hot and strong upward current of glass along the front wall 13k of the thresholdo The passage of electric current between the electrodes 11+8 and 11+9 counters any tendency for such strong flow of molten glass at the front side of the threshold to induce impurities forming in the compartment 151 at the rear of the threshold to pass into the compartment 152. The form of the electrode 11+8 can be selected to achieve a required predetermined electric current distribution along the rear side of the threshold. The quantity of molten tin in the electrode lk9 can easily be removed from the crucible and replaced by another metal or by a suitable metal salt, without interrupting the operation of the machine.

In the system shown in Fig. 17 » heating is achieved by means of three electrical resistance heaters 153 located above the threshold. There is very little heat loss through the top wall of the threshold. The resistance heaters can "be effective over the entire width of the kiln or over a part only of that width* The heat generated in the resistance heaters induces an upward flow of molten glass at the location of the threshold 131. The upward flow of glass continues to the surface region of the molten glass in the kiln and counters tendency for molten glass currents to flow over the threshold from compartment 151 to compartment 152.

When using heating means for creating a thermal "barrier transversely of the kiln, i.e., parallel with the plane containing the bottom portion of the drawn glass ribbon, it is not essential, whatever form of heating means be employed, for such heating means to extend over the full width of the kiln. For example, if such heating means extends only over a part of the kiln width which is co-extensive with the main part of the ribbon width, between its marginal zones, molten glass currents which flow back from the rear end wall of the kiln may flow laterally outwards past the ends of the said thermal barrier and carry impurities into the marginal portions of the ribbon but the said thermal barrier will protect the main central part of the ribbon from contamination by such impurities. As the marginal portions of the ribbon will be removed as waste in any event, such contamination is not a serious drawback.

Reference is now made to Pig. 18. The drawing shows part of a Pittsburgh-type machine comprising a lciln 159 having a sole l6o and a rear end wall l6l which incorporates a threshold l62, and a top rear portion 163 located slightly "below the level iSk of the molten glass in the kiln in order to permit a continuous overflow of molten glass at the rear of the lciln. In a trough between the threshold and the said top rear portion 163 there is a tungsten plate 165 1 cm in thickness, serving as an electrode. Over the kiln, there is the usual drawing chamber l66 bounded at the rear and front by L-blocks 167,168, principal coolers l69 and auxiliary coolers 170 located within the drawing chamber on opposite sides of the path along which the glass ribbon is drawn, and the tower section 171 through which the glass ribbon 172 is drawn upwardly while undergoing cooling. The glass ribbon 172 is drawn from the surface of the molten glass in the kiln via a meniscus 173» The front face of the glass ribbon is marked 17k and its rear face is marked 175» Instead of the traditional form of draw bar the machine incorporates a bar 176 having a recess 177 extending over nearly the entire length of the bar. A pool 178 of molten lead is held in this recess and serves as an electrode. The tungsten plate 165 and the pool 178 of molten lead are connected to the opposite poles of an alternating electric current source 179. The areas of the surfaces of the plate 165 and the pool 178 are important factors. They are such that the electric current density is not at any position on those surfaces in 2 excess of 0.5 amps/cm so that risk of bubble formation within the molten glass is minimal, but such that the current density is sufficient to cause the temperature of the glass over the threshold to be kept about 60°C higher than it would be in the absence of the electric current. The presence of the threshold 162 and the continuous generation of heat in the glass in the vicinity of this threshold causes the molten glass in contact with. the threshold and above the plate 165 to be kept in stable rotary movement about a horizontal axis in a direction which is clockwise in the aspect of the drawing. In consequence, impurities which may contaminate the molten glass in this region are kept displaced away from the drawing zone. The surface flow of molten glass which feeds into the rear side 17h of the glass ribbon 172 is kept substantially free of impurities forming in the region behind the threshold. The glass which goes to form that surface flow into the rear side of the ribbon is heated in the hot zone in the vicinity of the threshold and the viscosity of that glass can be kept comparable to the viscosity of the glass in the forward flow of glass feeding into the front face 175 of the glass ribbon. The current of molten glass which flows in contact with the pool 178 of molten lead is perfectly steady. This is in contrast to the flow of molten glass in contact with the top of an ordinary slotted draw "bar. In that case irregularities in the "bar surface disturb the glass flow and often cause irregularities in the thickness of the drawn glass ribbono 5 The extremities of the bar 176 are at a lower temperature than the pool 178 of molten lead and this lower temperature at the extremities of the bar assists in stabilising the position of the ends of the meniscus 173 via which molten glass flows into the side margins of the ribbon. 0 The continuous overflow of molten glass across the top rear portion 163 of the rear end wall l6l of the kiln, provokes a current of glass in the rearward direction which further helps to prevent impurities from passing from the region behind the threshold into the currents 5 of molten glass feeding into the ribbon. Such continuous rearward discharge of molten glass also serves to prevent the concentration of impurities in the region behind the threshold from building up to a high value.

The substantial thickness of the rear end wall l6l 0 of the machine shown in Pig. 18 , is of value in reducing heat loss from the glass in the kiln by conduction through the said wall and consequently helps to keep the glass near to the surface I6I. at the required temperature for feeding into the ribbon..

I. In the machine illustrated in Fig. 18, the density of the electric current between the electrodes must be sufficient to reheat the relatively cold currents of molten glass whic flow upwardly between the draw bar 176 and the rear end wall l6l of the kilno This factor affecting the choice of current density does not arise in the case of a machine as represented in Fig.l9o This figure shows a Pittsburgh-type machine which is similar to that shown in Fig. 18 save for certain modifications which will now be described.. The reference numerals used in Fig, 18 are used also in Fig. 19 for designating corresponding parts of the modified apparatus.

In the machine shown in Fig. 19 the rear end wall l6l is of even greater thickness, notably at a region located in an upper portion of its height but below the level of the molten glass in the kiln. The location, shape and cross-sectional dimensions of the rear end wall are such that an inwardly projecting ledge portion 180 thereof extends beneath the meniscus 173· This ledge portion 180 has a recess 181 in its top surface, the recess extending over nearly the entire internal width of the kiln. The recess holds a pool 182 of molten tin. Instead of molten tin, a pool of some other molten metal could be used or a pool of a suitable molten metal salt, e.g., a lithium salt. The top surface of the ledge portion 180 can be, for example, 20 centimeters beneath the surface 161+ of the molten glass in the kiln. The pool 182 of molten tin and the tungsten plate 165 are connected to opposite poles of a source 183 of alternating electric current.

The machine and process which are the subject of Fig.19 possess similar advantages to the machine and process described with reference to Pig.18 insofar as those advantages derive from the substantial thickness of the rear end wall l6l of the kiln, the presence of the threshold 162, the passage of electric current between the electrodes to maintain a hot zone above the threshold, and the continuous overflow of molten glass over the top rear portion 163 of the rear end wall. As distinct from the machine and process according to Pig. 18, there is in the machine and process according to Pig. 19 no upward flow of glass between the electrode beneath the drawing zone and the rear end wall of the kiln. In the system shown in Pig. 18, there is an upward flow of glass between the bar 176 and the rear end wall and this upward current mixes with the flow of molten glass forwardly over the bar. In the Pig. 19 system there is no such mixing of currents feeding the meniscus. The process illustrated by Pig. 19 can thus be regarded as one in which glass is drawn from a "mono-current" and this is particularly favourable to the drawing of sheet glass of a very high standard of homogeneity. In an actual embodiment according to Pig.19 an electric current density of only 0.3 amps/cm at the plate 165 was found to suffice for permitting a drawing speed which in the Pig. 18 process required a current density at that place of 0.5 amps/cm . Of 2 course a current density higher than 0.3 amps/cm , e.g., 2 a current having a density of 0.5 amps/cm , can "be used when employing a process according to Pig. 19, in which case the drawing speed can easily be increased to 1.8 or even 2.0 times the maximum drawing speed which is possible in a conventional Pittsburgh process.

When working according to Fig. 19, the molten glass which goes to feed the rear face 171+ of the glass ribbon 172 flows along the surface of the pool 182 of molten tin and is heated during its upward flow in the vicinity of the hot zone prevailing over the threshold 162. This flow of molten glass reaches the meniscus with but minimal contact with solid surfaces. These factors principally explain why so high a drawing speed is possible. Inasmuch as the flowability of the molten glass feeding into the rear side of the ribbon is comparable with that of the glass flowing into its front side, there is very good stratification of the glass in the drawn ribbon.

When performing a process according to Fig. 19 and also when performing a process according to Fig. 18, the rear face Ilk of the ribbon of drawn glass is notably flat. Because the extremities of the ledge portion 180, adjacent the side walls of the kiln, are somewhat cooler than the pool 182 of molten tin, they assist in stabilising the position of the end portions of the meniscus 173 via which the molten glass feeds into the margins of the ribbon.

Pig. 20 is an anamorphosic photographic image of part of an edge face of a sample of sheet glass according to the invention, the edge face being exposed by cutting the sheet glass along a line normal to the line of draw, the photographic image being called a "striascope" .

In the photographic image, seams of glass of different refractive indices appear as bands of different optical densities and the way in which differently refractive seams of glass are distributed is revealed by contour lines which can be seen in the photographic image and which correspond with boundaries between juxtaposed seams. It will be noted that these lines are mainly substantially parallel. The arrangement of the lines is such that they suggest a basic pattern of flat ellipses extending sensibly from one edge to the other edge of the drawn sheet glass. This suggestion derives particularly from the presence of shallow outwardly convex curves 186 extending in the lengthwise direction of the photographic image, and the presence of relatively smooth curves 187 of much smaller radii joining the ends of opposed shallow curves. It is worthy of note that in the case actually shown in Pig. 20, the above mentioned flat ellipses, one within another, have a substantially common centre, which is excentrated towards one of the faces of the sheet.

The remaining seams located near the other face could be thought of as forming shallow outwardly convex curves possibly belonging to larger ellipses* - 61+ - A sample of sheet glass such as the one represented, by Pigo 20 was tested in an inter erential microrefrac-tometer in which light was projected from a slit source in such a way as to form a series of straight parallel interference fringes. The sheet was placed in the light "beam in such a way that the light "beam entered one side edge face of the sheet and emerged from the opposite parallel side edge face of the sheet s the sheet being in a plane intersecting the interference fringes at k5°<· The interference fringes appeared as shown in Pig* 21, i.e. without any evident fault or break. The two oblique parallel lines intersecting the interference fringes represent the major faces of the glass sheet.

The distinctive characteristics of the sheet glass according to the invention as referred to in connection with Pigs. 20 and 21 become very apparent when those figures are compared with Pigs. 22 and 23. Pig. 22 shows a photographic image called "striascope" produced under the same conditions as the image in Pig. 20, of part of an edge face of a sample of sheet glass drawn by a classical Pittsburgh type process. Pig. 23 shows the effect which the presence of a sample such as drawn by a classical Pittsburgh type process had on the appearance of the interference fringes when the sample was tested in the same interferential microrefractometer used in testing the sheet represented by Pig. 21, and under the same conditions. As appears from Pig. 22 the contour lines do not exhibit any kind of elliptical pattern* In some places of the photographic image, the lines converge to form a sharp angle, in contradistinction with the smooth curves of Fig* 20, When placed in the light beam in the interferential microrefractometer the sheet glass caused a very marked fault or break 189 to appear in the interference fringes, as shown in Fig#23» This break is indicative of an abrupt change in the refractive index as between adjacent seams of glass located centrally of the thickness of the glass sheet.

Pig. 21+ shows a typical brush line pattern on a face of a sheet of glass drawn by a classical Libbey-Owens drawing process. Those brush lines are detected by interferometry using the known Fizeau fringes. The sheet of glass to be tested is disposed on a polished sheet of glass in such a way that the two sheets form an extremely small angle with each other. The line of intersection of the two sheets must be perpendicular to the direction of draw of the glass sheet to be tested.

The polished sheet of glass must have a planeity such that the tolerance of thickness is in the range of The faces of this polished sheet must be as truly parallel as possible.

When the two sheets so disposed are irradiated by rays which are sensibly perpendicular to their aces , these rays upon return show a pattern of alternately white and black fringes, as can be seen in Fig. 24.

Bach black line is representative of a line of equal thickness of the sheet. These lines of equal thickness show small waves of high frequency, which may be called "saw teeth". The presence of these "saw teeth" is indicative of the existence of "brushline" defect and allows it to be ascertained that this defect is in the nature of very small waves of the order of 0.3 mu of thickness, with a width of from 0.1 to 1.0 mm. When the two main faces of a sheet of glass according to the invention were subjected successively to examination for defects in precisely the same way as the sample of glass drawn by the Libbey-Owens process , no brush lines were detected on either face. The sample of sheet glass according to the invention and subjected to the said examination was a sample sheet glass drawn by a process according to the invention in which there was a thermal barrier in the molten glass in the kiln at a position located directly rearwardly of the drawing zone.

As an alternative, the brush lines can be detected by a method using reflection by projecting a beam of light onto the face of the glass sheet at an angle of incidence of the order of 65°, the axis of the beam being in a plane normal to such face and normal to the line of draw of the glass sheet, to cause light rays to be reflected from the said face onto a light-diffusing screen placed about 1 m from the glass sheet. The photographic images reproduced as Pigs. 20 and 22 were produced "by means of striascopic apparatus as shown schematically in Fige 25· This apparatus comprises an incandescent white light source 191, an achromatic condenser 192 located near to the light source, an objective 192 , a Foucault system 193 and a mask plate 19 in which there is a vertically disposed rectangular slit. Each of the photographic images is the image of the light pattern transmitted in the stria-scope through a strip 196 , 10-20 mm in width, separated from the appertaining drawn glass ribbon by cuts running across the full width of the ribbon normally to its line of draw. The strip was mounted in a transparent tube 197 filled with a liquid 198 having a refractive index very close to that of the glass. A suitable liquid is ethylsalicylate or mononitrobenzene , but there are also numerous other suitable liquids. The projected light rays passing through the slit in the mask plate 13k are recorded by a light-sensitive film 199· The striascope produces a photographic image of 3-10 times magnification of the thickness of the sheet* In order to produce a photographic record of the distribution of seams of glass of different refractive indices over the whole cross-section of the sample strip 196 , the strip is progressively displaced in its own plane and in a direction parallel with its own longitudinal axis (i.e., along a line normal to the plane of the drawing) so that the strip moves progressively through the path of the rays which impinge on the light-sensitive film and this film is simultaneously displaced in its own plane along a path normal to the plane of the drawing and in a direction opposite to the direction of displacement of the strip 196. In consequence, successive portions of the film are successively exposed through the above mentioned slit. This slit has its longer dimension vertical and has a width of from 0. 2 to 1 mm* The linear speed of the film 199 can be, e.g., 10 to 20 times smaller than the linear speed of the strip 196 so that the photographic image of the whole strip is magnified in thickness but reduced in lengthc Thus the striascope produces an anamorphosic photographic record.

The displacement of the sample strip 196 and the light-sensitive film 199 can be mechanically synchronised. For example, the said strip and the said film can be displaced by a common motor through a reduction gearing which is variable to permit the relative speed to be pre-set within a given range.

In making a photographic record in a striascope as above described it is suitable to use a "Copex Copy" film as marketed by Agfa-G-evaert of Mortsel, Belgium and to develop the film with an ordinary metol-hydroquinone developer as commonly used for the development of photographic prints on light-sensitive paper.

Reference is now made to Pig. 26. The interferential refractometer schematically represented by this figure and using the known method of Nomarski comprises a light source 200 , a condenser lens 201 , a mask 202 with a slit aperture 203 » an objective 20U , a birefringent double prism unit 205 , generally called a "Wollaston" prism, with front and rear polarising filters 206 and 207, and an optical focussing device 208 which may, e.g., be formed by an eye piece of an ordinary microscope or a device which focusses the transmitted rays onto a light-sensitive recording material. In a test a Nachet 300 microscope was used with a 3x objective and a 6x eye-piece , equipped with a photographic Polaroid chamber having a chamber index of 0.8. The film used was a black and white Polaroid roll film, 2 1/2 x 3 1/k inches, sensitivity 3000 ASA, type 37 , the exposure time being 1 second.

In the absence of a sample to be tested, the parallel light beam from the slit aperture is divided into differently polarised and differentially retarded parts and the transmitted rays are focussed to form an interference pattern comprising a series of straight parallel bands as represented in Pig. 21. Further details about the construction and functioning of an interferential microrefractometer of this kind can be found in the aforesaid paper entitled "Objectif interfdrentiel a prisme de Wollaston" published by Techniques de l'lngdnieur, of 21 , rue Cassette, Paris VI,. Prance .

In order to use the refractometer for testing a sample 209 of sheet glass, the edge faces of the sample are polished and it is placed in the parallel light beam from the slit aperture so that the beam traverses the sheet from one side edge face to the opposite side edge face and so that the sheet is in a plane intersecting the parallel interference bands. When a sample of sheet glass according to the invention is thus placed in the light beam, the continuity of the interference bands is not affected, whereas a sample of sheet glass drawn by a classical Pittsburgh type drawing process causes a marked break in the bands as shown in Pig 23* It is to be understood that the invention is in no way limited to the various embodiments which have been hereinbefore described by way of example and that numerous modifications are possible within the scope of the invention.

Claims (1)

1. 38776/3 T^ W WHAT IS CLAIMED IS: 1· A process of manufacturing sheet glass by continuously feeding molten glass into a kiln to establish a continuous forward low of glass to a drawing zone at which molten glass is continuously upwardly drawn from the surface of the molten glass in the kiln in the form of a ribbon* characterised in that in at least one place which in plan aspect of that kiln is spaced inwardly from a boundary of the surface of the molten glass in the kiln* the molten glass in the kiln is heated to maintain at that place a thermal barrier formed by an upward low of molten glass which rises to said surface from a position in the vicinity of a wall portion of or in the kiln so that molten glass between the said barrier and the said boundary is substantially prevented by such wall portion from flowing beneath said barrier, the depth of the molten glass bath above the said wall portion being less tthhaann tthhee ddeepptthh ooff tthhee ssaaiidd bbaatthh iinn tthhee aaddjjaacceenntt tthheerreettoo** 22·· AA pprroocceessss aaccccoorrddiinngg ttoo CCllaaiimm 11»» wwhheerreeiinn mmoolltteenn ggllaassss aatt tthhee surface of said forward flow feeds directly into the bottom of the ribbon at its front side whereas molten glass at a lower level of such forward low rises at a position behind the drawing aone and forms an oppositely directed surface flow which feeds into the ribbon at its rear side* characterised in that a said thermal barrier is created at a position which coincides with the said position where molten glass rises "behind the drawing zone« 3« A process according to claim 2, characterised in that a said thermal barrier is created which extends at least over a portion of the kiln width which is co-extensive with the width of the ribbone 1+. A process according to any preceding claim, characterised in that at least one said thermal barrier is created at a position which in plan aspect of the kiln is situated adjacent a side boundary of the surface of the molten glass in the kiln and at a position from which there is surface flow of molten glass towards an edge or margin of the ribbon<> 5· A process according to any preceding claim, characterised in that there is a said thermal barrier which is confined to an upper portion of the depth of the kiln. 6· A process according to any of claims 1 to i+, characterised in that there is a said thermal barrier which extends over the full depth of the kiln. 7. A process according to any preceding claim, characterised in that there is at least one said thermal barrier which is maintained above a threshold completely immersed in the molten glass. 8. A process according to claim 7, characterised in that there is at least one said thermal barrier which is maintained above a threshold completely immersed in the molten glass and wherein there is an upward flow of molten glass which rises against said threshold and continues its upward movement above the level of the top of such threshold. 9. A process according to any claim 7 or 8, characterised in that there is at least one said thermal barrier which is maintained above a hollow threshold and the said upward flow of molten glass at that place is maintained by generating heat in the space within the threshold. 10. A process according to any of claims 7 to 9, characterised in that there is at least one said thermal barrier which is maintained above a threshold con-stituted by a single solid wall and the said upward flow of molten glass at that place is maintained by heating the bottom of such wall. 11. A process according to any of claims 7 to 11, characterised in that there is at least one said thermal barrier where said upward flow of molten glass is maintained by heating means which is incorporated in or forms part of the or a wall forming a said threshold or part thereof. 12. A process according to any of claims 7 to 11, characterised in that there is at least one said thermal barrier where said upward flow of molten glass is maintained by heating means which is in contact with - 71+ - the molten glass adjacent a said threshold. 13. A process according to claim 12, characterised in that there is at least one said thermal barrier where said upward low of molten glass is maintained by heating means which is disposed on a side face and/or on the top face of a said threshold, li+o A process according to any of claims 7 to 13, characterised in that there is at least one said thermal barrier which is maintained above a threshold, and the upward flow of molten glass at that place is maintained by heating means located within the body of molten glass and spaced from the thresholde 15 A process according to any of claims 7 to 11+, characterised in that there is a said threshold which is upwardly extended by a plate. 16. A process according to any of claims 1 to 7, characterised in that there is at least one said thermal barrier where the upward current of glass is maintained by generating heat within the kiln at that place. 17. A process according to any preceding claim, characterised in that there is at least one said thermal barrier which is maintained by passing an electric current through the molten glass between electrodes, 18» A process according to claim 17, characterised in that there is at least one said electrode formed by a pool of molten metal or molten metal salt, 19. A process according to claim 17 or 18, characterised in that at least one said electrode is disposed above a threshold completely immersed in the molten glass. 20. A process according to claim 17 or 18, characterised in that a said heating current is maintained between electrodes disposed on opposite sides of a threshold completely immersed in the molten glass. 21. A process according to any of claims 17 to 20, characterised in that there is at least one said electrode disposed beneath the position at which the ribbon is drawn from the surface of the molten glass in the kiln. 22. A process according to claim 21, characterised in that beneath the position at which the glass ribbon is drawn from the surface of the molten glass in the kiln, there is an element in which an electrode in the form of a quantity of molten metal or molten metal salt is held. 23. A process according to any preceding claim, characterised in that glass is withdrawn from the surface of the molten glass in the kiln at a position behind a said thermal barrier to induce an outward surface current of glass through said barrier. 2-+· Apparatus for use in drawing sheet glass, said apparatus comprising a kiln having a feed end at which the kiln can be continuously fed with molten glass, and - means for continuously drawing a ribbon of glass upwardly from'he surface of the glass at a drawing zone in the kiln* characterised in that means is provided for locally heating the molten glass in the kiln in at least one place which in plan aspect of the kiln is spaced inwardly from a boundary of the said molten glass surface in order to maintain at that place a thermal barrier formed by an upward flow of molten glass which rises to the said surface from a position in the vicinity of a wall portion of or in the kiln wheeby aafcd- aH-portion ■Lmtiira m tiiti imifrwrttifl nirrm ^4 A n» 4rv **m -P ¾ *wwt beneeefa-i from a position in the vicinity of a wall portion of or in the kiln* the depth of the molten glass bath above the said wall being less than the near depth of the said bath in the kila* 25· Apparatus according to Claim 24» wherein the drawing zone is spaced from that boundary of the surface of the molten glass in the kiln which is opposite the feed end of the kiln, characterised in that a said local heating means is provided or maintaining a said upward low of glass at a place which in plan aspect of the kiln is between the drawing zone and the said opposite boundary reached by the surface of the molten glass in the kiln when the apparatus is in use. 26· Apparatus according to Claim 25» characterised in that a said local heating means is provided for (Maintaining a said upward flow of glass between the drawing zone and the said opposite glass surface boundary* such upward flow occurring at least over a portion of the kiln width which is co-extensive with the width of the ribbon* 27· Apparatus according to any of claims 2k to 26 , characterised in that a said local heating means is provided for maintaining a said upward flow of molten glass in at least one place which in plan aspect of the kiln is adjacent a side boundary to which the surface of the molten glass in the kiln extends y/hen the apparatus is in use. 28· Apparatus according to any of claims 21+ to 27, characterised in that a said local heating means is provided which is arranged so that it is effective for maintaining an upward flow of molten glass over the full depth of the kiln. 29. Apparatus according to any of claims 2k to 27 , characterised in that a said local heating means is provided which is arranged so that it is effective for maintaining an upward flow of molten glass in an upper portion of the depth of the kiln. 30. Apparatus according to any of claims 2k to 29, characterised in that there is at least one threshold located so as to "be completely immersed in the molten glass in kiln when the apparatus is in use and so that the or a said heating means maintains a said upward flow of molten glass above such threshold. 31. Apparatus according to claim 30 , characterised in that there is heating means for maintaining a said upward flow of molten glass from a position against a side face of a said threshold. k 32 o Apparatus according to claim 30 or 31 , characterised in that there is at least one said threshold which is hollow and means is provided for generating heat within such threshold. 5 33 e Apparatus according to any of claims 30 to 32 , characterised in that there is at least one said threshold formed "by a single solid wall and means is provided for heating the bottom of such wall to create a said upward flow of molten glass above such threshold. 10 3k» Apparatus according to any of claims 30 to 33 * characterised in that there is at least one said threshold which has or is formed by a wall which incorporates heating means. 35 o Apparatus according to any of claims 30 to 3k» 15 characterised in that there is at least one said threshold having a surface on which means for heating the glass is disposed so as to be in contact with the molten glass in the kiln when the apparatus is in use. 36 . Apparatus according to any of claims 30 to 35 » 20 characterised in that there is heating means for maintaining an upward flow of glass rising above a thresholds said heating means being disposed adjacent to or spaced from said threshold and in a position so as to be in contact with the molten glass in the kiln when the 25 apparatus is in use. 37 o Apparatus according to any of claims 2i+ to 36 , characterised in that there is a said heating means for heating molten glass in the kiln to maintain a said upward flow of molten glass, which heating means generates heat directly within the kiln<> 38 0 Apparatus according to any of claims 21+ to 37 , characterised in that there are electrodes between which electric current can be passed through molten glass in the kiln for maintaining a said upward flow of molten glass, 39 e Apparatus according to claim 38 , characterised in that there is at least one said electrode formed by a pool of molten metal or molten metal salt. 1+0. Apparatus according to claim 38 or 39 , characterised in that there is at least one electrode disposed above a threshold located so as to be completely immersed in the molten glass in the kiln when the apparatus is in use. 1+10 Apparatus according to claim 38 , 39 or 1+0, characterised in that there are electrodes disposed on opposite sides of a threshold located so as to be completely immersed in the molten glass in the kiln when the apparatus is in use* l+2e Apparatus according to any of claims to 1+1, characterised in that a said electrode is disposed beneath the position at which the ribbon is drawn from the surface of the molten glass in the kiln when the apparatus is in use. 3 o Apparatus according to claim 1+2, characterised in that "beneath the said drawing position there is a draw bar which incorporates or holds a said electrodeo 44* Apparatus according to claim 42 or 43, characterised in that the electrode beneath the said drawing position is incorporated in or held by an element which is integral with or connected to the rear end wall of the kiln. 1+5· Apparatus according to any of claims 38 to 44, characterised in that there is at least one electrode made of solid metal or of an electrically conductive refractory material. -4-6. Apparatus according to claim 45 j characterised in that there is at least one electrode made of a refractory precious metal, molybdenums, tungsten or Sn02s with or without incorporated doping agent. 47o Apparatus according to any of claims 38 to 46 , characterised in that there is at least one electrode composed of molten metal or molten metal salt. 8. Apparatus according to claim 47 , characterised in that there is at least one electrode composed of tin or lead. 49- Apparatus according to any of claims 38 to 48 , characterised in that there is at least one reservoir for holding a molten electrode in contact with molten glass in the kiln, said reservoir communicates with an extension at a cooler region, and material in said extension and in electrically conductive contact with such molten electrode is connected by a cable to a source of E.M.F. 50. Apparatus according to any of claims 21+ to 1+9 , characterised in that there is at least one threshold which is located so as to be completely immersed in the molten glass in the kiln when the apparatus is in use and which is located between bottom portions of the kiln which are at different levels . 51· Apparatus according to any of claims 21+ to 50 , characterised in that the kiln is provided with at least one skim opening for the withdrawal of molten glass from the surface of molten glass in the kiln, such skim opening being located behind a position where a said upward flow of molten glass will be maintained when the apparatus is in use. 52. A process for drawing sheet glass, substantially as herein described with reference to the accompanying drawings. 53 · Apparatus for use in drawing sheet glass, sub-stantially as herein described with reference to the accompanying drawings . I+0 Sheet glass obtained by a process according to any of claims 1 to 23 and 52. 55· Sheet glass produced bv/a drawing process, such sheet glass being characterised in that the distribution of seams of glass of dof erent refractive indices in a cross-section whictf extends over the full width - 82 - «■ /J-