GB2063852A - Coloured photochromic glasses - Google Patents

Abstract

A method for introducing a surface colouration into a transparent photochromic glass which comprises heat treating the glass at a temperature in excess of 200 DEG C. in a gaseous reducing environment to reduce the luminous transmittance of a surface layer of the glass to a value below that of the untreated glass at one or more wavelengths higher than the fundamental absorption peak of metallic silver in the glass, and optionally modifying the surface colouration by chemically removing a portion of the surface layer is disclosed.

Description

SPECIFICATION Coloured photochromic glasses This invention relates to coloured photochromic glasses.

The production of photochromic glasses or phototropic glasses, as such have been variously termed, was first disclosed in United States Patent No. 3,208,860. As is explained therein, photochromic glasses demonstrate the capability of darkening, i.e. changing colour, when subjected to actinic radiation, normally ultra-violet radiation, and then returning to the original transmission thereof when removed from the source of actinic radiation. By far the largest application for this type of glass has been in the ophthalmic field. Thus, for example, photochromic ophthalmic lenses may exhibit a transmittance of in excess of 90% when worn indoors, but, when the wearer steps outside into the sunlight, the lenses may quickly darken to transmittances well below 50%. Upon returning indoors, the lenses fade to the original transmittance.

Although other photochromic agents are known, the only commercially-marketed glasses have utilized crystals of silver halides and, particularly, crystals of AgCI and AgBr as the photochromic materials. Likewise, while other base glass composition have been investigated with some success, the only commerciallymarketed glasses have contained substantial amounts of silica. In general, the commercial glasses have had base compositions within the alkali metal aluminoborosilicate system. United States Patent No. 3,208,860, supra, explicitly refers to those compositions containing silver halide crystals as constituting the preferred embodiment.

The addition of transition metal oxides, such as CoO, NiO, Cur203, CuO, Fe2O3, V205, and MnO, and rare earth metal oxides, such as Pr203 and Er2O3, to glass compositions to impart colour thereto is well-known in the art. Ophthaimic lenses, including photochromic lenses, tinted in that manner are presently available commercially. Nevertheless, as is apparent, that means of tinting involves the careful addition of such colourants in precisely-controlled amounts in order to ensure uniform colouring from piece-to-piece and melt-to-melt. Moreover, this procedure may demand precise regulation of redox conditions during melting and forming of the glass.Accordingly, a process whereby a desired colour could be imparted to a glass without the need for specific colourant additions would obviously be highly attractive from a practical point of view. It will be appreciated that, where ophthalmic lenses are the products involved, the tinting process should not deleteriously affect the photochromic behaviour of the glass. Furthermore, when the lenses are required to be chemically strengthened or thermally tempered, the colour must be of such permanence that it will be retained within the glass after exposure to those strengthening practices.

United States Patent Nos. 3,892,582 and 3,920,463 relate to developing a tint in photochromic glasses wherein silver halide crystals constitute the photochromic agents. The former describes subjecting photochromic glass articles having compositions encompassed within United States Patent No. 3,208,860, supra, to a reducing atmosphere, customarily containing hydrogen, for periods ranging from 15 minutes at 300"C. to 4 or 5 minutes at 6000C. Strict observance of those heat treating parameters is required because of the colouring mechanism involved.

Thus, this reference notes that in almost all photochromic glasses there is inherently an excess of the active photochromic element present. Hence, in the cited compositions, there would be an excess of silver halide crystals. The reduction process is designed to act upon the photochromic agent and, preferably, only upon the excess thereof such that there will be no substantial degradation of photochromic behaviour.

Where a slight decrease in photochromic characteristics may be tolerated, however, reduction of some of the active photochromic element may be permitted. Nevertheless, the reference explicitly warns against the use of temperatures that are too high for periods that are too long, since the photochromic agent will be reduced to such an extent that the desired photochromic behaviour in the glass will be lost. Furthermore, there is a specific caveat against utilizing reducing conditions that are so severe that oxides in the base glass compositions are reduced.

United States Patent No. 3,920,463 is directed to the same basic reduction process, but discloses subsequently exposing the thus-reduced glasses to ultra-violet radiation. This exposure is said to produce darker and deeper tints than may be achieved by the reduction treatment alone.

United States Patent No.4,118,214 discloses an improvement in the production of polychromatic glasses.

The conventional process for producing such glasses comprises two sequences of exposure to high energy or actinic radiation, followed by a heat treatment in air. This reference discloses replacing the second exposure and heat treatment in air with a heat treatment conducted in a reducing environment at a temperature of at least 350"C., but below the strain point of the glass.

The desired colouration in polychromatic glass results from the presence of colour centres therein, the colour centres consisting of microcrystals of an alkali metal fluoride (customarily NaF) containing a silver halide selected from AgCI, AgBr and Agl having metallic silver deposited within or upon the surface of the microcrystals. The glasses consisted essentially of Na2O, Ag, F, a halide selected from Cl, Br and I and optionally Al203 and/or ZnO.

An atmosphere of hydrogen was said to be the most effective reducing environment, although other agents were said to be operable. The use of temperatures at or above the strain point of the glass was to be avoided since such temperatures destroyed the colour centres.

United States Patent No. 4,125,405 describes red silver halide-containing glasses, produced under reducing melting conditions, which are slightly photochrornic. These glasses, however;are insufficiently photochromic for commercial use in photochromic products. Attempts to enhance the photochromic performance thereof by heat treatment typically destroy the red colour of the glass.

Second-generation silver halide-containing photochromic glasses exhibiting improved darkening and fading characteristics have also been recently introduced. One family of such glasses has been described in United States Patent No. 4,190,451.

In another recent development, effort has been directed toward the development of lenses which photochromically darken in the top portion only, thus providing a gradient darkening behaviour. Examples of the production of such lenses may be found in United States Patent Nos. 4,036,624; 4,062,490 and 4,160,655.

As is evident from the above discussion, colouration in photochromic and polychromatic glasses was achieved by reducing silver ions to metallic silver. Such colouration is essentially a relatively low temperature process, i.e. temperatures below the strain point will be employed or very brief exposures at higher temperatures may be operable.

More specifically, the yellow observed upon the heat treatment of photochromic glasses under reducing conditions in the manner disclosed in United States Patent Nos. 3,892,582 and 3,920,463 is attributed to an absorption band caused by the precipitation of metallic silver in the glass during heat treatment. In silver-containing glasses free of other precipitated phases, the silver absorption band is manifested as an absorption peak centered at about 390 nm in the violet region of the spectrum. In the reduction-fired photochromic glasses reported in US Patent Nos. 3,892,582 and 3,920,463, which contained precipitated silver halide in addition to the matrix glass, absorption peaks are reported in the blue region of the spectrum at from 430 to 460 nm.

The hue and intensity of the induced colour in these prior art glasses probably dependent upon the position and intensity of the treatment-induced absorption peak. The deepest yellow colours were caused by strong absorption peaks at from 430 to 460 nm, while the light pink colour is now thought to have been caused by the same fundamental absorption peak as it first appeared in weak form at about 500 nm following a mild heat treatment.

While reduction firing treatments such as used in the prior art constituted a simple and convenient means for imparting surface colouration to photochromic glasses, they offered only a very limited range of colour selection. Attempts to intensity the colours provided, for example by using more severe heat treatments, appeared merely to shift the fundamental absorption peak towards the violet, resulting in more intensely yellow glass.

The United States Food and Durg Administration has adopted strength standards for ophthalmic ware such that glass products for that application must either be thermally tempered or chemically strengthened.

Thermal tempering contemplates heating a glass article to a temperature at or approaching the softening point of the glass and then quickly chilling it. Chemical strengthening comprehends contacting a glass article (commonly containing alkali metal ions) with a source of larger ions (usually an alkali metal ion of a larger ionic radius) at an elevated temperature below the strain point of the glass to cause the larger ions to migrate into the glass surface and replace the smaller ions present therein. This ion exchange reaction is conventionally undertaken for several hours, frequently overnight.

Both of these processes have the adverse side effect of altering the colour or hue developed in the glass by the reduction of silver ions. This situation may be remedied, of course, by conducting the strengthening operation prior to subjecting the glass to the reduction heat treatment. However, that remedy runs counter to the present stream of production since the glass strengthening process is now undertaken immediately before the lenses are inserted into the customer's frames. Moreover, the heating required during the reduction treatment would modify the strength developed via the thermal tempering or chemical strengthening technique.

The production of glasses having various tints has been sought by wearers of eyeglasses. For example, sportsmen have purchased the so-called "shooter's glass", which has a yellow tint, for use as both prescription and non-prescription lenses. The utility of such a product to a reduce haze and glare observed by the wearer, lies in designing and maintaining the tint within a narrow range of transmittances. As ideal product would combine a tine having a closely-controlled transmittance in a glass demonstrating photochromic behaviour, this tint demonstrating excellent thermal stabilty, i.e. it will be essentially unaffected by long term heat treatments at temperatures in the vicinity of the strain point of the glass or by short term exposures to heat treatments approaching the softening point of the glass.

Another desirable product would be a photochromic ophthalmic lens exhibiting a fixed tint colour gradient in combination with uniform photochromic darkening behaviour. Such a lens would darken uniformly in bulk upon exposure to actinic radiation and fade in the absence thereof, and would additionally have an invariant fixed tint gradient or other selectively applied colour pattern superimposed on the lens which would not change in response to changes in lighting conditions. In this way a lens exhibiting a photochromic darkening in all lens portions and also a gradient which would always be apparent to the observer, could be provided. Important requirements of such a lens would be relatively uniform photochromic darkening capability, a fixed-tint gradient or other selective colouring pattern and freedom from the refractive power aberrations caused by the lens fabrication procedure. Of course, processes risking the introduction of variations in surface smoothness or curvature which could affect the optical properties of the lens would be unacceptable for both sunglass and prescription ophthalmic lens products.

A principal object of the present invention is to provide a means for producing in a single photochromic glass composition several shades of a given tint, the differences in colouration being due to a major variation in luminous transmittance with a minor shift in chromaticity and the hues of which demonstrate excellent thermal stability.

Another principal object of the present invention is to provide a photochromic glass exhibiting a surface tint or hue different from the predominantly yellow tint of the prior art photochromic glass.

A specific object of the present invention is to provide a means for producing in a single photochromatic glass composition several shades of a yellow tint and for adjusting the luminous transmittances of the glass to a desired level, the shades demonstrating excellent thermal stability.

Another specific object of the present invention is to produce a photochromic glass body having a relatively fixed colour coordinates, but a controlled transmittance gradient across the face thereof, the colour exhibiting excellentthemal stability.

Yet another specific object of the present invention is to produce a semi-finished lens of photochromic glass exhibiting a colour in one surface of the lens which demonstrates excellent thermal stability.

Still another object of the present invention is to produce a photochromic glass body which may be highly absorbing of radiation having wavelengths below about 450 nm and exhibits a colour demonstrating excellent thermal stability.

Another object of the present invention is to produce a photochromic glass body containing a conventional glass colourant in the composition thereof, the hue of which may be modified to yield a tint which demonstrates excellent thermal stability.

Another object of the present invention is to provide a selectively tinted photochromic lens comprising a lens portion supporting a surface tint and another portion free of such tint or having a different tint.

The present invention provides a method for introducing a surface colouration into a transparent photochromic glass which comprises heat treating the glass at a temperature in excess of 200"C. in a gaseous reducing environment to reduce the luminous transmittance of a surface layer of the glass to a value below that of the untreated glass at one or more wavelengths higher than the fundamental absorption peak of metallic silver in the glass, and optionally modifying the surface colouration by chemically removing a portion of the surface layer.

Referring to the accompanying drawings: Figure 1 presents a plot of chromaticity coordinates determined on several of the Examples reported below.

Figure2 delineates several spectral transmittance curves measured spectrophotometrically on several of the Examples recorded below.

Figure 3 depicts a plot of chromaticity coordinates determined on several of the Examples recited below.

Figure 4 sets forth a plot of chromaticity coordinates calculated on several Examples discussed below.

Figure 5 includes several spectral transmittance curves measured spectrophotometrically on several of the Examples reported below.

Figure 6comprises a plot of the spectral transmittance coefficient for two of the Examples discussed below.

Figure 7 plots undarkened transmittance v light wavelength for a number of surface-coloured photochromic glass articles provided according to the present invention.

Figure 8 plots undarkened transmittance v light wavelength for a surface-coloured photochromic glass article subjected to a number of prior art reduction firing treatments.

Figure 9 plots undarkened transmittance v light wavelength for a photochromic glass article in both untreated and various treated states.

The present invention provides a method for inducing surface colouration in transparent photochromic glasses in addition to or in place of the relatively pure yellow colouration resulting from the use of prior art reduction heat treatments. As mentioned above, in photochromic glasses such treatments were typically effective to develop only the fundamental absorption peak of metallic silver in the glass, that peak varying in intensity and wavelength, but typically falling within the from 430 to 460 nm range.

In accordance with the present invention, utilizing a heat treatment of sufficient duration at temperature in excess of 200"C in a gaseous reducing environment, the luminous transmittance of a surface layer on the article is reduced to a value below that of the untreated glass at one or more wavelengths higher than the fundamental absorption peak of metallic silver in the glass. Such reduction in luminous transmittance may result from a relatively broad reduction in transmittance over the higher wavelength visible range or from the development of new relatively narrow absorption bands in that range, depending upon the reduction conditions and glass compositions employed.

In accordance with a first embodiment of the present invention, according to which a rather broad reduction in luminous transmittance at higher wavelengths is provided, it has been discovered that this may be achieved by subjecting photochromic glasses, wherein silver halide crystals comprise the photochromic agent and which contain lead oxide as a necessary component, to a strongly reducing gaseous environment at temperatures above the strain point of the glass and, preferably, at temperatures approximating or even somewhat exceeding the annealing point ofthe glass. At those elevated temperatures, not only are silver ions in the glass reduced to metallic silver, but lead ions therein are also reduced to metallic lead which is postulated to either coat or alloy with the reduced silver particles.

This embodiment of the present invention appears to be applicable essentially irrespective of the base composition of the photochromic glass so long as AgCI, AgBr and/orAgl crystals comprise the photochromic elements and there is an amount of lead oxide present therein which, when reduced in accordance with the present invention, will provide the desired colour. For example, United States Patent No. 3,548,060 describes glasses having base compositions within the Al203-B203-RO system, i.e. the glasses consist essentially, by weight, of from 30 to 86% B2O3, from 2 to 35% Al203 and from 12 to 45% of an alkaline earth metal oxide.United States Patent No. 3,703,388 discusses glasses having base compositions within the La203-B203 field, i.e. the glasses consist essentially, by weight, of from 15 to 75% La2O3 and 13 to 65% B203.

United States Patent No. 3,834,912 discloses glasses having base compositions within the PbO-B203 system, i.e. the glasses consist essentially, by weight, of from 14.2 to 48% B2O3, from 29 to 73% PbO, from 0 to 15% alkaline earth metal oxides, and from 0 to 23% ZrO2, Al203 and/or ZnO. United States Patent No. 3,876,436 is directed to glasses having base compositions within the R2O-AI203-P205 field, i.e. the glasses consist essentially, by weight, of at least 17% P2O5, from 9 to 34% Al2O3, not more than 40% SiO2, not more than 19% B203 and at least 10% alkali metal oxides.United States Patent No.3,957,498 is drawn to glasses having base compositions within the R2O-AI203-SiO2 system, i.e. the glasses consist essentially, by weight, of from 13 to 21% alkali metal oxides, from 17 to 25% A1203 and from 45 to 56% SiO2. Furthermore, as noted above in the discussion of United States Patent No. 3,208,860, the presently commercially-marketed, photochromic glasses have base compositions within the alkali metal aluminoborosilicate system.

That reference cites, as preferred compositions, glasses consisting essentially, by weight, of from 4 to 26% Al2O3, from 4 to 26% B2O3, from 40 to 76% SiO2 and at least one alkali metal oxide selected from 2 to 8% Li2O, from 4 to 15% Na2O, from 6 to 20% K2O, from 8 to 25% Rb2O and from 10 to 30% Cs2O. Such glasses contain, by weight on the basis of chemical analysis, at least one halogen in the minimum effective proportion of 0.2% chlorine,0.1 bromine and 0.08% iodine, and a minimum of silver in the indicated proportion of 0.2% where the effective halogen is chlorine, 0.05% where the effective halogen is bromine, but the glass contains less than 0.08% iodine and 0.03% where the glass contains at least 0.08% iodine.Where a transparent article is desired, the total silver will not exceed 0.7% and the total of the three recited halogens will not exceed 0.6%. The sum of the recited base glass constituents, silver and halogens will compose at least 85% of the composition.

From laboratory investigations, it appears that the minimum amount of lead, expressed in terms of PbO, required to provide the desired effect upon transmittance when the glass is treated in accordance with the first embodiment of the present invention is at least 0.5% PbO and, most preferably, in excess of 1% PbO. A range of colours varying over the spectrum from bright yellow to hues approaching orange may be developed and close control of the colour produced and the luminous transmittance of the glass is possible, thereby permitting excellent reproducibility of colour shading. The actual colour and luminous transmittance generated are somewhat dependent upon glass composition, notably PbO content, but are principally a function of the parameters of the reduction treatment.

Contrary to the statements in United States Patent Nos. 3,892,582 and 3,920,463, the photochromic properties of the glasses are not seriously degraded so long as the reduction treatment temperature does not significantly exceed the annealing point of the glass for a substantial period of time. As judged empirically, temperatures much in excess of 50"C above the annealing point of the glass for periods in excess of about one hour do appear to adversely affect the photochromic behaviour.

An atmosphere of pure hydrogen has been found to be the most effective environment with regard to the speed of reaction. Hence, the reaction appears to be founded in a diffusion effect such that the colouration commences at the surface and moves inwardly. Because of this phenomenon, the longer the period and/or the higher the temperature employed, the deeper the colouration will penetrate into the glass. Moreover, the strong temperature dependence of this phenomenon permits a colour gradient to be set up across the surface of the glass. Nevertheless, while pure hydrogen is preferred from the point of reaction speed, other considerations may suggest the substitution of other reducing environments, such as carbon monoxide, cracked ammonia and forming gas therefor.In general, then, the glass will be exposed to the gaseous reducing atmosphere at the prescribed temperatures to reduce lead ions therein to metallic lead and allow formation of metal particle colour centres, this time being dependent upon the composition of the glass, the environment utilized and the temperature employed. For example, where pure hydrogen is used, an exposure as brief as a few minutes may be adequate at the higher temperatures, while several hours may be required at temperatures near the strain point of the glass. Furthermore, of course, the depth of colour penetration desired in the glass is a factor that must be considered, this depth being governed by the law of diffusion.

The present invention permits the fabrication of semi-finished articles. For example, an ophthalmic lens blank may be finished (ground to the proper prescription and polished) on one face and the entire blank or only that face thereof exposed to the reducing environment to develop an integral surface layer thereon contain metallic lead particles to provide the desired colouration therein. Subsequently, the final prescription will be ground into the previously-unfinished face of the blank. Whatever colouration was produced in the latter face would be removed during the finishing thereof, but the final lens would retain the colouration from the initially-finished face.

Because it is contemplated that the present invention will have utility in producing photochromic ophthalmic lenses, i.e. ophthalmic lenses demonstrating reversible luminous transmittances, the preferred practice of this embodiment of the present invention involves producing the highly-absorbing surface layer on the rear face only of the lenses in order to avoid absorption of actinic radiation when the wearer steps out into the sunshine.

Although the present invention is operable in glass systems where the PbO content is very high (e.g. from 29 to 73% PbO in United States Patent No. 3,834,912, supra), the colourations derived from high PbO contents are not advantageously different from those of much lower PbO contents. It is required, however, to have at least 0.5%, preferably greater than 1%, PbO present and silver halide crystals will constitute the photochromic agents. With the above-delineated preferred compositions of United States Patent No.

3,208,860, the maximum PbO content will customarily not exceed 10%.

It will be appreciated, as mentioned above, that the present invention lends itself to producing shapes where only a portion of the area thereof is tinted or a gradient of colour is developed across the area of an article. United States Patent No. 4,072,490 illustrates an apparatus and process which may be readily modified according to the present invention.

Furthermore, the present invention permits the production of coloured photochromic glass bodies which are highly absorbing of ultra-violet radiations. These glasses are of particular utility when medically prescribed as ophthalmic lenses for a malady, such as retinitis pigmentosa, where protection from a strong illumination and particularly ultra-violet radiation is essential. A two-step treatment is required to achieve that purpose. First, the photochromic glass is fired in a reducing atmosphere in accordance with the practice described in United States Patent No. 3,892,582. That is, the glass is fired at a temperature and for a period sufficient to reduce Ag+ ions to silver metal. The preferred method for carrying out that process contemplates the use of firing temperatures below the strain point of the glass.Thereafter, the thus-treated glass is fired in a reducing environment at temperatures in the vicinity of the annealing point of the glass and higher to reduce lead ions to metallic lead in a shallow surface layer. This sequential firing of the glass at different temperatures gives rise to a layer of metallic lead particles being formed over a layer of metallic silver particles. The silver particles cause a sharp cutoff in transmittance to occur at a wavelength of about 450 nm resulting in strong absorption in the blue portion of the visible spectrum and down into the ultra-violet.

This circumstance renders the glass especially suitable as a filter for ultra-violet radition. However, even total absorption by silver particles cannot yield a low transmittance glass in the visible portion of the spectrum since the human eye is most sensitive to wavelengths at about 555 nm (yellow-green). In contrast, the lead particles absorb generally across the visible spectrum. Consequently, the luminous transmittance displayed by the glass is determined to a large extent by the thickness of the lead particle layer. In summary, this embodiment of the present invention contemplates the production of photochromic glass articles, most commonly ophthalmic lenses, having an integral surface layer on the back glass thereof containing metallic silver and metallic lead particles.The method permits the production of such articles which absorb strongly in the ultra-violet portion of the spectrum and which demonstrate various colour shades without resorting to hot glass forming processes.

In a second embodiment of the present invention, in accordance with which relatively narrow absorption bands are developed at wavelengths above the fundamental silver absorption wavelength in the glass, it has been found that coloured photochromic glass articles exhibiting, for example, orange, red, purple or blue surface colouration in the undarkened state may be provided by the reduction heat treatment of silver-halide containing photochromic glasses under appropriate thermal conditions. These results are obtained using a somewhat lower range of heat treatment temperature than was utilized for the purpose in the prior art, in order to minimize photochromic phase (silver halide) melting during the reduction treatment. Additional factors affecting results are the composition and thermal history of the photochromic glass starting material used in the colouring step.

The wide range of surface colouration observed in photochromic glasses produced according to this second embodiment of the present invention is attributed to the development during heat treatment of strong absorption bands in the glass surface, centered at wavelengths above 460 nm and frequently in the range of from 510 to 580 nm, which shift the hue of the glass out of the yellow and into the orange, red violet or blue regions of the spectrum. This represents a significant departure from the surface-coloured photochromic glasses disclosed in the prior art, which exhibited strong absorption only at from 430 to 460 nm and relatively weak absorptions at the wavelengths required to produce non-yellow glasses.

Viewed in terms of spectral light transmittance characteristics, the surface-coloured photochromic glass articles of the prior art, at least in the ophthalmic lens form commonly employed, tended to exhibit induced absorption peaks located to the left of the line CB in Figures 7 to 9 of the accompanying drawings, as most clearly shown in accompanying Figure 8. The present invention provides surface-coloured photochromic glass articles having induced peaks positioned to the right of the line CB, as illustrated in accompanying Figures 7 to 9.

Specifically, then, this second embodiment of the present invention provides a surface-coloured photochromic glass article produced by a process which comprises heat treating silver halide-containing photochromic glass article under reducing conditions at a temperature not exceeding 450"C, that treatment being continued to develop specific light absorption characteristics in the glass article. Those characteristics are such that following the reduction heat treatment, the glass article exhibits, in at least one cross-sectional dimension in the undarkened state, a spectral transmittance curve comprising at least one treatment induced absorption peak at a wavelenght above the fundamental silver absorption wavelength, typically found at from 430 to 460 nm.Thus, the treatment-induced peak typically has a peak location and intensity such that it falls within the spectral transmittance region to the right of the line CB in Figure 1 of the accompanying drawings, as more fully described below.

Such an article is produced by a process which comprises the step of heat-treating the article under reducing conditions at temperatures not exceeding 450"C to develop at least one treatment-induced absorption peak having the characteristics described above.

Absorption characteristics such as noted in the present glasses were first induced in second generation photochromic glasses of the type described in United States Patent No.4,190,451. However, the novel absorption effects are not limited to such glasses, having been observed in other types of photochromic glasses exposed to reduction heat in the manner described below.

It is believed that the unusual colouration effects observed in photochromic glasses subjected to treatments according to this second embodiment of the present invention are caused by the chemical reduction of silver in contact with silver halide microcrystals in a region very near the surface of the glass, with the observed colour being determined by the geometric form and arrangement of metallic silver on these microcrystals. This would be consistent with the experimentally observed fact that, using a given reduction heat treatment, a particular photochromic glass may exhibit a number of different absorption peaks depending upon the process originally used to develop the microcrystalline photochromic silver halide phase in that glass.

Furthermore, it has been discovered that the surface colouration which is normally imparted to photochromic glasses by reducing atmosphere heat treatment, herein referred to as reduction heat treatments, is sufficiently close to the surface of the glass that selective removal of portions of the coloured glass surface may be accomplished without unacceptably altering the optical properties of that surface.

Thus, it has been found that the colours resulting from such treatments ordinarily reside in a very thin layer, typically of from 10 to 100 microns thickness, on the surface of a treated glass lens, and that the removal of this surface layer by chemical means does not noticeably effect the refractive power of the lens surface or the surface quality.

Accordingly, the present invention further relates to a method for treating a surface-tinted photochromic glass article produced by heat treatment in a reducing gaseous environment as described above to provide a selectively tinted glass article, which method comprises chemically removing selected regions of the coloured surface layer to modify the apparent colour of that layer and thus the glass lens. The portion of the coloured surface layer chemically removed from the lens may be selected to provide a colour design or, preferably, a colour gradient which could range from relatively intense colour at the top of the lens to no colour at the bottom. The portion removed may also be controlled as to thickness, so that colour modification may constitute either a limited reduction in colour intensity or complete removal of the colour.

Furthermore, it will be appreciated that, where desired, a known tinting agent for glass may be included in the composition. The combination of such a tint with the colouration produced via the inventive reducing treatment may, of course, yield a wide variety of colour hues and shades.

The first embodiment of the present invention will now be described utilizing three photochromic glasses which are commercially marketed by Corning Glass Works, Corning, New York, U.S.A., for ophthalmic lenses. Approximate compositions of each are recited below in terms of weight percent. Glass A is marketed under the designation Corning 8097, Glass B is distributed under the designation Corning 8111 and Glass C is sold under the designation Corning 8105. Approximate valuers for the softening point (Soft.), annealing point (Ann.) and strain point (Str.) in C" are also reported for each glass.

A B C SiO2 55.6 56.46 55.52 B203 16.4 18.15 16.10 At203 8.9 6.19 8.90 Li2O 2.65 1.81 2.65 Na2O 1.85 4.08 1.83 K2O 0.01 5.72 BaO 6.7 - 6.70 CaO 0.2 PbO 5.0 - 5.04 ZrO2 2.2 4.99 2.07 Ag 0.16 0.207 0.175 CuO 0.035 0.006 0.0128 Cl 0.24 0.166 0.325 Br 0.145 0.137 0.50 F 0.19 - 0.2 TiO2 - 2.07 Soft. 675 662 675 Ann. 511 500 510 Str. 473 468 475 Circular lens blanks having a diameter of about 70 mm and a thickness of about 6 mm were pressed from Glass C, cut into quarters and polished to a thickness of 3 mm.Two samples of each were placed into an electrically-heated tube furnace which was connected to a source of pure hydrogen. The furnace was purged with a current or nitrogen gas, filled with pure hydrogen gas and the samples were then exposed at temperature of 415"C., 435"C., 460"C. and 525"C. for 30 minutes to pure hydrogen flowing at a rate of about 10 cc/second using a pressure slightly in excess of atmospheric pressure. After removal from the furnace, the samples were polished on one side to provide a thickness of 2 mm and one set of specimens was chemically strengthened by immersion for 16 hours in a bath of molten 60% KNO3.40% NaNO3 (by weight) operating at 400"C.

Undarkened colour and photochromic properties were determined utilizing a conventional tristimulus colorimeter and laboratory exposure/photometer system. Each sample was exposed to the source of ultra-violet radiation for 20 minutes at room temperature, i.e. from 20 to 25"C., and then removed from the radiation for five minutes. Table I below records the luminous transmittances exhibited by each sample before darkening (To)/ after darkening for 20 minutes (TD20) and after fading for five minutes (TF5). Table I also listthechromaticitycoordinates (x, y) of the undarkened specimens.

Figure 1 of the accompanying drawings plots the chromaticity coordinates of the undarkened glasses on a colour mixture diagram and accompanying Figure 2 records the spectral transmittances of the chemically strengthened (chem. stren.) samples after being thermally faded, i.e. after heating for 35 minutes at 97"C.

TABLE 1 Hydrogen Chem.

Treatment Stren. To TD20 TF5 X Y Example 1 41 5"C - 75 20 36 0.3768 0.3803 Example 2 435"C - 75 20 36 0.3830 0.3886 Example 3 460"C - 75 20 35 0.3862 0.3993 Example 4 525"C - 59 33 40 0.3534 0.3694 Example 1 415 C + 75 20 36 0.3734 0.3737 Example 2 435"C + 73 20 36 0.3886 0.3923 Example 3 460"C + 73 20 36 0.3949 0.4040 Example4 525"C + 61 32 41 0.3651 0.3845 A comparison of Example 4 with the other Examples in Table I and accompanying Figures 1 and 2 immediately demonstrates substantial differences existing therebetween. The spectral transmittance curves set forth in accompanying Figure 2 are especially instructive. The treatment at 525"C (above the annealing point of the glass) in a hydrogen atmosphere was high enough to cause reduction of lead in addition to reducing the silver ions. This reaction caused the glass to become absorbing throughout the visible portion of the radiation spectrum and led to the loss of the sharp absorption band which peaks beyond 450 nm and is definitive of the silver metal particles. The reduction of the lead ions also produces a colour-shift towards green.

A comparison of the colour and photochromic properties exhibited by the chemicallyatrengthened specimens and those not subjected to that treatment clearly indicates that the present process is not adversely affected thereby.

Circular lens blanks similar to those described above with respect to Glass C were pressed from Glass A, cut into quarters and ground and polished to a cross-section of about 3 mm. The samples were thereafter fired in an atmosphere of pure flowing hydrogen for 0.5 hour at temperatures of 400"C, 455"C, 480"C and 530"C, utilizing the apparatus and technique outlined above.

Colour and photochromic properties were again determined on specimens polished from one side to a thickness of about 2 mm using the above-described method and apparatus. Each sample was exposed to ultra-violet radiation for 20 minutes at room temperature and subsequently removed from the exposure for 5 minutes. Table II below records the luminous transmittances displayed by each specimen before darkening under ultra-violet radiation (To) and the chromaticity coordinates (x, y) of the undarkened specimens.

Figure 3 of the accompanying drawings graphically illustrates the spectral transmittances of the undarkened samples. Accompanying Figure 3 also records the spectral transmittance curve of a speciment which had received no reducing environment treatment whatever.

TABLE II Example Hydrogen No. Treatment To x y 5 400"C 65.9 0.4080 0.3954 6 455"C 60.9 0.4467 0.4427 7 480"C 56.5 0.4153 0.4190 8 530"C 27.8 0.4049 0.4063 Table II makes apparent the relatively small colour change, but large shift in luminous transmittance, which takes place between 480"C and 530"C. Figure 3 notes the strong absorption peaks of from 450 to 500 nm representative of the presence of silver particles in the curves for the 400"C and 455"C treatments.That phenomenon loses its identity at higher treatment temperatures and is replaced by a structureless absorption throughout the visible wavelengths which decreases as the wavelength increases. Thus, a longer hold at 480"C (slightly above the strain point of the glass) would remove the absorption relic observed in the curve.

Lens pressings of Glass A and Glass B were quartered and polished to a thickness of 2 mm. The specimens were fired for 5, 10, 20 and 40 minutes in an atmosphere of pure flowing hydrogen at a temperature of 520"C, i.e. about 50C" above the strain point of the glass employing the equipment and process described above.

The luminous transmittances exhibited by each specimen before darkening under ultra-violet radiation (T,), the chromaticity coordinates (x, y) and colour purity (%) were determined utilizing the apparatus and technique described above. Those data are reported in Table Ill below.

TABLE Ill Example Treatment Colour No. Glass Time To x y purity 9 A 5 77.7 0.3832 0.4130 46 10 A 10 72.2 0.3980 0.4236 50 11 A 20 63.8 0.4084 0.4296 52 12 A 40 55.7 0.4196 0.4316 54 13 B 5 81.2 0.3802 0.4211 47 14 B 10 78.3 0.4033 0.4525 62 15 B 20 76.2 0.4197 0.4700 70 16 B 40 72.7 0.4343 0.4813 73 Figure 4 of the accompanying drawings plots the visible transmittance spectra of Examples 9 to 12 and Figure 5 of the accompanying drawings records the transmittance spectra of Examples 13 to 16.

A comparison of Examples 9 to 12 (a lead-containing glass) with Examples 13 to 16 (a non-lead glass) indicates three significant differences: (a) the non-lead glass displays no extinction of the silver band in the transmittance spectrum after relatively long hyrogen firing; (b) the non-lead glass loses very little transmittance at long wavelengths even after relatively long hydrogen firing; and (c) the non-lead glass demonstrates rather large changes in chromaticity accompanied by relatively small changes in luminous transmittance.

Those distinctions dramatically illustrate the substantive effect which the reduction of lead ions to metallic lead particles has upon the colour and transmittance of the glass. The above data and the curves of accompanying Figure 5 underscore the capability of the present method to closely control the shades developed in glasses containing both silver and lead.

Lens blanks of Glass C were ground and polished on the back surface to the desired curvature, thereby preparing semi-finished lenses. Pairs of lenses were prepared utilizing the following firing schedules in an atmosphere of pure hydrogen: (a) 22 hours at 420"C plus one hour at 560"C (Example 17); and (b) 22 hours at 460"C plus 0.5 hour at 560"C (Example 18).

The front surface was thereafter ground and polished to yield a lens having a cross-section of about 3 mm. In the resulting lens, all tinting is provided by a thin layer in the back surface. Accordingly, the photochromic performance of the glass is not affected.

Figure 6 of the accompanying drawings comprises a plot of the spectral absorption coefficients for Examples 17 and 18. Example 17 cuts slightly more blue radiation than does Example 18. This difference is believed to be because the long term exposure at 460"C, even though below the strain point of the glass, was sufficient to effect some reduction of lead ions to metallic lead which, in turn, modified the silver absorption.

Stated in another way, the double firing practice caused strong absorption by silver in a deep layer to thereby remove transmittance at wavelength below 450 nm. The higher temperature employed in the second step reduced the overall luminous transmittance of the glass.

Regarding the second embodiment of the present invention directed to coloured photochromic glasses exhibiting narrow absorption peak at longer wavelengths, a treatment-induced absorption peak is defined as one induced by the surface reduction of a photochromic glass as described herein. Such a peak is one which is not present in the parent photochromic glass from which the surface-coloured product is made. A surface-coloured photochromic glass article is therefore one wherein the surface colour differs from the bulk colour of the article, if any, a condition which may readily be determined by comparing the spectral transmittance characteristics of the article before and after the removal of a small amount of glass therefrom.

The peak location or position of an absorption peak is defined by its wavelength and intensity, with the wavelength being taken to be that wavelength at which a minimum in light transmittance through the undarkened glass article is observed to result from the peak. In some cases, however, the absorption bands giving rise to such peaks may be sufficiently close in wavelength that one absorption peak is manifested only as a shoulder on another absorption peak. The position of the former peak in that circumstance may be resolved in accordance with conventional spectral analysis techniques.

For the purpose of exemplifying the absorption characteristics of some prior art glasses, Figure 8 of the accompanying drawings sets forth spectral transmittance curves for a series of undarkened surface-coloured photochromic glass ophthalmic lens blanks provided in accordance with prior practice. These lens blank were composed of Corning Code 8097 photochromic glass, commercially available under the trade mark "PHOTOGRAY" photochromic glass from Corning Glass Works, Corning, New York, U.S.A., and were produced by the heat treatment of uncoloured photochromic glass lens blanks in an atmosphere of 100% H2 for 10 minutes at various heating temperatures reported in the drawings.

As is evident from a study of accompanying Figure 8, the surface coloured glasses produced by such treatments have absorption characteristics which typically range from very slight absorption at about 510 nm upon mild heat treatment to increasingly intensde absorption at lower wavelengths upon more severe heat treatment. The slightly absorbing glass produced by treatment at 300"C appears pink in colour, while the more intensely absorbing glasses exhibit yellow colours.

Insofar as described in the prior art, products of these types never exhibited significant induced absorption at wavelengths above 510 nm and never exhibited strong induced absorption peaks in the form 460 to 510 nm range. Hence, the absorption peaks exhibited by these glasses, the positions of which are marked by points at the transmittance minima associated with the peaks, appeared to be limited to that region to the left of the line marked CB in accompanying Figure 8, hereinafter sometimes referred to as a colour barrier, and the colour of the glass products was limited accordingly.

In direct contrast to this behaviour, glass articles provided in accordance with this embodiment of the present invention may exhibit strong induced absorption bands to the right of the position of the colour barrier and into the green and yellow regions of the spectrum. Figure 7 of the accompanying drawings contains undarkened spectral transmittance curves for a number of such surface-coloured glass articles and illustrates the variety of different induced absorption characteristics exhibited thereby. The colour barrier CB in accompanying Figure 7, as in accompanying Figures 8 and 9, is a straight line connecting the 0% transmittance point at 460 nm and the 100% transmittance point at 510 nm on the linear scale transmittance-wave-length diagram. The absorption characteristics exhibited by the present glasses may range from a strong absorption peak to the right of the line CB in the blue region of the spectrum, providing an orange colouration in transmitted light (Curves 1 and 5 of accompanying Figure 7), to a broad absorption peak centered in the yellow region of the spectrum which imparts a blue colour to the glass (Curve 4 of accompanying Figure 7). The development of absorption peaks between these two extremes in otherwise colourless photochromic glasses may yield surface-coloured glasses exhibiting red and purple colouration in transmitted light, or mixtures of orange, red purple and/or blue colours.

As mentioned above, the absorption characteristics provided in accordance with the present invention are strongly dependent upon the temperature at which the glass is maintained during the reduction heat treatment. Specifically, it appears to be very difficult to provide orange, red, purple and/or blue colouration utilizing heat treatment temperatures substantially in excess of 450"C. To cite a specific example, if the glass corresponding to curve 1 in accompanying Figure 7 is heat treated in a reducing atmosphere at temperatures approaching and exceeding 450"C, the relatively strong absorption peak at about 510 nm is shifted to an absorption near 450 nm and the glass becomes lighter and more yellow in colour.For this reason, heat treatment at temperatures of from 200 to 450"C are preferred for use according to this second embodiment of the present invention.

It has also been determined that the surface colouration exhibited by the heat-treated glass is strongly dependent on the thermal history of the photochromic starting material. In the extreme case where the starting material is the glass corresponding to Curve 1 of accompanying Figure 7 which has been simply annealed rather than heat-treated art a photochromic phase development temperature, the reduction-heattreated product is yellow for all reduction heat treatments in the from 300 to 4500C range.

Base glass composition appears to have an equally important effect on the variety of surface colouration which may be obtained. While the glass corresonding to Curve 7 of the accompanying Figure 1 is orange following a reduction heattreatmentat400 Cforone hour, Corning Code 8097 photochromic glass such as was used to provide surface-coloured photochromic glass articles in accordance with prior art teachings is yellow in colour after an identical reduction heattreatment.

Photochromic glasses which are preferred for use in the production of surface-coloured photochromic glass articles in accordance with the present invention are those set forth in the aforementioned United States Patent No.4,190,451. Such glasses consist essentially, in weight percent, of from 0 to 2.5% Li2O,from O to 9% Na2O, from 0 to 17% K2O, from 0 to 6% Cs2O, from 8 to 20% Li2O + Na2O + K2O + Cs2O, from 14 to 23% B2O3,from 5 to 25% Al2O3, from 0 to 25% P2Os, from 20 to 65% SiO2, from 0.004to 0.02% CuO, from 0.15 to 0.3% Ag, from 0.1 to 0.25% Cl and from 0.1 to 0.2% Br, wherein the molar ratio of alkali metal oxides:B203 is from 0.55:1 to 0.85:1 and the weight ratio Ag: (Cl + Br) is from 0.65:1 to 0.95:1. As also noted in the above disclosure, such glasses may additionally contain, as optional constituents, up to 10% total of other selected oxides or elements for known purposes, including up to 6% ZrO2, up to 3% TiO, up to 0.5% PbO, up to 7% BaO, up to 4% CaO, up to 3% MgO, up to 6% Nub205, up to 4% La2O3 and up to 2% F.

Of course, other photochromic glasses have been found suitable for use in accordance with the present invention in varying degrees, depending upon the extent to which they exhibit induced absorption peaks at relatively long wavelengths. Another example of a composition field wherein examples of such behaviour have been observed in that described in United States Patent No.4,018,965, which includes photochromic glasses consisting essentially, in weight, percent, of from 57.1 to 65.3% SiO2, from 9.6 to 13.9% Al2O3, from 12.0 to 22.0% B2O3, from 1.0 to 3.5% Li2O, from 3.7 to 12.0% Na2O, from 0 to 5.8% K2O, from 6 to 15% total of Li2O + Na2O + K2O, a weight ratio of Li2O:Na2O + K2O not exceeding 2:3, from 0.7 to 3.0% PbO, from 0.1 to 1.0% Ag, from 0.15 to 1.0% Cl, from 0 to 3.0% Br, from 0 to 2.5% F, from 0.008 to 0.12% CuO, from 0 to 1% total of transition metal oxide colourants and 0.5% total of rare earth metal oxide colourants. As noted in United States Patent No. 4,018,965, such glasses are particularly suitable for the production of sheet glass for photochromic sunglass lenses.

The present invention is illustrated by the following Examples.

Example I A composition for a photochromic glass consisting essentially, in parts by weight, of about 56.46 parts SiO2,6.19 parts A1203r 18.15 parts B2O3, 1.81 parts Li2O,4.08 parts Na2O, 5.72 parts K2O,4.99 partsZrO2, 2.07 parts TiO2, 0.006 parts CuO, 0.207 parts Ag, 0.166 parts Cl and 0.137 parts Br is melted in a continuous melting unit, pressed into ophthalmic lens blanks and annealed using a peak annealing temperature of 470"C for an annealing interval of 10 minutes, following by cooling at furnace rate.Several of these annealed ophthalmic lens blanks are converted to photochromic lens blanks by heat treatment 660"C for 30 minutes, while several other of the annealed blanks are converted to photochromic blanks by heat treatment at 550"C for 65 hours. Small glass samples are then cut from the photochromic ophthalmic lens blanks and polished to 2 millimetre thickness.

A sample of the above described photochromic glass heat-treated at 550"C is placed in a furnace operating at 400"C and containing a flowing atmosphere of 100% H2. The glass sample is maintained in the furnace for one hour and then removed from the furnace and examined.

The surface-coloured sample resulting from this treatment appears bright orange in transmetted light and exhibits an undarkened spectral transmittance curve substantially conforming to that reported as Curve 1 in Figure 7 of the accompanying drawings. The characteristics of that curve include strong absorption peaks centered at 460 and 510 nm, with a transmittance of about 11% at 510 nm in the undarkened state.

Example 2 One of the glass samples receiving a 660"C photochromic development heat treatment as described in Example 1 above is exposed to the reduction heat treatment described therein, consisting of treatment in a flowing 100% H2 atmosphere at 400"C for 1 hour. Following the reduction heat treatment, the sample is found to exhibit a red colouration in transmitted light and to have a spectral transmittance curve substantially conforming to that designated Curve 2 in Figure 7 of the accompanying drawings. That curve is characterised by strong absorption peaks centered at about 460 and 540 nm, with the transmittance of the sample at 540 nm being about 9% in the undarkened state.

Example 3 Two photochromic glass samples which have received a 550"C photochromic development heat treatment is described in Example 1 are selected for reduction heat treatment. One of the samples, referred to as Sample 3, is exposed to a reduction heat treatment at 300"C for 1 hour in flowing 100% H2. The other sample, referred to as Sample 4, is subjected to a reduction heat treatment at 2000for 16 hours in flowing 100% H2.

When removed from the heat treatment furnace and examined in transmitted light, Sample 3 exhibits a purple colour of medium intensity while Sample 4 exhibits a light blue colouration. The spectral transmittance curves of these samples conform to Curves 3 and 4, respectively, in Figure 7 of the accompanying drawings. Sample 3 exhibits an absorption peak centered at about 535 nm in the yellow-green portion of the visible spectrum, having an undarkened transmittance at that wavelength of 29%. Sample 4 exhibits a broad absorption peak centered at about 565 nm in the yellow portion of the visible spectrum, exhibiting an undarkened transmittance at that wavelength of about 63.5%.

The usual effect of heating a surface-coloured photochromic glass article comprising one of these new absorption bands to a temperature above that at which it was coloured, as may occur during ion-exchange strengthening orthermal tempering, is to shift the induced absorption bands toward the violet region of the spectrum and thereby to shift the colour of the glass towards the yellow. In cases where it is desired to avoid such shifts, it may be possible to chemically or otherwise strengthen the photochromic glass prior to the colour-inducing reduction heat treatment and thereafter to reduction-heat-treat the glass without significantly affecting its strength. This practice is illustrated by the following Example.

Example 4 A pair of photochromic glass ophthalmic lens blanks, commercially available from Corning Glass Works as Corning Code 8111 lens blanks and having a composition substantially conforming to the composition of the samples described in Examples 1 to 3 above, is selected for treatment. This lens blank pair is ground and polished to a prescription and edged for framing, and is then chemically strengthened by immersion in a molten salt ion exchange strengthening bath containing KNO3 and NaNO3 at a temperature of 400"C for 16 hours.

Following ion exchange strengthening, the lenses are plunged into a tube furnace operating at a temperature of 4300C, maintained in that furnace under an atmosphere of 100% H2 for 15 minutes and then removed from the furnace and examined. The treated lenses are found to exhibit a vermillion colouration in transmitted light, due to a strong, medium-width absorption peak centered at about 510 nm in the green region of the spectrum. The undarkened transmittance of the lenses at 510 nm is about 44%.

When framed to provide a pair of photochromic sunglasses, these lenses provide particularly good performance out-of-doors under green background conditions, because of the relatively low transmittance thereof with respect to green light combined with the significantly higher transmittance thereof of blue, yellow and red light. These transmittance characteristics improve object-background contrast for many objects viewed against a green background.

The use of the present invention to provide a surface-coloured photochromic glass article from a photochromic glass article containing a bulk glass colourant in the form of a dissolved oxide is illustrated by the following Example.

Example 5 A pair of photochromic glass ophthalmic lens blanks having a composition substantially conforming to that of the photochromic glass Example 1 above, but additionally containing about 0.09 parts NiO and 0.01 parts CoO, by weight, as dissolved oxide glass colourants, is provided for treatment. These lens blanks are commercially available from Corning Glass Works as Corning Grade 8115 lens blanks and are light brown in colour.

The blanks are ground and polished to a specific ophthalmic prescription. The resulting lens pair is then plunged in a furnace operating at a temperature of 420"C and containing an atmosphere of 100% H2, maintained therein for an interval of 15 minutes to impart surface colouration thereto and then removed.

The coloured lenses thus provided, exhibiting a colour attributable to a combination of bulk and induced surface colouration, are subjected to an ion-exchange strengthening treatment substantially conforming to that treatment described in Example 4 above, comprising a 16-hour exposure to a molten salt bath at a temperature of 400"C, and are thereafter removed, washed and examined. The resulting strengthened lenses are copper (red-brown) in colour, exhibiting a rather broad induced absorption band centered at about 485 nm and having a transmittance at that wavelength of about 25% in 2 mm thickness. They exhibit good photochromic darkening and facing response.

The use of Corning code 8115 glass lenses having a composition such as above described constitutes a particularly preferred embodiment of the present invention, as does the use of Corning code 8114 glass, a light grey glass of the same base composition, but containing 0.017 parts NiO and 0.020 parts CoO, by weight, in place of the NiO and CoO in the former glass. Nevertheless, other photochromic glasses may alternatively be used to provide surface coloured photochromic glass articles according to the present invention, as illustrated in the following Examples.

Example 6 A composition for a photochromic glass which consists essentially, in weight percent, of about 59.83% SiO2, 15.01% B2O3, 9.42% Al2O3, 1.84% Li2O, 3.5% Na2O, 5.8% K2O, 4.6% CaO, 0.5% Ag, 0.5% Cl and 0.06% CuO is melted in a small continuous melting unit and formed into a glass article. The article thus provided is annealed at a peak annealing temperature of about 470"C for an annealing interval of 10 minutes and then cooled at furnace rate.

Small glass samples are then cut from this article and polished to approximately 2 mm thickness. One of the samples is further heat treated at a temperature of 575"C for about 60 minutes to provide a photochromic glass sample therefrom.

This photochromic sample is positioned in a furnace operating at a temperature of 400"C and containing an atmosphere of 100% H2 for a treating interval of one hour. It is then removed from the furnace and examined.

The colour of this sample is transmitted light is dark orange. The spectral transmittance curve of the sample corresponds substantially to that shown as Curve 5 in Figure 7 of the accompanying drawings. That curve exhibits a strong absorption peak centered at about 470 nm in the blue region of the spectrum and a transmittance at that wavelength, in a sample thickness of about 2 mm in the undarkened state, of about 6%.

Example 7 Athin photochromic sunglass lens, formed of drawn sheet glass and having a composition, in parts, by weight, of about 58.6 parts SiO2, 17.5 parts B2O3, 11.5 parts Al2O3, 7.7 parts Na2O, 2.0 parts K2O, 2.2 parts PbO, 0.3 parts Ag, 0.37 parts Cl, 0.13 parts Br, 0.022 parts F, 0.025 parts CuO, 0.041 parts NiO and 0.029 parts CoO, is provided for treatment. This lens is light grey in colour in the undarkened state, having a thickness of about 1.5 mm. and an undarkened spectral transmittance curved substantially conforming to the curve labelled 'Untreated' in Figure 9 of the accompanying drawings.

Surface colouration is imparted to this lens by plunging it into a furnace operating at 420"C and containing an atmosphere of 100% hydrogen for 15 minutes. Following this treatment the lens is removed from the furnace and examined.

The treated lens is grey-brown in colour in transmitted light and has an undarkened spectral transmittance curve substantially conforming to that of the curve labelled "420"C" in Figure 9 of the accompanying drawings. Hence the glass exhibits a treatment-induced absorption peak at a peak location of about 490 nm which reduces the transmittance of the undarkened glass at that wavelength to about 42%.

The importance of treatment temperature on the induced absorption effects observed in reduction-heattreated photochromic glasses may be demonstrated by treating a grey sunglass lens of a composition identical to that just described at 465"C in flowing hydrogen for 15 minutes. The product of this treatment is yellow-grey in colour and has a transmittance curve substantially conforming to the curve labelled "465"C" in Figure 9 of the accompanying drawings. That curve indicates that the induced absorption peak is shifted to about 460 nm art a point to the left of the line CB in accompanying Figure 9 by this treatment, resulting in the strong yellow colour component observed in this glass.

In providing a selectively tinted photochromic glass article by surface removal in accordance with the present invention, it has been found that acidic media containing F- ions may conveniently serve as chemical agents for selectively removing surface material from coloured lenses. However, the surface coloured photochromic lens to be subjected to the chemical removal step of the process is preferably one wherein the coloured surface layer has a depth not exceeding 100 microns, in order to minimize the effects of colour layer removal on lens surface smoothness.

The identity of the chemical used for selective surface removal is not critical. Various agents which are effective to dissolve the selected photochromic glass without introducing surface defects, such as frosting or pitting may be used. Preferred agents for use with silicate photochromic glasses of commercial composition are aqueous solutions of HF.

The rate of surface removal from the glass may be controlled by controlling the composition of the removal medium, for example, by dilution or by the introduction of modifying acids or salts, for example, and also controlling the temperature of the medium during the removal step. Routine variations of these parameters may readily provide conditions suitable for extremely rapid removal of colou red surface glass, e.g. total colour layer removal within seconds, or a relatively slow dissolution rate requiring many minutes for complete colour layer removal.

Control over the area or configuration of the surface portions to be subjected to chemical removal may be exercised using the various masking techniques commonly employed for the hydrofluoric acid etching of glassware. Such could include the use of paraffin-based masking media, or other methods for preventing contact between the removal medium and the glass surface.

Alternatively, and in accordance with a presently preferred procedure, the lens may be merely partly dipped into the removal medium to obtain colour layer modification or removal and then removed and rinsed to halt the removal process. This technique is particularly useful for producing a gradient tinting effect in a lens because the sharpness of the gradient may readily be controlled by varying the dipping procedure.

Hence, a sharp gradient or abrupt change in tint between top and bottom portions of a lens may be obtained by rapid partial immersion and a rapid removal and rinsing after the immersion interval, while a gentle gradient or gradual reduction in tint from upper to lower portions of a lens may be obtained by gradual immersion and withdrawal. The avoidance of a sharp colour demarcation line between immersed and non-immersed lens portions in such dipping procedures is aided if the lens is wetted with water prior to dipping.

Example 8 A thin photochromic sunglass lens, formed of draw sheet glass and having a composition, in parts, by weight, of about 58.6 parts SiO2, 17.5 parts B2O3, 11.5 parts Awl203,7.7 parts Na2O, 2.0 parts Li2O, 1.5 parts K2O, 2.2 parts PbO, 0.3 parts Ag, 0.37 parts Cl, 0.13 parts Br, 0.022 parts F, 0.025 parts CuO, 0.150 parts NiO and 0.014 parts CoO, is provided for treatment.This lens is light brown in colour in the undarkened state, having a thickness of about 1.5 mm, and being commercially available as Corning Code 8103 glass from Corning Glass Works of Corning, New York, U.S.A.

In order to provide a coloured surface layer on this lens, it is heated under reducing conditions by plunging it into a tube furnace containing an atmosphere of 100% hydrogen, operating at a temperature of 550"C, for 30 minutes. It is then removed from the furnace for examination, and is found to exhibit a dark brown surface colouration in transmitted light.

In order to provide a selectively tinted lens from this surface coloured lens, a portion of the coloured surface layer is chemically removed by first wetting the lens with water and then partly immersing it in an aqueous HF solution containing about 16%, by weight, HF for an immersion interval of about 2 minutes.

The lens was immersed in the solution to a depth sufficient to cover approximately half of the lens, with immersion and withdrawal being controlled so that a relatively gradual transition between the untreated and fully treated sections of the lens surface was obtained.

The product of this treatment was a photochromic sunglass lens exhibiting relatively uniform photochromic darkening upon exposure to ultra-violet light, and also a fixed tint or colour gradient, visible in both the undarkened and darkened states, which graded in the undarkened state from relatively dark brown in the top portion of the lens to very pale yellow in the bottom portion thereof. Most importantly, no visually detectable variations in the refractive power of the lens surfaces were observed to result from the step of chemically removing a part of the coloured surface layer from the lens.

Example 9 In order to provide a constrasting colour to the bottom portion of a fixed-tint gradient sunglass lens such as provided in accordance with Example 8 above, such a lens may be recoloured following the selective removal of the brown surface layer therefrom. For the purpose of recolouring the bottom portion of such a lens, originally pale yellow in colour, the entire lens may be reheated under reducing conditions by placing it in a tube furnace containing 100% H2 and operating at a temperature of 350"C for an interval of 1 hour. If the lens is then removed from the furnace and examined, the bottom portion of the lens will normally be found to exhibit a light purple colour in transmitted light, which contrasts strongly with the dark brown colour of the top of the lens.

In carrying out a reheating surface colouration step such as just described, it is preferred that the reheating used to provide the second or subsequent coloured surface layers on the glass be carried out at a temperature lower than that at which the first or other prior coloured surface layers have been applied thereto. This is because the use of a lower heating temperature will ordinarily prevent any modification of the colour of previously applied layers, provided that the previous layers were applied at high temperatures.

Thus, in making a multicolour product, the first surface colour to be developed in the glass will normally be that colour requiring the highest temperature reduction heat treatment and subsequent colours will be applied in order of decreasing reduction heat treatment temperature.

Of course, in the reducing atmosphere heat treatment of photochromic glasses containing PbO as described above, colouration due to the reduction of PbO to metallic lead is present in the glass in addition to that attributable to silver. This colour is also confined to the glass surface and is removable, being even less penetrating than the silver colour. With appropriate control of the surface removal treatment, it is therefore possible to remove the lead colouration from certain surface regions and the lead and silver colourations from other surface regions, to provide brown, yellow and clear sections in a single glass article with only one heat treatment.

Claims (18)

1. A method for introducing a surface colouration into a transparent photochromic glass which comprises heat treating the glass at a temperature in excess of 200"C. in a gaseous reducing environment to reduce the luminous transmittance of a surface layer of the glass to a value below that of the untreated glass at one or more wavelengths higher than the fundamental absorption peak of metallic silver in the glass, and optionally modifying the surface colouration by chemically removing a portion of the surface layer.

2. A method as claimed in claim 1 in which the surface colouration is due to a broad change in luminous transmittance of the glass surface with a minor shift in the chromaticity thereof, promoted by exposing a portion of the glass article, containing at least 0.5% PbO, to the gaseous reducing environment at a temperature of from above the strain point to about 50C". above the annealing point of the glass to cause silver ions to be reduced to metallic silver particles and lead ions to be reduced to metallic lead particles in the surface of the glass, optionally before or after thermally or chemically strengthening the glass.

3. A method as claimed in claim 1 in which the surface colouration is due to the development of at least one treatment-induced absorption peak having a peak location at a wavelength higher than the fundamental absorption peak of metallic silver in the glass, promoted by exposing a portion of the glass to the gaseous reducing environment at a temperature of from 200 to 450"C. to cause the chemical reduction of silver in contact with silver halide microcrystals in the surface of the glass.

4. A method as claimed in claim 1 or claim 2 in which the glass comprises, in weight percent on the oxide basis, from 0.5 to 10% PbO, from 4 to 26% Awl203, from 4 to 26% B2O3, from 40 to 76% SiO2, and at least one alkali metal oxide selected from 2 to 8% Li2O, from 4 to 15% Na2O, from 6 to 20% K2O, from 8 to 25% Rb2O and from 10 to 30% Cs2O.

5. A method as claimed in claim 1 or claim 3 in which the glass comprises, in weight percent, from 0 to 2.5% Li2O, from 0 to 9% Na2O, from 0 to 17% K2O, from 0 to 6% Cs2O, from 8 to 20% total of Li2O + Na2O + K2O + Cs2O, from 14 to 23% B203 from 5 to 25% Al2O3, from 0 to 25% P2O5, from 20 to 65% SiO2, from 0.004 to 0.02% CuO, from 0.15 to 0.3% Ag and from 0.1 to 0.25% Cl and optionally up to 10% total of one or more other oxides or elements selected in amounts not exceeding the indicated proportions from up to 6% ZrO2, up to 3% TiO2, up to 0.5% PbO, up to 7% BaO, up to 4% CaO, up to 3% MgO, up to 6% Nub205, up to 4% Lea203, up to 2% F, up to 1% of one or more transition metal oxide colourants and up to 5% of one or more rare earth metal oxide colourants, the molar ratio of alkali metal oxide(s) B203 being from 0.55:1 to 0.85:1 and the weight ratio of Ag:(CI + Br) being from 0.65:1 to 0.95:1.

6. A method as claimed in any of claims 1 to 5 in which the gaseous reducing environment comprises pure hydrogen.

7. A method as claimed in any of claims 1 to 6 in which the treatment lasts for from a few minutes to several hours.

8. A method as claimed in any of claims 1 to 7 in which a gradient colouration is produced in the glass across the surface thereof.

9. A method as claimed in any of claims 1 to 8 in which the glass comprises a tinting agent.

10. A method as claimed in any of claims 1 to 9 in which treatment comprises: (a) exposing at least a portion of the glass to a gaseous reducing environment at a temperature below the strain point of the glass to cause silver ions in the surface of the glass to be reduced to metallic silver particles; and (b) subjecting the previously-exposed portion of the glass to a gaseous reducing environment at a temperature of from above the strain point of the glass to 50"C. above the annealing point of the glass to cause the reduction of lead ions to metallic lead particles as a layer over the metallic silver particles.

11. A method as claimed in any of claims 1 to 10 in which the glass is a boroalumino-silicate glass.

12. A method as claimed in any of claims 1 to 11 in which a portion of the coloured surface layer is chemically removed by exposing the surface to an aqueous HF-containing solution.

13. A method as claimed in any of claims 1 to 12 in which, following the chemical removal of at least a portion of the coloured surface layer, the glass is reheated under reducing conditions to provide a second coloured surface layer on at least a portion of the surface of the glass.

14. A method as claimed in claim 13 in which the glass is reheated under reducing conditions at a temperature below that at which it was first heated under reducing conditions.

15. A method as claimed in any of claims 1 to 14 substantially as herein described.

16. A method as claimed in any of claims 1 to 15 substantially as herein described with reference to any one of the Examples and/or the accompanying drawings.

17. A glass having a surface colouration introduced by a method as claimed in any of claims 1 to 16.

18. The invention substantially as herein described.