IL40405A - Process of colouring or modifying the colouration of a glass body containing reducing ions by diffusion - Google Patents

Abstract

1400824 Colouring glass GLAVERBEL 11 Sept 1972 [1 Oct 1971] 42127/72 Heading C1M [Also in Division B5] A process of colouring or modifying the colouration of a glass body containing reducing ions by causing a substance to diffuse into surface layers of the glass body from a contacting medium is characterized in that the said medium comprises a mixture of metal salts containing not more than 50% by weight of a salt providing reducible metal ions of a type capable of being reduced by said reducing ions, and in that after diffusion of reducible metal ions into said body from said medium, the final colouration is achieved by heating the body to a temperature above the strain point of the glass. The glass body may be formed from a soda-lime glass. The reduction is not relied upon for achieving the final colouration and in some cases there is no significant colouration of the glass body at the end of the first treatment stage. The process finds particular application in colouring bodies of flat soda-lime glass which may be produced by the so-called "float" process in which the reducing ions (Sn<SP>++</SP>) diffuse into the glass from the molten material the glass floats on. Alternatively the glass contains the reducing ions (Fe<SP>++</SP>) prior to being drawn into a glass body using a conventional drawing machine. The treatment medium may contain less than 10% by weight of salt(s) providing the reducible metal ions and such ions are wholly or predominantly silver ions. Such ions produce a yellow colouration. Other reducible metal ions used are of copper and gold. Preferably the treatment medium is a salt medium in molten state, the medium being agitated if necessary as by propellers or bubbling gas therethrough. However the treatment medium may comprise the mixture of metal salts in vapourifed condition. The molten medium may be sprayed on to the body, but preferably the body is immersed in a quantity of the molten medium. Only part of the surface of the body (flat or curved sheet) may be coloured or modified in colour and in such cases those parts of the surface not to be so modified are shielded from the treatment medium by a temporary coating. In further alternatives the medium may be kept in contact with the surface portion to be treated by retaining such medium within an endless retaining wall contacting the body along the boundary of the surface area to be treated or the medium may be caused to stream along the surface area to be coloured. The treatment medium advantageously contains one or more zinc salts which promote the diffusion of the reducible metal ions into the surface of the glass body. The said reducible metal ions preferably derive from a metal salt selected from nitrates, chlorides and sulphates. The balance of the treatment medium comprises alkali metal salts, e.g. NaNO 3 , KNO 3 , Na 2 SO 4 , K 2 SO 4 . When silver salts are used in the treatment medium the first treatment step is preferably carried out while the body is at a temperature less than 350‹ C.; in the second step the body is heated to a temperature of at least 500‹ C. after the diffusion of reducible metal ions thereon. The colouration development in the second stage may be promoted by bringing the treated surface of the body into contact with a reducing gas, e.g. H 2 . The salt with which the salt providing the said reducible metal ions is mixed may comprise a metal salt providing metal ions which diffuse into the glass body in exchange for other ions. After heating the glass body the body may be rapidly cooled to produce compressive surface stresses therein. In one embodiment during the heating of the body while at a temperature above the strain point of the glass ions, e.g. Li<SP>+</SP>, which confer a lower coefficient of thermal expansion on surface layers of said body are caused to diffuse into such surface layers in exchange for other ions. The body may be tempered either during or subsequent to the colouring treatment. During the final heat treatment the body may be bent. [GB1400824A]

Description

PROCESS OF COLOURING OR MODIFYING THE COLOURATION OF A GLASS BODY CONTAINING REDUCING IONS BY DIFFUSION. -ΠΤΓΙΠ Dn3 ~P ·>3ΏΠ ΓΠΖΠΙΠ ΚΠΛ 7ΰ ]Π3 "n:Pu7 IN -]iiX? ητ?πιι T!vis i voivLion relates to a process of colouring or modifying the colouration of a glass body containing reducing ions by causing a substance to diffuse into surface layers of the glass body from a contacting medium.

Coloured glass bodies can be made by forming them .from a vitrifiable batch incorporating appropriate colouring compounds. This procedure is practicable in but a limited number of circumstances due to the need to use a different batch composition for every different glass colour to be produced. In the glass manufacturing industry and in particular in the production of flat glass it is usally much more practicable to colour the glass body during or after its formation so that the colouring treatment can be controlled independently of the production of the vitrifiable batch and of the glass-forming process.

Various methods are known for colouring a glass body during or after its formation. Such methods include the coating of the glass body with a film of a metal or metal oxide by vacuum deposition. It is possible by this method to produce a very thin film which docs not greatly reduce the light-transmissivi ty of the body.

However, the coating film is liable to be damaged or removed by abrasion or mechanical or chemical action.

A more satisfactory known process involves the diffusion of colouring substances into the glass at elevated temperature. In this way the glass can be coloured over a certain depth from its surface and the colour cannot be removed by scratching of the glass surface. However the known diffusion process is limited in its applicability for obtaining required colourations of a range of different glasses. For example, there are numerous metals which can be caused to diffuse in ionised form into a body of glass and which will impart different colours to such a body, but some of them, although they are of potential interest and value by reason of the particular colour which tye can confer on the glass, give only a feeble colouration when an attempt is made to colour ordinary drawn sheet glass with them by the known process. At least, this is so if economically viable treatment conditions are observed.

A particular example of a potentially valuable colouring metal is silver. However this metal gives only a very feeble yellow colouration to sheets of ordinary drawn sheet glass when used as colouring agent in diffusion colouring processes as hitherto proposed unless the arount of silver, in the form of a salt, used in the treatment medium, or the duration of the treatment is excessive. This feeble yellow colouration is not sufficient to be of real significance for the purpose of screening off actinic light.

The present invention aims to provide a diffusion type process which has a high degree of flexability in respect of the hues and colour densities which can be imparted to bodies of a range of different glass compositions, including ordinary sheet glass.

According to the present invention there is provided a process of colouring or modifying the colouration of a glass body containing reducing ions by causing a substance to diffuse into surface layers of the glass body from a contacting medium, characterised in that the said medium comprises a mixture of metal salts containing not more than 50% by weight of a salt providing reducible metal ions capable of being reduced by said reducing ions to bring about colouration or colour modification of said body, and in that after diffusion of reducible metal ions into said body from said medium, the final colouration is achieved by heating the body to a temperature above the strain point of the glass.

The process according to the invention is much more flexible than the diffusion-type colouring process known hitherto in that it is possible by the process according to the invention to obtain a range of colourations of bodies composed of a range of different glasses, under viable industrial conditions. This is due to the performance of two distinct processing steps, namely a diffusion step in which the amount of colouring substance available for diffusing into the glass body is restricted and a developing step in which colouring substance which has diffused into the body is caused to colour or modify the colour of the said body, or in which the colouring or colour modifying effect of that substance is developed.

In addition to its said flexibility, the process according to the invention has the inportant advantage that the amount of colouring agent required for achieving a given colouration can be much lower than would otherwise be necessary. In the first stage of the process, at least some of the reducible metal ions which diffuse into the glass body are reduced by said reducing ions contained in such body, but this reduction is not relied upon for achieving the final colouration and in fact there may in some cases be no significant colouration of the glass body at the end of the first treatment stage. For example when treating ordinary drawn sheet glass with a treatment medium containing silver ions, the amount of silver in the treatment medium can be very low, and even insufficient to give any preceptive colouration of the glass in the first treatment stage. In that case the process can be industrially performed at a very lo cost.

In the most important applications of the invention, the treated body is formed from soda-lime glass of ordinary composition. Such glasses are substantially colourless and by performance of the present invention colourations can be imparted to them which are determined solely by the colouring process.

The invention is very useful for colouring bodies of j£lat soda-lime glass and bodies formed from flat f soda-lime glass. The problem of colouring glass bodies to achieve a nicely controlled predetermined colouration to meet specified optical requirements arises most often in the production of articles made of or made from flat soda-lime glass, e.g., in the production of flat or curved glazing panels or lenses of sunglasses, The invention can for example be successfully employed for colouring flat soda-lime glass formed by the so-called "float" process in which the molten glass batch is caused to forn a floating layer on a quantity of material of higher specific gravity, or for colouring soda-lime rolled glass.

In the most important embodiments of the invention, the process is employed for colouring a body composed of or formed from flat drawn soda-lime glass, e.g., a flat panel of drawn glass or a curved glass panel formed by bending such a flat panel. The benefits of the invention are very great indeed when it is used for colouring such a body. Drawn soda-lime glass usually contains only a 2+ very minor proportion of reducing ions, e.g., Fe so that if an attempt is made to colour such a body with certain substances by a known diffusion process it is not possible to achieve satisfactory colourations under viable industrial conditions. The colouration is often too weak to achieve the required purpose. When employing a process according to the invention a much stronger colouration can be achieved at much lower cost. This is r- » because the special two-stage treatment enables quite dense colourations to be achieved for a relatively low consumption of colouring substance and without an unduly large consumption of power for heating purposes. It has been found that a wide range of colouration densities can be achieved in a total treatment time of less than 2 hours and even less than 5 hours even when using a minor amount of a colouring substance which when used in the known diffusion processes gives only a very feeble colouration even when such substance is used in much larger quantities.

The use of a treatment medium which comprises not only a salt which provides the reducible metal ions for colouring purposes but also one or more other salts is valuable not only because the proportion of colouring substance available for diffusing into the glass is restricted (the maximum amount of the salt providing the reducible metal ions in accordance with the invention being 50% by weight of the treatment medium) but also because the proportion by weight of such salt can be chosen from a wide range of values in accordance with the results required. In other words the said proportion provides a control parameter in addition to processing temperature and processing time. For any given colouring substance, these parameters jointly influence the depth of penetration of the reducible metal ions of the colouring substance, as well as the final colouration.

In certain processes according to the invention, which are of particular importance, the treatment nediun contains less than 10% by weight of salt(s) providing the reducible metal ions and such ions are wholly or predominantly silver ions. The use of silver for colouring glass is of special importance, and notably so for colouring bodies co-nposed of or formed from flat drawn soda-lime, glass. When the silver ions are reduced within the glass or during the subsequent heating of the glass body the glass acquires a yellow colouration. The yellow colouration is indicative that the body has certain actinic-light-absorbing properties. It is surprising that the process according to the invention enables a significant yellow colouration and thue a significant actinic-light-absorbing power to be conferred on a body composed of drawn soda-lirne glass of ordinary composition, under viable industrial conditions, when using a treatment medium containing such a relatively small amount of silver.

In certain processes according to the invention, the treatment medium contains less than 3% by weight of said salt(s) providing the said reducible metal ions, such ions being silver ions. In such processes the treatment medium can be of very low cost. The presence of such a minor proportion of silver does not prevent the process from giving a significant yellow colouration to bodies composed of or formed from flat drawn glass of an ordinary soda-lime type, In fact such a result can even be achived even when the proportion of silver salt in the Eiediuu is very much lower than that value.

In certain optimum processes according to the invention, which are for example applicable for treating ordinary drawn soda-lime glass (window glass) or float glass, the treatment medium contains less than 0.5 by weight of salt(s) providing said reducible metal ions, such salt(s) being a silver salt or salts. In certain other optimum processes according to the invention, wherein float glass is treated, the treatment medium contains less than 0.05%, e.g., 0.01% by weight of salt(s) providing said reducible metal ions, such salt(s) being a silver salt or salts. Inasmuch as reliance is not placed on the diffusion of colouring substance into the gl for achieving the final colouration, because colouration development is brought about in a subsequent stage, the diffusion of reducible metal ions into float glass can take place in the continuous float glass production line notwithstanding the relatively high speed of the glass ribbon, and without any necessity to take special measures to promote the rate of diffusion of said reducible metal ions into the glass.

While emphasis has been placed on silver as a particularly valuable colouring substance, the invention is not confined to processes in which that substance is used.

In some processes according to the invention, the said reducible metal ions in the treatment medium are wholly or predominantly copper ions. The use of copper is advantageous for achieving strong colouration of a glass body. The colouration achieved is useful, e.g., for decorative purposes.

The invention is also useful for bringing about colouration of a glass body using gold as the colouring substance .

Preferably the treatment medium which comprises a mixture of salts, is in molten state, The use of a medium in molten state is recommended for achieving a highly satisfactory diffusion of reducible metal ions into the glass body under normal atmospheric pressure conditions However, the invention can be performed with a treatment medium comprising the mixture of metal salts in vapourised condition.

The invention may be performed by sprinkling or spraying the medium in molten condition onto the body to be treated. Preferably however the body is immersed in a quantity of the molten medium. Immersion is preferred because generally speaking it enables a uniform temperature to be more easily maintained over the treated surface area. On the other hand the immersion technique; is mainly suitable in cases in which the whole surface of the body is to be coloured or modified in colour. The invention is of course not restricted to such cases.

The invention includes processes in which only part of the surface of the body, e.g., one face of a body in the form of a flat or curved sheet, is coloured or modified in colour. If only one surface portion is to be coloured the body may be immersed in the treatment medium if the remainder of the surface of the body is shielded from the treatment medium, e.g., by a temporary coating or covering. Alternatively only the part of the body to be treated may be immersed if the shape of the body permits that to be done. As a further alternative, the medium may be kept in contact with the surface portion to be treated by retaining such medium within an endless retaining wall contacting the body along the boundary of the surface area to be treated. As a further alternative, the medium may be caused to stream along the surface area to be coloured. The procedure of flowing the medium along the body can also be adopted in cases in which the whole surface of the body is to be coloured.

When the diffusion treatment is performed by holding a quantity of the treatment medium in molten condition in contact with the body to be coloured, the uniformity of the treatment can be promoted by agitating the medium, e.g., by means of propellers. As an alternative, agitation may be effected by injecting gas into the medium.

During the diffusion of reducible metal ions into the body from a molten medium, the required concentration of such reducible metal ions may be maintained by bringing about the electrolytic dissolution of a quantity of solid or liquid substance providing the required ions.

Advantageously, the treatment medium incorporates one or more zinc salts. It has been found that the presence of one or nore zinc salts in the medium can promote the diffusion of given reducible metal ions into the surface of a glass body under given conditions. By adding a suitable zinc salt to a medium comprising a copper salt, the colour can be deepened to green or even grey.

Advantageously, the said reducible metal ions derive from a metal salt selected from the group: nitrates, chlorides, sulphates. In general such salts are very easy to prepare, handle ond use. Moreover such salts are molten and do not decompose at the temperature involved .

In the case that the salt(s) providing said reducible metal ions is or are a silver salt or salts, the first treatment step, i.e., the step in which said reducible metal ions are caused to diffuse into surface layers of the body, preferably takes place while the body is at a temperature less than 350°C. By maintaining the temperature below that value the objective of limiting the diffusion of the reducible metal ions into the body in the first stage preparatory to a controlled development of colour in the subsequent stage is promoted.

The second stage of the process comprises heating the body to a temperature above the strain point of the glass. In preferred embodiments of the invention, in which the body is a body composed of or formed from flat drawn soda-lime glass, the body is heated to a temperature of at least 500°C after the diffusion of reducible tnetal ions therein. By heating the body to at least 500°C for the second stage of the process, colourations of useful density can be achieved in moderate or even very short periods of tine even when using a treatment medium in which the reducible metal ions are silver ions and are present in a very low concentration in the treatment medium, e.g., in a concentration of less than 0.5% by weight. In certain tests it was found that the quantity of the reducible silver ions whifch diffused into the glass in the first stage of the process were not reduced, although a certain amount of reducing ions remained available in surface layers of the glass. The effect of the heating in the second stage of the process is probably to increase the activity of the reducible and reducing inns so that the colouration, which may even be practically invisible at the end of the first stage of the process, is developed. It is not suggested that that is necessarily the real or the sole reason for the success of the process. Probably other phenomena play an important role such as the development of crystallites. Whatever be the reason, however, it is the fact that significant colouration can be realised by the combination of the two stages, even when using a molten salt containing a comparatively small t quantity of a salt of the colouring metal. VThen using a silver salt to provide silver ions for diffusing into float glass, the quantity of silver salt in the treatment medium can be very small. For example the medium may contain as little as 0.01% of silver nitrate.

Depending on the concentration of silver ions in the treatment medium and/or on other influencial factors such as the presence or not of an adjuvant and possibly a zinc salt, it is possible to achieve satisfactory colourations for actinic light screening purposes in periods of time ranging for example from h hours to a few minutes with second stage treatment temperatures ranging from 500°C to 600°C.

The colouration development in the second stage of of the process can be promoted by bringing the treated surface of the body, during at least part of the period of duration of the second stage treatment, with a reducing fluid, e.g., a reducing gas such as hydrogen. This feature is of particular interest in the case that copper is the colouring substance.

The salt or salts with which the salt or salts providing the said reducible metal ions is or are mixed to form the treatment medium may serve a purpose additional to that of acting as a diluting agent for the salt or salts of the colouring substance; or substances. For example, the diluting agent may comprise a metal salt providing metal ions which diffuse into the glass body in exchange for other ions to bring about some other modification of its surface properties.

In certain processes according to the invention, after the said heating of the glass body to achieve said final colouration, the body is rapidly cooled to produce compressive surface stresses therein. The colouring ' and tempering of the body can thus conveniently be effected in the course of a single treatment process.

In other processes according to the invention, the processes are characterised in that during the heating of the body while it is at a temperature above the strain point of the glass following the diffusion of said reducible rnetal ions into the body, ions, e.g., lithimm ions, which confer a lower coefficient of thermal expansion on surface layers of said body are caused to diffuse into such surface layers in exchange for other ions so that compressive surface stresses are set up when the body is subsequently cooled. The body is thereby chemically tempered and in consequence has an increased resistance to breakage under tensile loadings of its surface.

The body nay of course bo tempered in some other way, either by a treatment which takes place at least partly during the colouring treatment, or by a treatment which is performed subsequently to the colouring treatment. Thus the body can be thermally tempered by heating and they rapidly cooling the body after it has been allowed to cool following the second stage of the colouring treatment As a further alternative, after the completion of the colouring process, the body m y be chemically tempered by causing ions to diffuse into the body from a contacting medium, in exchange for smaller ions at a t mperature insufficiently high to allow complete relaxation of compressive surface stresses in the glass.

In ion exchange treatments as aforesaid it is preferable to use a medium comprising an alkali metal salt as the chemical tempering agent providing the cations for diffusing into the glass in exchange for other ions. For example it is very suitable to use a molten potassium salt. The alkali metal salts useful in such ion exchange tempering treatments can for example be selected from nitrates, chlorides and sulphates.

The process according to the invention can of course be used not only for colouring uncoloured glass but also for modifying the colouration of a glass body which is already coloured. The process as applied to an already coloured glass body is precisely the same as the process applied to a colourless glass. The modification of colouratinn may involve a modification of the existing colour or tint or merely an intensification or deepening or strengthening of an existing colour or tint, e.g., a colour or tint imparted by a preceding process, which latter process m y of course also be a process according to the invention.

When performing the second stage of the process at a sufficient elevated temperature, e.g., above 600 C the body being treated can in suitable cases, e.g., in the case that the body is a body of flat glass, be bent during this treatment stage. The performance of a bending operation in course of the colouring treatment is of interest for speeding the mass production of articles, for example for speeding the mass production of vehicle windscreens. By performing a tempering treatment in course of the colouring process, a panel of flat glass can be given a yellow colouration, tempered and bent to the required curvature in a relatively abort period of time.

Processes according to the invention can be performed for imparting different colourations to different surface portions of a glass body. Thus, by way of example, -a body of flat glass can be subjected to processes according to the invention for imparting different colourations to the surface layers of glass at its opposite faces.

For example different faces can be given yellow colourations of different densities.

The invention includes a body of glass which has been coloured or subjected to colouration modification by a process according to the invention.

The invention includes a vehicle windscreen comprising sheets of glass or at least one sheet of glass and at least one sheet of plastics, which glass sheet or at least one of which glass sheets if there is more than one has been coloured or subjected to colouration modi ication at at least one of its faces by a process according to the invention, such sheets being united at their margins or over their whole areas to an intervening layer of adhesive, glue and/or plastics. An expoxy resin glue or adhesive may be used. Advantageously the intervening layer comprises a preformed sheet such as a sheet of polyvinyl butyral. Particular importance is attached to windscreens comprising two said sheets of glass (at least one of them having thus been coloured or subjected to colouration modification) united to an intervening preformed sheet, e.g., a sheet of polyvinylbutyral .

The invention will be illustrated by various specific embodiments selected merely by way of non-limiting example.

Example 1 A ribbon of glass essentially having the following composition by weight : si°2 72.5%, A12°3 Na2° lZ*^' CaO 7' 5%» MgO % together with a minor proportion of K^O and of iron oxide, was formed by drawing in a Pittsburgh-type glass drawing machine. The ribbon was 3 mm in thickness.

Sheets of glass measuring 1 m x 0.5 rci were cut from this glass ribbon. The sheets xfcre pre-heated and then immersed in a bath of molten salt composed of 58% by weight KNO^ and k?S by weight NaNO^ to which AgNO^ had been added in a proportion of Όν,0 % of the aggregate weight of the said potassium and sodium salts.

The molten salt bath was at a temperature of 330°C and the sheets were immersed in the bath for a period of 1 hour.

After withdrawal of the sheets from the molten salt bath, the sheets, which were then still colourless, were heated at 550°C for 3 hours. The sheets were then cooled and washed. The sheets had acquired a yellow colouration.

In other tests, further sheets cut from the same drawn glass ribbon were subjected to the same treatment in a bath of molten salts and after withdrawal from this bath they were heated at a temperature of 600°C for five minutes. The sheets were then abruptly cooled and washed. The sheets were yellow in colour and by virtue of the abrupt cooling of the sheets their faces were in a state of compression so that they had a high resistance to breakage by flexure.

A third series of sheets cut from the same drawn glass ribbon, after treatment in the same manner in a bath of molten salts, were heated at 650°C and while at this elevated temperature the sheets were bent.

The bent sheets were yellow in colour.

Example 2 Sheets of glass identical with those treated in Example 1 were immersed in a bath of molten salts comprising l% NaNO^ and 57% KNO^ and 2% AgNO^ . The bath temperature was 300°C and the sheets were immersed in the bath for a period of 30 minutes.

After withdrawal of the sheets from the molten bath they were heated at 550°C for 1 hour. The sheets were then cooled and washed. The sheets were yellow in colour, whereas prior to the heating at 550°C the sheets were almost colourless.

In another test the same process was repeated using sheets of glass cut from the same glass ribbon, with however the modification that a molten salt bath composed of 90% NaNO^ and 10% AgNO^ was used. In this case a strong yellow colouration was imparted to the glass sheets by the subsequent heating step.

Example 3 Sheets of glass identical with those treated in Example 1 were immersed in a bath of sulphates of the following composition by weight: CuSO^ 10. 5%, Na2S0/i 32.2%, ZnS0¾ ½7.6%, 9.7%. The temperature of the bath was 550°C and the period of immersion was 20 minutes.

As a result of this treatment the glass had acquired a very faint greenish tint.

The sheets were then subjected to a thermal treatment in an atmosphere of hydrogen at 650°C for 10 minutes. This further treatment caused a change in the colouration of the glass sheets. The sheets acquired a pink colour.

Example Three sample batches of glass sheets of different conpositxon were subjected to a treatment according to the invention. The sample batches, A, B and C, had the following composition: Sample A Sheets of this sample were of ordinary composition similar to that of the sheets treated in Example 1.

Sample B Si02 80% Al^ 2%, B∑0 13%, Na^O 3.5%, X^ 1%, together with a small quantity of iron oxide.

Sample C The glass sheets of this sam le were composed of heat-reflecting glass of green colour containing a higher proportion of iron oxide than the ordinary glass (Sample A) The sheets of each of the samples were immersed in a bath of aNO containing 0.15% by weight of A O^„ The bath was at a temperature of J^O°C and the immersion period was 2 hours.

After washing the sheets they were heated at 600°C on a forme on which they acquired a cylindrical curvature, Inspection of the sheets of the different samples showed that the glass of ordinary soda-lime composition (Sample A) and the borosilicate glass sheets (Sample B) , which were colourless prior to the heating ot 600°C, had acquired a yellow tint and had a light transmissivity of the order of 70/ό, while the heat-reflecting glass (Sample C) which was still green after the treatment in the bath of molten salts, had become deep brown in colour and was much more strongly light-absorning than sheets of ordinary glass. The light- transmissivity of the treated sheets of Sample C was of the order of 20%.

Example 5 A ribbon of flat glass having a thickness of 3 mm was formed by the "float" process.

The ribbon of flat glass contained reducing tin ions in the surface layers of glass at that face of the ribbon which had been formed in contact with the molten tin in the float tank.

In conventional manner the ribbon of float glass was conveyed from the float tank through an annealing gallery. During its passage through this annealing gallery the ribbon of glass was exposed to contact with a mixture of molten salts at OO°C which was projected in the form of an aerosol onto the lower face of the glass ribbon. The mixture of molten salts had the following composition: k2% NaNO^ , 58% KNO^, 0.2% AgNO^. The period of contact of each section of the glass ribbon with the aerosol mixture was several seconds .

After cooling the float glass ribbon the glass (which was substantially colourless) was cut into sheets which were then subjected to heating at 550°C for 15 minutes. As a result of this treatment the sheets acquired a pale yellow colouration at the faces which had been formed in contact with the molten tin in the float tank.

Example 6 A ribbon of float glass was formed, the glass having the following composition by weight: Si02 71% A1203 1% Na≥0 14% CaO 9% MgO % together with minor amounts of The sheets were immersed for 5 minutes in a mixture of molten salts maintained at a temperature of 300°C. The mixture of molten salts had the followigg composition by vreight: 58% KNO^, 2% NaNO^, together with AgNO^ in a proportion of 0.2% of the aggregate weight of the said potassium and sodium salts. The sheets were still substantially colourless. After cooling the sheets and washing them they were subjected to heating according to the following schedule: 500°C for 2 hours 550°C for 15 minutes 600°C for 20 seconds.

The treated sheets had a strong yellow colouration which was confined to the faces of the sheets which had been formed in contact with the molten tin in the float tank.

In other tests, sheets of glass of identical composition were subjected to the same treatments with however the modi ication that the bath of molten salts used in the first step contained only 0.01% pf AgNO^. When the sheets were subjected to the thermal treatment they developed a yellow colouration but in this case the yellow colouration was paler.

Example 7 Sheets of glass identical with those used in Example 1 were immersed for 1 hour in a mixture of molten salts at 300°C and composed of NaNO^ and 0.2% by weight of AgNO^. After cooling and washing the sheets, which were colourless, they were heated at 560°C for 2 hours. The sheets were slowly cooled.

As a result of the thermal treatment the sheets acquired a yellow colouration.

The sheets were then immersed in a bath of molten NO^ at 20°C for 8 hours. After withdrawal of the sheets from this molten potassium nitrate bath they were cooled and washed. The colouration of the sheets had not changed but the sheets had acquired a higher tensile strength as a result of the diffusion of potassium ions into surface layers of the glass. The strengthening was the same at the two faces of the glass and the resistance of the sheets to breakage by flexure, expressed in terms of the tensile strength of the tensioned face, -2k- was of the order of 100 kg/mm Example 8 Sheets of glass identical with those treated in Example 1, were immersed in a bath of molten salts having the following composition by weight: 58% NO^ and k2% NaNO^, to which AgNO^ has been added in a proportion of 0.5% of the aggregate weight of the potassium and sodium salts. The molten salt bath was at a temperature of 330°C and the immersion period was 1 hour.

After withdrawal of the sheets from the molten salt bath, the sheets, which were still substantially colourless, were plunged into a second bath of molten salts having the following composition by weight: 58% NaCl, 40% NaNO^ and 2% LiNO.^. This bath of molten salts was at a temperature of 580°C and the immersion period was 2 hours. The sheets were then cooled and washed.

During the immersion of the sheets in the second bath, sodium ions in surface layers of the glass were substituted by lithium ions deriving from the molten salt bath. As a result of this ion exchange, the surface layers of the glass sheets acquired a lower coefficient of thermal expansion than that of the interior layers of glass so that after cooling of the sheets the surface layers of the glass were in a state of compression. Consequently the sheets had a high resistqnce to breakage by flexure corresponding wi a tensile loading of the tensioned face of about 97 kg/mm2.

During the treatment in the second bath, the sheet acquired a yellow colouration.

Claims (10)

WHAT IS CLAIMED IS:

1. · A process of colouring or modifying the colouration of a glass body containing reducing ions distributed through said body f causing silver or copper ions to diffuse into surface layers of the glass body from a contacting medium* characterised in that the said medium is composed of a mixture of molten salts containing not more than 50 by weight Of a salt providing silver or copper ions capable of being reduced by said reducing ions to bring about colouration or colour modification of said body, and in that after diffusion of silver or oopper ions into said body from said medium, the final colouration is achieved by heating the body to a temperature above the strain pftint of the glass*

2. · A process according to claim 1, characterised in that said body is a body of soda-lime glass of ordinary composition.

3. · A process according to claim 2, characterised in that said body is a body composed of or formed from flat drawn glass.

4. A process aocording to any preceding claim, characterised in that the said medium contains less than 10 fey weight of salt(s) providing the said reducible metal ions and suoh ions are wholly or predominantly silver ions*

5. · A process according to claim ¾·, characterised in that said medium contains less than 3 by weight of saltfe) providing the said reducible metal ions, such ions being silver ions. -2?

6. · A process according to claim 5» characterised in that said medium contains less than 0*5$ by weight of salt(s) providing the said reducible metal ions, such ions being silver ions*

7. · A process aocording to any preceding claim, characterised in that said medium incorporates one or more zinc salts*

8. A process aocording to any preceding claim, characterised in that said medium contains a salt or salts providing said reducible ions, selected from the group: nitrates, chlorides, sulphates*

9. * A process according to any preceding claim, characterised in that the salt(e) providing said reducible metal ions is or are a silver salt or salts and the diffusion of said reducible metal ions into said body takes place while the body is at a temperature less than 350°C

10. A process according to any preceding claim, characterised in that said body is a body composed of or formed of flat drawn soda-lime glass and is heated to a temperature of at least 500°C after said diffusion of reducible metal ions therein* 11· A process according to any preceding olalm, characterised in that after the said heating of said body to achieve said final colouration, the body is rapidly cooled to produce compressive surface stresses therein· 12. A process according to any of claims 1 to 10, characterised in that during the heating of said body while said body is at a temperature above the strain point of the glass following the diffusion of reducible metal ions into said body, ions, e*g*, lithium ions, which confer a loxtrer coefficient of thermal expansion on surface layers of said body are caused to diffuse into such surface layers in exchange for other ions so that compressive surface stresses are set up when the body is subsequently cooled* 13· A process according to any preceding claim, characterised in that said body is su jeoted to bending during the period in whioh its temperature is above the strain point of the glass* l^. A process of colouring or modifying the colour of a glass body, substantially according to any of the Examples herein* 15. A body o glass which has been coloured or modified in colour by a process according to any preceding claim* 16* A vehiole windscreen comprising sheets of glass or at least one sheet of glass and at least one sheet of plastics, which glass sheet or at least one of which glass sheets if there is more than one, has been ooloured or subjected to colouration modification at least one of its faces by a process according to any of claims 1 to l^* 17* A vehicle windscreen according to olaim 16, comprising two said sheets of glass (at least one of them having thus been coloured or subjected to colouration modification) united to an intervening preformed sheet, e*g*, a sheet