NO134333B - - Google Patents

Description

The present invention relates to a method for producing plate glass by continuously feeding molten glass to a furnace to create a forward flow of glass towards a drawing zone, in which molten glass is continuously drawn up from a meniscus in the surface of the glass in the furnace in the form of a band and where molten glass in the furnace is locally heated. The invention also relates to an apparatus for carrying out the method, comprising an oven with a feed end where the oven is continuously

lig can be fed molten glass, and where there is a device for continuously pulling a band of glass upwards from a meniscus in the surface of the glass in a drawing zone, as well as devices for local heating of the molten glass in the furnace.

When drawing glass as indicated above is used, the heating and flow conditions in the furnace are of critical importance for the quality of the drawn glass. It is in all cases necessary that these conditions are such that a largely stable meniscus is obtained on the surface of the glass in the drawing zone, but the shape and maintenance of such a meniscus in no way ensures that the drawn glass will be of good quality . Glass is drawn into the meniscus from the surface areas of the molten glass surrounding the meniscus,

and the difference in temperature in the glass that always exists between the surface areas at different distances from the boundary walls of the furnace combined with the rather complicated current pattern,

tends to prevent the formation of a band that is truly flat and of substantially one thickness across its entire width, and these temperature differences can lead to optical defects on

due to flows that are composed of glasses with different viscosities. These difficulties are more marked as the pulling speed increases.

The aforementioned problems arise in all drawing processes in which glass is drawn from the surface of the molten glass in the furnace as opposed to processes in which molten glass is extruded into glass from below the surface of the molten glass in the furnace as in the classic Gourcault process. In such extrusion processes, the flow pattern in the glass is completely different and the problems mentioned above do not arise.

In short, the drawing method according to the present invention can be divided into two categories according to the furnace depth in the drawing zone. On the one hand, one can make use of a shallow furnace from which the glass is drawn from the entire depth of molten glass in the drawing zone. This category of process includes the classic Colburn process in which the glass strip drawn upwards from the furnace is bent over a bending roller and passed through a horizontal hardening zone. On the other hand, a deep furnace may be used in which the overflow of molten glass flowing to the drawing zone flows over a return stream of colder glass coming from the terminal end region of the furnace. This category of process includes the classic Pittsburgh process in which the glass strip is pulled upwards through a vertical pulling tower.

Numerous modifications of these classic methods are possible within the aforementioned broad categories. E.g. the glass strip in any given type of process may be drawn from the furnace at an angle to the vertical and a strip drawn from a deep furnace may be bent over a bending roller instead of being drawn through a vertical drawing tower.

The demand for high-quality glass and greater production speeds has stimulated a continuous search by manufacturers for ways to create better heat and current conditions in the drawing plant, and numerous proposals for these purposes have been put forward in recent years.

It has thus been proposed to heat the bottom and side wall parts of the oven externally to particularly high temperatures in order to reduce current reduction along the walls. This aid does not produce conditions that are favorable for the production of high-quality sheet glass. In reality, there is an increasing risk of the drawn glass being contaminated by refractory grains or containing glass bubbles. This tendency for the refractory material to corrode or erode increases with the increase

of the temperature in the refractory material.

The present invention aims to provide

an improvement in the drawing of sheet glass by affecting the basic flow pattern inside the molten glass body. The invention specifically aims to provide an increased pulling speed without the usual increased risk of corrosion and erosion of refractory material which has hitherto been encountered.

According to the present invention, there is provided a method for producing plate glass by continuously feeding molten glass to a furnace to create a forward flow of glass towards a drawing zone, in which molten glass is continuously drawn up from a meniscus in the surface of the glass in the furnace in the form of a band, and where molten glass in the furnace is locally heated, and it is characterized by the fact that in at least one place in the furnace, below the glass surface and at a distance on the far side (behind) and/or to the side of the meniscus, a heat barrier formed by an upward flow of molten glass rising to and reaching the surface from an area near a wall or bottom of or in the furnace and adjacent to a glass overflow is provided, so that molten glass on the far side of the barrier is substantially prevented from flow back through and under the heat barrier.

The method according to the invention provides the important advantage that in at least one area around the meniscus at the bottom of the drawn glass strip, molten glass which is about to be fed into the meniscus has a lower viscosity and therefore a greater flowability without the glass flows towards a heated furnace 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 mass of molten glass which is excluded from access to the drawing zone by the heat barrier. This exclusive amount of molten glass is kept continuously in motion due to a rather intricate system of convection currents that exist on this side of the heat barrier. These currents help to avoid or reduce the stagnation in any part of the molten glass which tends to produce solidified glass grains which may be entrained towards the drawing zone. It is a further important advantage of the method that molten glass which flows downward along a side or end wall of the furnace behind the heat barrier and is cooled during such downward flow is prevented from flowing under the heat barrier and directly exerting a cooling effect on the molten glass which in the plan direction of the oven is in front of the barrier. Such a cooling effect would adversely affect the rate of flow of molten glass into the belt.

The invention is an important contribution to achieving a good layering in the glass that makes up the band, so that high-quality glass can be drawn at greater speed. The extent to which this result occurs depends partly on the place or places in which the aforementioned heat barrier is created, and on the extent of this barrier or barriers, in the horizontal plane.

In certain embodiments of the invention in which the process is of a type in which molten glass on the surface of the forward stream is fed directly into the bottom of the front side of the belt, while molten glass at a lower level in this forward stream rises to a position past the drawing zone and forms an opposite surface current which is fed into the belt on its backside, whereby a heat barrier is created at a position which coincides with the position in which molten glass rises behind the drawing zone. A heat barrier in this location has a particularly marked good effect. Over the significant bulk of the band width, excluding its edges, the backside of the band originates wholly or mainly from this counter-directed surface current. If the glass in the flow of the opposite surface is significantly less fluid than the glass in the surface of the forward flow, the drawing speed must be kept low if the drawn glass is to be of acceptable quality. If the pulling speed increases beyond a certain value, which depends to a large extent on the current resistance in the glass that feeds the back of the strip, the glass is deformed or plate glass is formed which is optically very defective. A drawing speed which is considerably higher than what is normal will be possible, however, can be achieved if a heat barrier according to the invention is formed along a zone which extends across the furnace in a position so that some of the heated glass which flows upwards in this position serve to shape the oppositely directed surface flow of glass into the rear of the meniscus. In these designs, the heat barrier may be spaced from the rear end wall of the oven, but this is not important because, as mentioned below, a part of this wall which is below the glass surface can be shaped so that it protrudes inwards towards or up to a position below the drawing zone so that in this case the heat barrier can be placed over a submerged part of it

rear end wall.

Depending on the extent of this heat barrier across the width of the furnace, it will be possible for the glass flow to move from an area near the center of the rear end wall of the furnace,

and moving past the ends of the heat barrier towards the end portions of the drawing zone from which the edges of the tape are drawn. All impurities that are drawn inward to the drawing zone in this way are therefore directed towards these end parts in the drawing zone and do not contaminate the central part of the drawing zone from which the main part of the usable part of the tape is drawn.

Preferably, one of these heat barriers is placed on

a place located behind the drawing zone and across the full width of the oven and at least over a part of the oven width that is at least equal to the width of the belt. In this case, all or virtually all of the molten glass body which is near the rear end wall of the furnace near the flux line and which would normally be drawn inwards to the drawing zone by the currents caused by the drawing operation, is effectively isolated by the heat barrier.

A remarkable improvement in the drawing process can, however, be achieved by forming a heat barrier according to the invention in a position which, in the plan of the furnace, is close to a side boundary of the surface of the molten glass in the furnace, and in a position from which there is surface flow of molten glass against an edge or mark on the tape. By means of a heat barrier so placed, the fluidity of the molten glass feeding the corresponding edge or margin of the strip is improved due to the heating of this glass and due to a reduction in the frictional resistance. The flow of glass feeding the edge or margin of the glass is protected from the flow of more viscous glass that has been cooled by contact with the side wall of the furnace near the flux line. Consequently, the width of the marginal parts of the tape that must be discarded when the tape is cut is reduced.

In the event that a heat barrier is placed near a side boundary on the glass surface as mentioned above, it is of course preferable that a corresponding heat barrier is also formed near the other side boundary on this surface so that equal heat and current conditions determine the formation of both side edges, or marginal parts on the tape.

It is of course very advantageous that the heat barrier

according to the invention is created near the two side boundaries of the molten glass surface and also behind the drawing zone. In this case, the glass feed on the rear side of the belt and its edges can be kept in a very fluid state compared to the main surface flow forward which directly feeds the front side of the belt, and the allowable drawing speeds will be maximum.

According to certain important embodiments of the invention, there is at least one heat barrier which functions as described above and which is maintained above a threshold which is completely immersed in the molten glass. The threshold serves positively to localize the rising currents due to the local heating of the glass. The bulk of the glass behind the sill tends to be kept in a stable rotational motion about a horizontal axis, which also helps to avoid or reduce any tendency for solidified grains to form in the bulk of the glass. Furthermore, the threshold also serves as a mechanical barrier against displacement under the heat barrier of all such grains of solidified material or of all refractory grains by which the vitreous body could possibly be contaminated.

As the threshold is submerged and therefore not in contact with the air above the molten glass, the threshold is not as susceptible to corrosion by molten glass flows as the refractory material in the flux line blocks.

It is an advantage that the upward flow of glass forming a thermal barrier begins below the top level of the threshold if this is arranged so that glass rises towards the threshold, and continues its upward movement above the top level of this threshold. The threshold then serves better to stabilize the upward flow of glass.

The side surfaces of the threshold may be vertical or inclined to the vertical or one of the surfaces may be vertical and the other inclined. The height and shape of the threshold influence the direction of the upward flow paths for the glass that forms the heat barrier and thus the flow pattern in the glass flows that feed the belt.

Where a threshold is provided, the threshold can be hollow, and the heat required to create the heat barrier can be developed in the space inside the threshold. The heating device is thus protected by the threshold from direct contact with the molten glass, which would limit the choice of heating devices. As heating devices can be used e.g. gas or oil burners, or electrical resistance elements.

Alternatively, a threshold consisting of

of a simple compact wall. In this case, the heat that will create the heat barrier can be produced at the foot of this wall.

It is also an advantage when using a submerged threshold that you can use heating devices that are included in or form part of the threshold, or a wall that forms the threshold, or part of the threshold. For example, such a wall can be formed in part by one or more electrically conductive refractory elements through which an electric current can be sent that generates heat using the Joule effect. This method is advantageous in creating a strong heating effect which is localized where it is most effective in preventing a flood of glass flows over the threshold.

To maintain the heat barrier in place for a

threshold, it is also possible to use heating devices which are in direct contact with the molten glass near the threshold. The use of a heating device which is in contact with the molten glass, but which does not actually form part of the threshold, is applicable to create heat in a well-defined zone or while leaving the threshold free of any direct heating function, and in this way increases the choice of construction specifications. As an example, you can use heating elements that are placed on a side surface and/or on the top surface of the terkel.

Alternatively or in addition, you can make use of heating elements, e.g. electrical resistance elements, which are located inside the molten glass body and at a distance from the threshold. Such an arrangement is advantageous in reducing any risk of corrosion of the threshold. Where it is necessary to

raising the temperature of the glass to a certain level in a particular area at a distance from the threshold, this arrangement makes it possible for the heating to be achieved with a lower energy consumption than if the heat were to be generated in or immediately on the threshold.

In a method in which the oven is provided with a threshold can

this last be extended upwards by means of a plate to

increase the height of the mechanical barrier that prevents the inflow of impurities, such as solidifying materials or bubbles. This disc

can be made of metal, e.g. molybdenum. It is advantageous if the top of the plate is placed as close as possible to the free surface of the molten glass in the furnace.

A heat barrier according to the invention can of course be created in one or more places as needed, by generating heat inside the oven by means of electric resistance heating devices or other devices, regardless of whether a threshold is provided or not at this location or locations, however, the provision of the threshold is of particular interest because it helps to stabilize the thermal barrier.

A very beneficial way to create a heat barrier

is to send an electric current or currents through the molten glass in the area to be heated, between suitably placed electrodes. This type of heating system produces the necessary calories directly in the molten glass itself and the glass can be kept at a necessary high temperature while the electrodes have a lower temperature level which can be low enough to largely avoid any risk of corrosion on the electrodes due to the molten glass.

Electrodes for use in a heating system utilizing the electrical conductivity of the molten glass may be in the form of plates or rods. However, it is very advantageous to use electrodes which are formed from pools of molten metal or molten metal salts. Molten metal or molten salt electrodes can have a large surface area which further gives the advantage that they have a very low frictional resistance to the flow of molten glass in contact with the electrodes.

A very satisfactory heating system makes use of electrodes, at least one of which is placed above a threshold which is completely immersed in the molten glass.

Another possible arrangement for the electrodes which is very suitable in certain cases is an arrangement with electrodes on both sides of the threshold. By using electrodes placed on this side, a very large volume of molten glass covering the threshold can be heated directly with relatively 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 obtained with a low current density, which is desirable to avoid bubble formation in the glass.

In case a metal plate is used to increase

the height of the mechanical barrier formed by the threshold and the heating is accomplished by means of an electric current or currents passing through the glass between electrodes placed on both sides of the threshold, the plate will form an equilibrium potential surface in the electric field, and the plate can have a shape that is chosen so that a necessary favorable direction is obtained for the electric current.

In certain embodiments of the invention, the molten glass in the furnace is heated locally so that a heat barrier is created by sending electric current through the molten glass between electrodes, one of which is placed below the place where the tape is pulled from the surface of the molten glass in the furnace. In this way, a heat barrier can be maintained very close to the drawing zone.I

a very satisfactory arrangement is a drawing rod arranged under the drawing point, and an electrode in the form of a certain amount of molten metal or metal salt is held in this drawing zone. The presence of a pool of molten metal or molten metal salt in this position is advantageous because of the low frictional resistance exerted on the molten glass flowing into the meniscus.

Another feature of the method which is of value comprises producing a surface stream of molten glass over the heat barrier and in a direction away from the drawing zone, and withdrawing excess glass from an area behind this barrier. The outer surface flow complements the effect of the heat barrier by counteracting any tendency for impurities to be drawn inwards towards the draw zone. Such an outward surface flow can be produced by drawing glass via at least one foam hole in a side wall of the furnace in an area behind the heat barrier.

The invention comprises a device for use during the drawing of sheet glass, which device comprises a furnace having a feed end into which the furnace can be continuously fed with molten glass, and a device which continuously draws a band of glass upwards from the surface of the molten glass in a drawing zone in the furnace, characterized in that there is a device for local heating of the molten glass in the furnace at least at one location which, viewed from above, is spaced within a boundary in the molten glass surface to maintain at that location a heat barrier formed by an upward flow of molten glass rising to the surface from a position near a wall portion of or in the furnace, such that this wall portion serves to prevent molten glass behind the barrier from flowing below it.

The provision of a local heating device according to the invention makes it possible to create a warm zone which makes the glass, which feeds the belt from at least one area around the meniscus, more fluid, while at the same time forming a heat barrier which prevents colder glass from flowing into the tape from the wall part behind the heat barrier in the oven. Consequently, glass which shows good layering properties and which is generally of high quality can be drawn at a greater speed with the device according to the invention.

In a preferred arrangement according to the invention

the drawing zone is located at a distance from the boundary of the surface of the molten glass in the furnace which is opposite to the feed end of the furnace, and a local heating device is provided to maintain the said upward flow of glass at a place located in the plan of the furnace between the drawing zone and the opposite limit reached by the surface of the molten glass in the furnace when the device is in use. The advantage of this and other optional features of the devices mentioned below will be understood from the above description in terms of the advantages of the corresponding features of the methods.

Advantageously, however, the device according to the invention can include devices which maintain an upward flow of molten glass at least in one place which in the plan of the furnace is close to a side boundary to which the surface of the molten glass in the furnace extends when the device is in use. It is particularly advantageous if the device comprises devices which maintain such an upward flow of glass near each side boundary of the molten glass surface, and in certain devices according to the invention there are, in addition to the heating device which creates an upward flow in these places as mentioned above, devices for to create a hot zone directly behind the draw zone. In this case, the flow of molten glass into the back of the belt and into its side edges or margins can be promoted so as to enable very high drawing speeds to be achieved.

A device according to the invention can be of a type that comprises a shallow furnace, so that the drawing device is designed to draw glass from the entire depth of molten glass in the furnace. In this case, the device which creates a local heating zone according to the invention can be arranged so that it is effective in maintaining an upward flow of molten glass in this zone over the entire depth of the molten glass in the furnace.

A device according to the invention can alternatively

be of a type consisting of a deep furnace where the drawing device is designed to draw glass from the upper part of the molten glass in the furnace. In this case, the device which creates a local hot zone, according to the invention, is arranged so that it effectively maintains an upward flow of molten glass in this zone at least in the upper part of the depth of molten glass in the furnace, and the device is arranged so that upward flow of molten glass in the hot zone takes place from a position in the vicinity of at least one side wall part, so that flow of molten glass under the heat barrier is largely prevented.

Certain devices according to the invention include

a threshold which is positioned so that it is completely immersed in molten glass in the furnace when the device is in use, and so that the heating device

ning maintains an upward flow of molten glass above this threshold. Preferably, there is a heating device which maintains an upward flow of molten glass from a position below the top level of the threshold such that the upward flow occurs along the threshold and continues above its top level.

Preferably, it is the part of the bottom of the oven that is located

is behind the threshold higher than the part in front of the threshold. IN

in this case, the depth of the unused glass behind the thresher is reduced

electricity. In the case of a deep furnace process, it is further

a further advantage is that the molten glass in the lower levels of the furnace in front of the threshold can be cooled more effectively, which promotes a more positive downward movement of the glass forming the submerged return flow.

Preferably, the device comprises a threshold which is hollow

and a device inside the threshold that creates the heat that is necessary

reversed to produce the local heating zone at this location.

In other advantageous embodiments, the threshold consists of a

simple compact wall, and the device that creates the local heating

zone at this location is arranged so that it creates heat at the base of this wall.

In order to produce this local heating zone, certain devices according to the invention are equipped with heating devices which form part of or form part of the threshold, or a wall which forms the threshold or part of the threshold. As an example, the wall can be shaped in part by one or more electrical . conductive, refractory elements, such as one or more tin oxide blocks, through which electrical heating current can be sent. Such heating devices may have a surface which is flush with or which is retracted or protrudes from the adjacent part of the threshold surface.

Preferably, the device comprises heating devices which form a local heating zone and a threshold in this zone, which heating devices are located inside the furnace so that they are not in reality in contact with the molten glass near or at a distance from the threshold. For example, the heating elements can be placed above the threshold so that the heating of glass in this area can be more intense, or the temperature rise in this area can be different from what it would be if the heating elements were located below or in the threshold. Of course, the heating elements can also be arranged in a lower level

to give the glass near the threshold is upward impulse.

According to a very satisfactory arrangement, there is a local heating device for heating molten glass in the furnace to maintain an upward flow of molten glass, which heating device creates heat directly inside the furnace.

In the most preferred embodiment of the device in

according to the invention, the heating device which maintains the heat barrier comprises electrodes between which electric current can pass through the molten glass in the furnace. Such electrodes may be in the form of plates or rods, but preferably they are made of pools of molten metal or molten metal salt. In one arrangement, at least one electrode is placed above the threshold, but preferably the heating devices comprise electrodes located on opposite sides of the threshold.

It is advantageous to place an electrode under the position in which the glass strip is pulled from the surface of the molten glass in the furnace when the device is in use. For example, a draft counter can be arranged in this position, and this draft counter can contain or have an electrode. An electrode which is placed under the pulling position as mentioned above, can in some cases be included in or held by an element which goes together with, or is connected to, the rear end wall of the oven. In some cases, the presence of a free path between the electrodes along which molten glass can flow upward into the path of the electric heating current from a lower level in the furnace is advantageous in producing better heating and dynamic current conditions inside the furnace. The use of electrodes for heating the molten glass makes it unnecessary to send heat energy through refractory materials with the consequent risk that they will be heated to such an extent that there is a great risk of corrosion of the refractory materials due to the molten glass. there is less risk of turbulent uncontrollable convection currents being induced in the glass than when using heating devices that entirely use convection currents in the glass to heat the glass in the heating zone.

The use of electrodes which are placed on opposite sides of the lid cell enables a larger volume of glass to be heated in the heating zone and is the origin of a very marked upward current. Electrodes with a very large surface area can be conveniently placed in these positions, which is desirable to avoid high current density and the formation of bubbles. It is advantageous that the top of the threshold is quite close to the surface level of the molten glass so that it provides a relatively large current density above the threshold, and to make the threshold as effective as possible as a barrier against inward movement of impurities into the drawing zone.

When using electrodes on opposite sides of the threshold and at the bottom of the furnace, it is advantageous that the bottom of the furnace, behind the threshold, is at a higher level than in front of the threshold due to the fact that in this case, in addition to the advantages attributed to such a level difference as mentioned above, it has the advantage that the electrodes can be closer together.

In the event that electrodes are used, at least one electrode can be made of a compact material or of an electrically conductive refractory material, preferably using refractory noble metals such as molybdenum, tungsten and SnC3/, with additives being used if necessary. It has been found that these materials behave satisfactorily in molten glass at high temperatures even when an electric current is passed over the surface of the material in contact with the glass. Furthermore, the compact electrodes can be in a shape that is chosen so that a predetermined current density distribution is achieved.

It is particularly advantageous to use electrodes consisting of molten metal or molten metal salt. If molten metal or salt heavier than the molten glass is used, the electrodes are placed below the molten glass and help reduce the frictional resistance to the flow of molten glass in the furnace. It is possible to use electrodes consisting of molten metal or molten metal salt with a lower specific gravity than the molten glass. Such electrodes in no way damage the surface currents in the molten glass. Such electrodes can be easily replaced over time and their thickness can be changed when necessary. Furthermore, their electrical properties can be modified by changing their chemical compositions without interrupting plate glass production.

Suitable molten metals as a heavier than< glass are tin and lead. These metals have a high electrical conductivity.

According to an optional but very advantageous feature, there is a reservoir containing an electrode consisting of molten metal or metal salt, in contact with the molten glass in the furnace, and. this reservoir has an extension leading to a cooler area where metal or metal salt contained in this extension can be connected to a lead cable. In this way, the problem of maintaining a good electrical connection between the cable and an electrode in a very high temperature zone, e.g. on the traction zone side of the threshold. For 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 state, or at least in a cooler state, and the cable may be connected to the solid, or cooler tin. A molten metal salt electrode can similarly be contacted

a body with the same or another metallic salt in compact or at all events in a cooler state, provided the cooler salt is sufficiently electrically conductive.

The oven for a device according to the invention can be arranged with at least one foam opening in a position which is located in a boundary wall therein which is opposite to a position in which the device which creates a local heating zone is arranged. When the device is in use, a small amount of glass can be extracted continuously or intermittently from the surface of the molten glass via this foam opening. Removal of glass from the surface in this manner produces an outward flow through the top of the heat barrier and serves as a further check against the inward flow of impurities.

The present invention is characterized by the features reproduced in the claims, and will be described in more detail in the following with reference to the drawings in which: Fig. 1 is a vertical section through part of a Pittsburgh-type machine and the section is taken along the line I-1 in fig. 2,

fig. 2 is a plan view of the machine part shown in fig. 1,

fig. 3 is a vertical longitudinal section through part of

a Colburn machine,

fig. 4 shows a vertical longitudinal section through a part

of another machine of the Colburn type, where the cut is taken along the line IV-IV in fig. 5,

fig. 5 is a fragmentary section of a plan view of the machine part shown in fig. 4,

fig. 6 is a vertical longitudinal section of part of another Pittsburgh type machine,

fig. 7 is a vertical longitudinal section through part of

another Pittsburgh-type machine,

fig. 8 is a vertical section of a part of the machine in fig. 7, where the section is taken along the line VIII-VIII in fig. 7,

fig. 9 is a vertical longitudinal section through part of

another Colburn type machine,

fig. 10 is a vertical longitudinal section through part of another Colburn type machine,

fig. 11 is a vertical longitudinal section through part of another Colburn type machine,

fig. 12 - 17 are detailed drawings showing parts of ovens

in six different glass drawing machines in vertical section, parallel to the longitudinal axis of the furnace,

fig.18 is a vertical longitudinal section through part of another Pittburgh-type machine and

fig. 19 is a vertical longitudinal section through part of another Pittsburgh type machine.

Fig. 20 represents the cross-sectional contour pattern which appears on an anamorphic photograph (called "striascope") of a sample of plate glass according to the invention,

fig. 21 shows interference fringes formed by the light rays sent through samples on plate glass, shown in fig.

20 during the testing of such a sample in an interference microrefractometer according to the known "Nomarski" method, fig. 22 represents the cross-sectional contour pattern as seen on a striascope referred to above, of a sample of -plate glass drawn by a classic Pittburgh-type drawing process,

fig. 23 shows the appearance of interference fringes which. is formed by light rays transmitted through a plate glass sample drawn by means of a classical Pittburgh-type drawing method during the testing of this sample in an interference microrefractometer according to the "Nomarski" method described above,

fig. 24 is a typical brush stroke pattern recorded by photographic methods on a surface of a plate glass sample drawn by a classic Libbey-Owens type method,

fig. 25 is a schematic view of a striascope device for photographic recording of the homogeneity in glass in section through the glass plate, and

fig. 26 is a schematic view of the optical system i

an interference microrefractometer applicable according to the "Nomarski" method.

The device shown in fig. 1 and 2 comprise an oven

1 which contains a certain amount of molten glass. The lower part of the oven comprises a sole 2, a lower rear end wall part 3 and lower side wall parts 4 and 5. In the upper part, the plan dimensions are

the furnace extended by horizontal wall parts 6, 7 and 8, which project outwards from the walls 3, 4 and 5 and carry an upper end wall 9 and upper side walls 10 and 11. When the device is in use, molten glass is continuously fed into the furnace at that end (not shown) which is furthest away from the end walls 3 and 9, so that it holds the surface 12 of molten glass at the indicated level inside the furnace, while glass is drawn upwards from this surface in the form of a continuous band 13 which has side edges 14 and 15. Inside the oven it is placed

three horizontal electrodes, one above the other in parallel,

but with a distance inwards from the upper end wall 9 in the oven. A corresponding group of three horizontal electrodes 17 is arranged in parallel, but with a distance inwards from the upper side wall part 10,

and a third and corresponding group of three horizontal electrodes 18 is placed in parallel, spaced inwards from an upper side wall part 11 in the oven. The electrodes 17 are connected to a source 19 with direct current, the central electrode 17 being connected to one pole of such a source, and the top and bottom electrodes 17 being connected to the other pole of this source. The other electrode groups, i.e. the electrodes 16 and 18, are connected in a similar way to an electrical source which, however, is not shown.

The draft device above the oven comprises a draft chamber

and a drawing section through which the glass strip is drawn by rollers. These and other parts of the device are in accordance with well-known practice and do not need any description or illustration.

The electric power supplied to the electrodes in each of the groups 16, 17 and 18 is such that an electric direct current passes through the molten glass in the furnace, between the electrodes, with different potential in the group. The voltage is such that nowhere in the molten glass is the current density higher than 0.4 A/cm<2>and due to the passage of electric current through the molten glass, the temperature of the glass near each electrode group is about 40° C higher than it would otherwise be. Accordingly, a thermal barrier is formed and maintained at the location of each electrode group. At this location there is a continuous up-overflow of molten glass rising from a position near the underlying wall portion 6, 7 or 8 as the case may be. Some of the upflowing glass flows inwards along the surface of the molten glass in the furnace towards the meniscus at the bottom of the band 13, and the rest of this upflowing, molten glass flows outwards to the adjacent wall parts, 9, 10 or 11 respectively. Behind each heat barrier there is a relatively cool zone of molten glass, and the shape of the side and end walls of the furnace which form the wall parts 6, 7 and 8 which lie below the heat barriers, prevent glass from these relatively cool zones from flowing in and under the barriers. Accordingly, there is a much smaller risk of solidified glass grains that may have formed near the wall sections 6, 7, 8, 9, 10 and 11 and corroded refractory material that may have been separated from the flux line blocks in the wall sections 9, 10 and 11, from passing inwards into the glass streams that are fed into the meniscus at the bottom of the glass belt 13. The amount of glass in the relatively cool zones behind the electrode groups circulates about horizontal axes and this in turn reduces the risk of solidification and bubble formation in the molten glass in the lower level in these zones . Furthermore, the local heating of glass at the location of the electrode arrays reduces the viscosity of the glass flowing towards the back and towards the side edges of the belt, so that the width of the marginal parts of the belt is reduced for the given draw speed, and glass of a given standard quality can be drawn at a higher speed than in conventional procedures.

The increase in the usable width of the drawn tape when using a method according to the invention, can e.g. be as much as 10 cm.

In a modification of the method described with reference to fig. 1 and 2, only the electrodes 16 were used. In this case, the maximum draw speed was somewhat less due to the resistance provided by the somewhat higher viscosity of the molten glass fed into the core portions of the glass strip, and the width of the strip core portions that had to be discarded as slack was greater due to of contamination by solidified grains in the glass in these areas. The quality of the sheet glass at the operative pulling speed was, however, considerably better than when the plant was run in the same way without the use of the electrodes 16.

According to another modification, only electrode groups 17 and 18 were used. In this case, the maximum drawing speed at which ribbons having a given flatness standard and uniform thickness could be drawn was greater than in a conventional method without a heat barrier, and the width of the marrow portions exceeding the maximum allowable thickness was normal. However, it was found that there were several more defects in the central part of the drawn strip due to the presence of solidified grains and bubbles than when the electrodes 16 were used.

Reference is now made to fig. 3 which represents a part of a Colburn type glass drawing machine equipped for use of the method according to the invention. The device comprises a draft furnace, or crucible 20, which comprises a sole 21, a rear end wall 22 and side walls, of which only one wall 2 3 is shown in the drawings. The furnace is supported by pillars 24 and 25. Molten glass is continuously fed along the furnace from a glass melting furnace towards the rear end wall of the furnace and a glass band 26 is continuously pulled from the surface 27 of the glass in the furnace and sent around a bending roller 28 into a hardened tunnel. The curing tunnel, the rollers used to

passing the glass ribbon through the tunnel, the tip stones and other constituent parts of the machine, are not illustrated and do not need any explanation as they are according to standard practice.

The furnace 20 is shallow and the glass that continuously enters

in the drawn band 26, is drawn from the full depth of molten glass in the furnace-* ;Inside the furnace 20 in a position between the drawing zone and the rear end wall 22, there are two electric resistance heaters 29 which run across and across the entire width of the furnace, parallel to this pulling zone and the rear end wall. During the glass drawing process, electric current is continuously sent through the resistance heating elements 29 and 30 so that a local heating of molten glass takes place at this place in the furnace. The heating elements increase the local temperature in the glass by about 50°C. Accordingly, there is a continuous upward flow of molten glass to the surface 27 in the vicinity of the heating elements. This upward flow of molten glass takes place from a position near the underlying portion of the bottom wall 20 in the furnace. The basic flow pattern in the molten glass in the furnace in the vertical longitudinal plane is represented by arrows in the drawing. It will be seen that the flow pattern differs from that which takes place in a normal blast furnace process, in that the flow of glass towards the rear of the belt does not take place from the back end wall of the furnace, but from a position that is spaced inwards from this wall along the surface of the molten glass, which position is determined by the position of the heating elements 29 and 30. From the molten glass located behind the vertical transverse plane in which the heating elements are located, there is essentially no flow of glass into the meniscus due to the heat barrier. Flow of molten glass below the heat barrier cannot occur due to the upward flow of molten glass starting from the bottom of the furnace. The molten glass behind the heat barrier is kept in motion by continuously circulating convection currents. Any grains of solidified glass that may form near the rear end wall or/and any grains of corroded refractory material that may be separated from the refractory wall in this area are effectively prevented* from accessing the draw zone by this heat barrier. Because of this and the low viscosity of the glass that is fed into the back of the belt, the maximum drawing speed at which plate glass of good quality can be drawn is increased by about 30%.

Reference is now made to fig. 4. Here again only such parts of the machine are illustrated as are necessary to describe how the invention is included in it. Drawings show part of a furnace 31 into which molten glass is continuously fed from a glass melting furnace. This oven rests on columns 33 and 34. The area 35 above the oven is in reality closed, as this is the area

inside the .ordinary drawing chamber, but the drawing chamber with the lcjppe stones, the curing tunnel, the transport rollers by means of which the glass strip is carried and transported through the tunnel and other parts which are standard in this type of machine, are not shown because they are not necessary to understand the invention . The only part located above the furnace shown in the drawing is the so-called deflection roller 36, around which the glass strip is guided before it is led into the annealing tunnel.

The oven 31 has a rear end wall 37 and a sole consisting of sections 38 and 39 on both sides of a threshold 40 which extends across the oven, and consists of walls 41, 42 and 43. The threshold divides the lower part of the oven into an end chamber 44 and a front chamber 45. A series of electric resistance heating elements 46 pass upwards through the threshold so as to penetrate the molten glass in the furnace, and are connected to an electric current source, which source is not shown. The lower parts 47 of these electrodes located outside the furnace are lined with refractory material Glass is pulled upward from the surface of the molten glass in the furnace so that a meniscus 48 whose front and rear surfaces, called 49 and 50, are established on the glass surface and fed into the glass band 51 which have front and rear surfaces, respectively 52 and

53. In an actual embodiment of the method using the device illustrated in FIG. 4 and 5, the electrodes 46, which were placed at 25 cm intervals across the furnace, were connected to a voltage source so that the total power consumption that went down to heat the molten glass above the threshold 40,. was 30 kw. This amount of force held the glass in the area above the threshold

at a temperature that was 30°C above what would otherwise have been. This local heating of the melt was observed

glass produced a steady upward flow of molten glass along the walls 41 and 43 of the sill and continued up to the surface of the molten glass in the furnace. The flow of molten glass rising along wall 41 flowed outwards towards the rear end wall 37 of the furnace and sank down along this wall so that the amount of glass which filled the space 44 was kept in a circulating motion and was effectively isolated from the surface flow of glass that was fed into the rear side of the meniscus 48. This surface flow of glass into the rear side of the meniscus originated in whole or in part from the upward flow of molten glass along the front wall 43 of the sill. Grains of corroded refractory material or of solidified glass, which were formed in space 44, were completely or almost completely excluded from access to the drawing zone, that is, from the area from the surface of the molten glass in the furnace where the meniscus 48 was formed. This last result was further promoted by the fact that the feed rate of molten glass along the furnace from the glass melting furnace was somewhat (about 1%) greater than the withdrawal rate of molten glass from the furnace in the form of the belt 51. The excess glass was continuously discharged through a series of foam holes 54 which was formed in the end wall 37 at the level of the molten glass surface, and led into flow channels 55. A similar result could be achieved by making the holes 54 in the end wall 37 at a lower level. At whatever level the foam holes were, the emptying of glass through these holes could be period-wise instead of continuous.

Fig. 6 shows part of a Pittsburgh-type machine. The drawing shows a part of the furnace 60 which has a sole 61, a rear end wall 62 and side walls, of which only one wall 63 is shown in the drawing. Preferably, the end wall 6 2 is formed from a plurality of refractory elements with different compositions so that the temperature of the molten glass mass in the furnace can be controlled more easily, but this is not essential. The furnace is kept filled with molten glass up to a level 64, just enough 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 shaped so that it forms a threshold 66 which is completely immersed in the molten glass in the furnace and extends over the entire width of the furnace. The top surfaces of the parts of the wall 62 which are in front of the threshold 66 and are likewise immersed in the molten glass, have electrodes 67 and 68 which are formed of tungsten.

The furnace 60 is surrounded by the usual drawing chamber which is limited behind and forward by rear and front L-blocks 69 and 70, and through which molten glass is drawn continuously in the form of a belt 71. The drawing chamber is covered by a tower section through which the belt is drawn upwards by gripper rollers . These and other parts of the machine, such as the cooling devices provided inside the drawing chamber and the edge rollers between which the edges of the strip are gripped a short distance above the surface of the molten glass in the furnace, are not shown because they are in accordance with well established practice and are not of interest in the description of the invention.

The electrodes 67 and 68 which extend over the entire width of the furnace 60 are connected to the poles of an electric current source which is shown schematically in the drawing, so that an electric current is continuously sent through the molten glass between these electrodes. The electrical potential is such that the current density inside the glass is somewhat in excess of 0.6 A/cm 2.

The glass ribbon is fed by a forward flow of molten glass. Molten glass in the surface area of this forward flow is fed directly into the front of the belt as indicated by arrow 72, while some of the glass at a lower level in this forward flow continues past the drawing zone and then flows upwards to the surface of the nearby glass of the threshold 66, as indicated by arrow 73, before flowing back to the meniscus along the surface area of the molten glass mass. Some of the molten glass which flows forward to the region of the rear end wall 62 descends towards this wall and forms the submerged return stream 74. 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 near the threshold. Consequently, the layering in the glass in the belt is improved and the drawing can take place at a greater speed than with a conventional method. >

The increase in draw speed can be as much as 20-30% or even more. The amount of glass behind the threshold 66 is

effectively isolated from the flow of molten glass to the meniscus due to the heat barrier maintained above the threshold by the electrical heating current through the glass. The threshold itself prevents glass from flowing under this heat barrier. The heating of the glass near the threshold keeps the glass behind the threshold in circulation as indicated by flow line 75 and this helps to reduce the risk of contamination of the glass stream 73 with impurities such as grains of solidified glass and of corroded refractory material. The threshold itself, which is completely immersed in the molten glass, is only slightly exposed to corrosion. The restraint in the inward movement of impurities towards the draw zone is increased due to the outward surface flow resulting from the withdrawal of a quantity of molten glass as an overflow stream 76 via the weir 65.

It should be particularly noted that the distance of the drawing zone from the rear end wall 62 in the oven is considerably smaller than in conventional methods. By using the illustrated system, the mass of glass held in the furnace behind the vertical plane containing the meniscus can actually be reduced to about half compared to conventional Pittsburgh-type processes.

Reference is made to fig. 7 and 8 where the Pittsburgh type machine shown comprises a furnace 80 having a rear end wall 81, side walls of which a wall 82 is shown in the drawing, and a sole 54. A band of glass having rear and front surfaces 86 and 87 respectively, is drawn from the molten glass in the furnace. The molten glass enters the belt via a meniscus 88 having front and rear surfaces 89 and 90 formed in surface 91 of the molten glass in the furnace. The position of the meniscus 88 is stabilized by a drawbar 92.

The conventional superstructure comprising the drawing chamber and its associated parts and the tower section through which the glass strip is drawn is not shown in the drawings as they are of conventional construction.

The side edges of the band 85 have a greater thickness than the central main part of the band width in the same way as the side edges of the band shown in plan in fig. 2, but the greater thickness of the far side edge of the band shown in fig. 7, is not indicated on this figure.

The rear end part of the oven is locally expanded in the upper part. Fig. 8 shows the shape of one side of the oven in this rear end part. The other side of the furnace is of identical shape in that the local expansion of the furnace is symmetrical in relation to its central vertical plane in the longitudinal direction. Reference is made to fig. 8 where the rear end part of the oven side wall 82 has a reduced height and ends at the level 94. In the area of the upper level of this shallower side wall part there is a horizontal wall part 95 which extends outwards to the bottom of an upper side wall part 96. threshold 97 protrudes from the horizontal side wall part 95. This threshold extends over the entire distance between the rear end wall 88 of the oven and a wall 98 which forms a front boundary wall for the side extent of the oven. The horizontal wall part 95 carries tungsten plates 99 and 100 which are placed on both sides of the threshold 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 and 100, and thus over the threshold 97. A hot zone is thereby formed in this area which causes a continuous upward flow of molten glass along the sides on the threshold and up to the surface of the molten glass in the furnace. As can be seen; in Fig. 7, the electrode plates 99 and 100 do not extend completely over the entire distance between the walls 81 and 98. The plates can of course go over the entire distance if there is necessary. In reality, the dimensions of the plates are chosen to achieve a required predetermined electrical current density through the molten glass. The electrical currents through the molten glass located inside the side extensions of the furnace heat the streams of glass flowing from the rear end of the furnace into the end portions of the meniscus 88, from which the side edges of the glass strip are drawn. In conventional methods, the molten glass which flows to sled to the end regions of the meniscus from one side wall of the furnace near its rear end, tending to have a considerably lower temperature and therefore a higher viscosity. This is one of the factors that normally limits the maximum allowable draw speed. By heating these streams of molten glass by means of a heat barrier, the drawing speed can be increased and/or there is an increase in the width of the usable part of the strip over which the thickness is below an allowable maximum.

By means of a modification of the method described with reference to fig. 7 and 8, the metal plate electrodes can be replaced by ponds of molten metal, such as molten tin or by ponds of molten metal salt, with the advantage that the frictional obstacle to the flow of molten glass fed into the ends of the meniscus on its rear side is further reduced.

Under favorable conditions, it is possible by adopting this form of the furnace and heat barrier system described with reference to fig. 7 and 8, to achieve a drawing speed which is of the order of 1.8 to 2 times the maximum drawing speed obtained in conventional methods.

In fig. 9, a part of a Colburn-type machine is illustrated, namely a part of the furnace 102 from which molten glass is drawn and the bending roller 10 3 over which the drawn glass strip passes before it is passed through the hardening tunnel. The drawing chamber and curing tunnel and various other parts which are standard in machines of this type, but which do not concern the description of the invention, are omitted from the drawing. The oven 102 comprises a rear end wall 104 and a sole comprises sections 105 and 106, which are located respectively behind and in front of a threshold 107 which extends across the oven. The threshold 107 divides the lower part of the furnace into a rear chamber 108 and a front chamber 109. The sole section 105 and 106 are covered by layers 110 and 111 of molten tin which similarly extend over the entire inner width of the furnace. The layer 111 is held back in its front boundary by a frame 112. The side wall of the furnace has, instead of one of the normal refractory blocks, a block of tin oxide. The drawing shows the tin oxide block 114 which is included in the side wall 113 shown in the drawing. The other side wall has an identical block of tin oxide in a position directly opposite the block 114. The layers 110 and 111 of molten tin in the bottom of the furnace are connected to opposite poles of an electric current source as shown in the drawing. The poles from the electric current source which is connected to layer 110 are also connected to each of the tin oxide blocks which are part of the side blocks in the side walls of the furnace. Consequently, an electric current occurs along three paths inside the mass of molten glass that is in the furnace. An electric current path runs between the layers of molten tin 110 and 111 at the bottom of the furnace and consequently over the threshold 107. Another electric current path runs between the layer 111 of molten tin and the tin oxide block 114. A third electric current path runs between the same layer 111 and the opposite tin oxide biock in the other side wall of the oven. The electrical current paths between the layer 111 and the tin oxide blocks pass through masses of molten glass immediately near the inner surface of the side walls of the furnace. The surface area of the layers 110 and 111 of molten tin is such that the current density along the first of the paths is nowhere greater than 0.5 A/cm 2 , while the current density along the second and third paths nowhere exceeds 0.2 A/cm 2 With such low current densities, there is little risk of bubbles forming inside the molten glass. The heating of the glass by electric current passing along the first path causes molten glass to flow upward along both side surfaces of the threshold 107 and to continue moving upward to the surface of the molten glass. The electric current density along this path is at its highest in an area above the threshold and the glass is consequently heated most in this position. Any impurities, such as grains of solidified glass or grains of corroded refractory material that are entrained by the flow of molten glass at the rear of the sill 107, are not captured in the flow of glass to the draft zone, but are recycled back into the compartment 108 towards the rear end wall 104. In this room, the molten glass is kept in a stable rotating movement against the block. In the space 109, the molten glass flowing along the layer 111 of molten tin flows upwards along the face along the threshold 107, up to the surface area of molten glass and back along this surface area into the meniscus, through which the glass flows into the band 115. The continuous heating of glass in the zones running along the side walls of the furnace between the layer 111 of molten tin and the tin oxide blocks, such as 114, in these side walls, stabilizes the flows of glass along these walls, while at the same time reducing the frictional lag of the flows of molten glass along these walls and promoting the free flow of molten glass to the meniscus. When using such a method, a strip of glass of substantially constant thickness can be drawn at a speed which is 1.5 to 1.7 or more times the maximum drawing speed in conventional methods. Fig. 10 illustrates a modification of the method and device described with reference to fig. 9. The modification is that a top extension element is provided on the threshold 107, which extension element comprises a foot part 116 which carries a curved plate 117 which projects up to a short distance below the surface of the molten glass. This extension element is made of molybdenum, and the plate 117 forms an equipotential plane in the path of the electric current between the layers 110 and 111 of molten tin. The shape and dimensions of this plate influence the electric field lines and consequently the electric current density. Furthermore, plate 107 forms an additional barrier against the movement of contaminated glass streams across the threshold from compartment 108 to compartment 109. Fig. 11 shows parts of a shallow furnace, generally of the Colburn type, in which molten glass 120 in the furnace flows on molten metal lying in bottom of the oven. In the lower part of the furnace there are rear and front rooms, respectively 121 and 122, which at the bottom of the furnace have layers of molten tin 123 and 124, which rooms and which different amounts of molten tin are separated by a threshold 125. These amounts of tin are respectively connected to opposite poles of an alternating current source 126 so that a heat barrier is created in the position of the threshold. Molten glass is drawn from the surface of the molten glass in the furnace through a meniscus 127 into a belt 128.

A special feature of the illustrated device lies

in the construction of the refractory walls in the furnace which form channels and recesses 129, 130, which are kept filled with tin which respectively is a continuation of the molten tin layers 124 and 123. The tin 129 and 130 are in solid form and the dimensions and recesses in which this tin is held, is such that the current density through the metal into these recesses is not sufficient to melt the metal. Due to the fixed state of the metal in these positions, the connections to the alternating current source can be easily established, and the manifold problems which are well known to be involved in the connection of electrical cables to metal electrodes which have a very high temperature are avoided.

Figs 12-17 represent different forms of thresholds and associated heating devices to create a heat barrier.

Any of the thresholds and heating systems shown in these figures can be used in a Pittsburgh-type and in a Colburn-type method/and thus any of the systems can be used in all devices or methods already described with reference to fig. 1-11. When using a Pittsburgh or any deep furnace type process in which molten glass is not drawn from the full depth of molten glass in the furnace, it is of course necessary for the threshold to extend above the upper level of the submerged return stream of returning glass along the oven towards its rear end.

The threshold shown in all figures 12-17 is indicated by the number 131. The threshold is hollow and comprises a top wall part 132 and rear and front walls, 133 and 134 respectively.

In the system shown in fig. 12, the threshold is heated by flames 140 flowing out from a gas burner 141 which extends along the inside of the threshold and is placed so that the flames play against the top wall part 132 and the upper parts of the rear and front walls 133 and 134. The elongated shape of these walls is advantageous for an intense and uniform heat exchange between the threshold and the molten glass located at the scene of this threshold.

The top wall part 132 of the threshold which is shown in fig. 13 is formed by an electrically conductive sheet of tin oxide. The bottom surface of this plate is in contact with a non-oxidizing atmosphere. The tin oxide plate is connected to an electrical current source (not shown). Electric current flows through the plate, which has a uniform density over the entire area of the plate, so that there is a completely uniform heat distribution to the molten glass that is in contact with the top of the plate.

Reference is made to fig. 14, where the threshold shown is made of ordinary refractory material, but the top wall part carries a molybdenum plate 142 which has a width that is somewhat smaller than the width of the top wall part 132 of the threshold, taking into account molybdenum's good electrical conductivity. The molybdenum plate is connected to an electrical current source. The current density in the plate is uniform over its entire area, which ensures a uniform heat distribution and this heat is concentrated in the column of molten glass with a relatively limited horizontal cross-section, which is located above the plate 142. It is not necessary to have a special atmosphere in contact with the bottom of the metal plate.

In fig. 15, the top wall part 130 of the threshold is formed of • an electrically conductive sheet of tin oxide. In the corner angles between the sole section 143 and the rear wall 133 of the threshold on one side and the sole section 144 and the front wall 134 of the threshold on the other side, there are hollow wedges 145 and 146 which are made of tin oxide and extend along the entire length of the threshold. The hollow wedges 145 and 146 are connected to an alternating current source 145. The cross-sectional shape of the hollow wedges is chosen according to the required current density distribution inside the molten glass, taking into account its resistance properties near the threshold. During operation, electric current is passed through the molten glass between electrode 132 and 145 on the one hand and between electrode 132 and 146 on the other hand, these electric currents creating a heating zone which encloses the threshold. The arrangement illustrated in fig. 15, provides the advantage that it counteracts any tendency for the temperature of the molten glass along the walls 133 and 134 of the threshold to become too low or for the molten glass near these walls to stagnate, giving rise to the formation of solidified grains.

In the system shown in fig. 16, two electrodes 148 and 149 are placed along the entire length of the threshold. The electrode 148 which is connected to one pole of an electric current source 150 is a plate of platinum which is placed against the back wall 133 of the threshold. The second electrode 149 which is connected to the other pole of the electric current source comprises a molybdenum crucible containing an amount molten tin. This arrangement is suitable in the case where there is a very hot and strong upward flow of natural glass along the front wall 134 of the sill. The passage of electric current between electrodes 148 and 149 counteracts any tendency for the strong flow of molten glass on the face of the sill to allow impurities formed in the space 151 at the threshold pass into the space 152. The shape of the electrode 148 can be chosen so that it gets a necessary, predetermined current distribution along the rear side of the threshold. The molten tin in the electrode 149 can easily be removed from the crucible and replaced with another metal or a suitable metal salt, without interrupting the operation of the machine.

In the system shown in fig. 17 comes the warm-up

enabled by means of three electrical resistance elements 153 placed above the threshold. There is very little heat loss through the top wall of the threshold. The resistance elements can be effective

over the entire width of the oven, or over only part of the width. The heat developed in the resistive elements produces an upflow of molten glass at the site of the threshold 131. The upflow of molten glass continues to the surface area of the molten glass in the furnace and counteracts any tendency for molten glass streams to flow across the threshold from space 151 to room 152.

When heating devices are used to create a heat barrier across the furnace, i.e. parallel to the plane of the bottom part of the drawn glass strip, it is not important, whatever form of heating device is used, that this heating device extends over the entire width of the furnace. For example if this heating device extends only over a portion of the width of the furnace coinciding with the main part of the belt width, between its core zones, molten glass streams flowing backward from the rear end wall of the furnace may flow laterally outward past the ends of the heat barrier and introduce impurities into the cores of the tape, but the heat barrier will protect the main center of the tape from such impurities. Because the edge portions of the tape will be removed as clearance in any case, such contamination is not a serious defect.

Reference is made to fig. 18. The drawing shows part of

a Pittsburgh-type machine comprising an oven 159 having a sole 160 and a rear end wall 161 comprising a threshold 162,

and an upper rear portion 163 positioned slightly above the level 164 of the molten glass in the furnace to allow a continuous over-flow of molten glass at the rear of the furnace. In a trough between the threshold and the upper rear part 163 there is a tungsten plate 165, 1 cm thick, which serves as an electrode. Above the furnace is the usual drawing chamber 166 which is bounded behind and in front by L-blocks 167 and 168, main coolers 169 and auxiliary coolers 170 located inside the drawing chamber on opposite sides of the path along which the glass strip is drawn, and the tower section 171 through which the glass strip 172 is drawn upwards while undergoing de-cooling. The glass strip 172 is drawn from the surface of the molten glass in the furnace via a meniscus 173. The front side of the glass strip is indicated at 174 and its back side is indicated at 175.

Instead of the traditional form of drawbar, the machine has a bar 176 which has a recess 177 which extends almost the entire length of the bar. A pond 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 opposite poles of an alternating current source 179. The surface area of the plate 165 and the pool 178 are important factors. It is such that the electrical current density is not greater than 0.5 A/cm 2 anywhere on the surfaces so that the risk of bubble formation within the molten glass is minimal, but such that the current density is sufficient to bring the temperature in the glass above the threshold of staying about 60°C higher than it would be if there were no electrical 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 maintain a stable rotational movement about a horizontal axis in the clockwise direction of the drawing. Consequently, impurities that could contaminate the molten glass in this area are kept away from the drawing zone. The surface stream of molten glass which is fed into the back side 175 of the glass belt 172 is kept largely free of impurities formed in the area behind the threshold. The glass which forms this surface flow into the rear side of the belt is heated in the hot zone near the threshold, and the viscosity of this glass can be maintained so that it can be compared with the viscosity of the glass in the forward stream of the glass being fed. in the back side 175 of the glass band. The stream of molten glass flowing in contact with the pond 178 of molten lead is completely steady. This is in contrast to the flow of molten glass which is in contact with the top of an ordinary slotted drawing zone. In this case, irregularities on the counter surface will disturb the glass flow and often cause irregularities in the thickness of the drawn glass strip. The ends of the counter 176 have a lower temperature than the pool 78 of molten lead and this lower temperature at the ends of the counter helps to stabilize the position of the ends of the meniscus 173 from which molten glass flows into the side edges of the strip. The continuous overflow of molten glass across the top of the rear portion 163 of the rear end wall 161 of the furnace produces a rearward flow of glass which further assists in preventing impurities from passing from this area behind the threshold into the stream of molten glass that is fed into the belt. This

continuous backward discharge of molten glass also serves

to prevent the concentration of impurities in the area behind the threshold from building up to a large size.

The large thickness of the rear end wall 161 in the machine shown in fig. 18, is of value in reducing the heat loss from the glass in the furnace by conduction through the wall, and consequently helps to keep the glass near the surface 164 at the necessary temperature for feeding into the belt.

In the machine illustrated in fig. 18, the density must

on the electric current between the electrodes be sufficient to reheat the relatively cold streams of molten glass which flow upwards between the draft counter 176 and the rear end wall 161 of the furnace.

This factor influencing the choice of current density does not occur with a machine shown in fig. 19. This figure shows a Pittsburgh type machine similar to that shown in fig. 18, except for certain modifications which will now be described. The reference numbers used on 18 are also used on fig. 19 to indicate corresponding parts in the modified device.

In the machines shown in fig. 19, rear end wall 161 has even greater thickness, particularly in an area located in the upper part of its height, but below the level of the molten glass in the furnace. The location, shape and cross-sectional dimension of the rear end wall is such that a cornice part 180 thereof extends below the meniscus 173. This cornice part 180 has a recess 181 in its top surface, the recess extending over almost the entire inner width of the oven. The recess contains a pool of molten tin. Instead of molten tin, a dam of some other molten metal may be used, or a dam of a suitable molten metal salt, e.g. a lithium salt. The top surface of the cornice part 180 can e.g. be 20 cm below the surface 164

on the molten glass in the oven. The pond 182 of molten tin and the tungsten plate 165 are connected to opposite poles of a current source 183.

The device and method shown in fig.

19 has similar advantages as the device and method described with reference to fig. 18, in that these advantages derive from the large thickness of the rear end wall 161 of the furnace, the presence of the threshold 162 and the passage of electric current through the electrodes which maintain a hot zone above the threshold, and the continuous overflow of molten glass over the rear top part 16 3 on the rear end wall. In contrast to the device and method according to fig. 18, it is in this device and method according to fig. 19, no upward flow of glass between the electrodes under the roof zone and the rear end wall of the furnace. In the system shown in fig. 18, there is an upward flow of molten glass between the counter 176 and the rear end wall, and this upward flow mixes with the flow of molten glass forward over the counter. In the system of fig. 19 there is no such mixture of currents fed into the meniscus. The method illustrated in fig. 19

can thus be considered a method in which glass is drawn from a "monostream", and this is particularly advantageous when drawing sheet glass with a very high standard of homogeneity. In a real embodiment according to fig. 19, it was found that an electrical current density of only 0.3 A/cm 2 in plate 165 was sufficient to permit a draw speed as with fig. The 18 method required a current density at this location of 0.5 A/cm 2 . Of course, a current density higher than 0.3 A/cm 2 , e.g. a current which had a density of 0.5 A/cm 2 is used when applying a method according to fig. 19, in which case the draw speed can easily be increased to 1.8 or even twice the maximum draw speed possible in a conventional Pittsburgh process.

When working according to fig. 19, molten glass which feeds the back surface of the glass ribbon 174, 172 flows along the surface of the pond 182 of molten tin and is heated during its upward flow in the vicinity of the hot zone located above the threshold 162. This stream of molten glass reaches the meniscus, but with minimal contact with compact surfaces. These factors explain in principle why such a high pulling speed is possible. As the flowability of the molten glass that is fed into the rear side of the belt is comparable to the glass that flows into the front side, there is a very good layering of the glass in the drawn belt.

When using the method according to fig. 19 and also when using the method according to fig. 18, the rear surface 174 of the drawn glass ribbon is particularly flat. Due to the fact that the outer edges of the cornice part 180 near the side walls

in the furnace, is somewhat colder than the pool 182 of molten tin,

they help to stabilize the position of the meniscus 173 end parts through which the molten glass is fed into the edges of the belt.

Fig. 20 is an amamorphotic photographic image of part of an edge surface of a sample of a glass plate according to the invention, which glass plate is produced by cutting the plate glass along a line normal to the drawing line, and the photographic image is called "striascope".

In the photographic image, glass layers with different refractive indices appear as bands of different optical density, and the way in which the glass layers with different refraction are distributed is shown by means of contour lines which can be seen in the photographic image and which correspond to the boundaries between juxtaposed layers. It should be noted that these lines are largely parallel. The arrangement of the lines is such that they suggest a basic pattern of flat ellipses extending visibly from one edge to the other in the drawn plate glass. This suggestion derives in particular from the presence of shallow outwardly convex curves running in the longitudinal direction of the photographic image, and the presence of relatively smooth curves with much smaller radii that form the ends of the opposing shallow curves. It is worth noting that in the cases really shown in fig. 20, the above-mentioned flat ellipses, one within the other, have a largely common center which is displaced towards one of the plate surfaces. The other layers that are close to the second surface can be thought of as forming shallow outwardly convex curves that probably belong to larger ellipses.

A plate glass sample such as that shown in fig. 20 was tested in an interference microrefractometer in which light was sent from a slit source in such a way as to form a series of straight parallel interference lines. The plate was placed in the light beam in such a way that the light beam entered one of the plate's side edges and emerged from the opposite parallel side edge, on the plate, and the plate was in a plane that created interference lines at 45°. The interference lines appeared as shown in fig. 21, that is, without any obvious errors or violations. The two oblique, parallel lines that intersect the interference lines represent the main surfaces of the glass plate.

The characteristic properties of the plate glass according to the invention as mentioned in connection with fig. 20 and 21 become easily visible when these figures are compared with fig. 22 and 23. Fig. 22 shows a photographic image, called "striascope", which was produced under the same conditions as the image in fig. 20, of part of an edge surface of a sample of plate glass drawn by means of a classical Pittburgh-type method. Fig. 23 shows the effect that the presence of such a sample, drawn by means of a classical Pittburgh-type method, had on the appearance of the interference lines when the sample was tested in the same interference microrefractometer as was used when testing the plate in fig. 21 and under the same conditions. As seen in fig. 22, the contour lines do not show any kind of elliptical pattern. In some places in the photographic image the lines converge to form an acute angle, in contrast to the smooth curves in fig. 20. When placed in the light beam of the interference microrefractometer, the plate glass caused a very marked flaw, or a break to appear in the interference lines as shown in fig. 23. This break indicates a sudden change in the index of refraction between adjacent layers of glass located centrally in the thickness of the glass plate.

Fig. 24 shows a typical brush stroke pattern on the surface of a plate glass drawn by means of a classic Libbey-Owens drawing method. These brush strokes are detected by interferometry using the well-known Fizeau lines. The glass plate to be tested is placed on a polished glass plate in such a way that the two plates form a very sharp angle with each other. The cutting line between the two plates must be perpendicular to the direction of pull for the glass plate to be welded. The polished glass plate must have a flatness so that the thickness tolerance is in the range of X/4. The surfaces of this polished plate must be as precisely parallel as possible.

When the two plates thus arranged are irradiated by means of rays which are generally perpendicular to their surfaces, these rays, as they return, show a pattern of alternating white and black lines as seen in fig. 24. Each black line represents a line of equal thickness in the plate, these lines but of equal thickness show small waves of high frequency which can be called "saw teeth". The presence of these "saw teeth" indicates the presence of "brush stroke" defects and allows one to determine that this defect is in the form of very small waves of the order of 0.3 µu in thickness and with a width of 0.1 to 1 mm. When the two main surfaces of a glass plate according to the invention were subjected in turn to an examination for defects in exactly the same manner as the glass sample drawn by means of the Libbey-Owen method, no brush strokes were detected on either surface. . The glass plate sample according to the invention that was subjected to this scrutiny was a test glass plate drawn using a method according to the invention, in which there was a heat barrier in the molten glass in the furnace in a position just behind the drawing zone.

Alternatively, the brush strokes can be detected by a method using reflection, by sending a light beam down onto the surface of the glass plate at an angle of incidence of 65° with the axis of the beam in a plane normal to this surface and normal to the drawing line of the glass plate to cause light rays to be reflected from this surface towards a light-scattering screen which is placed approx. 1 m from the glass plate.

The photographic images were the same as shown in fig.

20 and 22, by means of a striascope apparatus shown schematically in fig. 25. This apparatus comprises a white incandescent light source 191, an achromatic condenser 19 2 which is placed near the light source, an objective 192, a Foucault system 19 3 and a masking plate 194 in which a vertical, rectangular slit is placed. Each of the photographic images is an image of the light pattern transmitted in the striascope through a strip 19 6 , 10-20 mm wide, which is cut from the respective glass ribbon with a cut that runs across the full width of the ribbon, normal to its draw line. The strip was placed in a transparent tube 197 which was filled with a liquid 198 having a refractive index very close to that of glass. A suitable liquid is ethyl salicylate or monotrobenzene, but numerous other suitable liquids are also available. The emitted light rays that passed through the slit in the mask plate 194 are recorded by a light-sensitive film 199. The Striascope produces a photographic image with 3-10 times the magnification of the plate's thickness. In order to produce a photographic record of the distribution of the glass layers of different refractive indices over the entire cross-section of the sample strip 196, the strip is displaced little by little in its own plane and in a direction parallel to its own longitudinal axis (that is, along a line normal to the drawing plane) so that the strip moves little by little through the beam path impinging on the photosensitive film, and this film is simultaneously displaced in its own plane along a path normal to the drawing plane and in a direction opposite to the direction of displacement of the strip 196, consequently exposed, one after the other , parts of the film in turn through the aforementioned slit. This slit has its largest dimension vertically and has a width of from 0.2-1 mm. The film's 99 linear speed can be e.g. 10-20 times less than the linear speed of the strip 196, so that the photographic image of the entire strip is enlarged in thickness but reduced in length. The striascope thereby produces an amamorphic photographic image.

The displacement of the test strip 196 and the light-sensitive film 199 can be mechanically synchronized. For example, the strip and film can be moved by a common motor through a reduction gear which is variable to enable the relative speed to be preset within a given range.

When making a photographic record in a striascope as described above, it is convenient to use a "Copex Copy" film, marketed by Agfa-Gewaert, Mortsel, Belgium, and to develop the film with a common methol-hydroquinine developer such as generally used for developing photographic copies on light-sensitive paper.

Reference is now made to fig. 26. The interference refractometer which is schematically shown in this figure and which uses the known Nomarski method, comprises a light source 200,

a converging lens 201, a mask 202 with a slit hole 203, an objective 204, a double line double prism unit 205, generally called a "Wollaston" prism, with front and rear polarizing filters 206 and 207, and an optical focusing device 208 which can, for example , be in the form of the eyepiece of an ordinary microscope, or a device which focuses the emitted rays onto a light-sensitive recording material. In one test, a Nachet 300 microscope with a 3x objective and a 6x eyepiece equipped with a polaroid camera having a chamber index of 0.8 was used.

A black and white Polaroid roll film 63.5 x 82.6 nuii', light sensitivity 3000 ASA, type 37 was used and the exposure time was one second.

Without a sample to be tested, the parallel light beam from the slit hole is split into different polarized and differently retarded parts and the emitted rays are focused so that they form an interference pattern comprising a series of straight parallel bands as shown in fig. 21. Further details regarding the construction and operation of this type of interference micro-refractometer can be found in the above-mentioned paper entitled "Objectif interférentiel à prisme de Wollaston" published by Techniques de 1'Ingénieur, 21 rue Cassette Paris VI, France. In order to be able to use the refractometer for testing a sheet glass sample 209, the edge surfaces of the sample are polished and placed in the parallel light beam from the slit hole so that the beam passes through the plate from one side edge surface to the opposite side edge surface, and so that the plate is in a plane that intersects the parallel interference bands. When a glass plate sample according to the invention is placed in this way in the light beam, the continuity of the interference band is not affected, whereas a plate glass sample drawn using a classic Pittburgh-type drawing method caused a marked break in the band, as shown in fig. 23.

Claims (10)

1. Process for manufacturing sheet glass by continuously feeding molten glass to a furnace to create a forward flow of glass towards a drawing zone, in which molten glass is continuously drawn up from a meniscus in the surface of the glass in the furnace in the form of a ribbon, and where molten glass in the furnace is locally heated, characterized in that at least one place in the furnace, below the glass surface and at a distance on the far side (behind) and/or to the side of the meniscus, a heat barrier formed by an upwardly directed stream of molten glass rising to and reaching the surface from an area near a wall or bottom of or in the furnace and adjacent to a glass weir, so that molten glass on the far side of the barrier is substantially prevented from flowing back through and under the heat barrier. 2. Method as stated in claim 1, characterized in that molten glass on the surface of the forward flow is fed directly into the bottom of the belt on its front side, while molten glass at a lower level in this forward flow rises in a position behind the draw zone and forms a oppositely directed surface flow feeding into the belt on its rear side, and in that the heat barrier is created in a position coinciding with the position in which the molten glass from the lower level rises behind the draft zone. 3. Method according to claim 2, characterized in that a heat barrier is formed which extends at least over the part of the oven width that coincides with the belt width. 4. Method according to any one of the preceding claims, characterized in that at least one heat barrier is formed over a hollow threshold and that the upward flow of molten glass at this location is maintained by creating heat in the space inside the threshold. 5. Method according to any one of the preceding claims, characterized by at least one heat barrier which is maintained by passing an electric current through the molten glass between electrodes. 6. Method according to claim 5, characterized in that a heating current is maintained between electrodes which are placed on opposite sides of a threshold which is completely immersed in the molten glass. 7. Apparatus for carrying out the method stated in one or more of the preceding claims, comprising a furnace (1, 10, 60, 80, 102, 159) with a feed end where molten glass can be continuously fed to the furnace, a device for continuously drawing a band of glass upwards from a meniscus in the surface of the glass in a drawing zone in the furnace, and devices (16, 17, 18, 29, 30, 46, 67, 68, 99, 100, 110, 111, 114, 116, 117, 123, 124, 129, 130, 141, 142, 145, 146, 148, 149, 153, 165, 178, 182) for local heating of the molten glass in the furnace, characterized in that heating devices are placed below the surface at a distance on the far side and/or to the side of the meniscus, thereby creating a heat barrier formed by an upward flow of the molten glass rising to the surface from an area near a wall section (7, 8, 21, 42, 66, 94, 95, 97, 105, 106, 107, 125, 132, 161, 162, 176, 180) of or in the furnace adjacent to a glass overflow, whereby the mentioned wall part serves to prevent molten glass behind the barrier from flowing back under it. 8. Apparatus as stated in claim 7, characterized by a heating device (67, 68) to maintain an upward flow of molten glass from a position up against the side surface of the threshold (66). 9. Apparatus as set forth in one of claims 7-8, characterized by electrodes (16, 17, 18, 110, 111), between which electric current can pass through molten glass in the furnace to maintain an upward flow of molten glass. 10. Apparatus as stated in claim 9, characterized in that at least one electrode (110, 111) which is formed by means of a pool of molten metal or molten metal salt.