CA1118624A - Photochromic glasses suitable for simultaneous heat treatment and shaping - Google Patents

Abstract

PHOTOCHROMIC GLASSES SUITABLE FOR SIMULTANEOUS
HEAT TREATMENT AND SHAPING

Abstract of the Disclosure The instant invention is related to the manufacture of photochromic glasses having base compositions with m a very narrow interval of the alkali metal boroaluminosilicate system wherein silver chloride and/or silver bromide crystals impart photochromic properties. The glasses are notable for their rapid fading characteristics and relatively low tem-perature dependence of darkening. The compositions are especially suitable for a production process which contemplates simultaneously shaping articles from glass sheet and developing photochromic properties therein.

Description

Back~round of the Invention The field of photochromic glasses is founded in United States Patent ~o. 3,208,860 which discloses the production of silicate-based glasses that exhibit darkening when exposed to actinic radiation, customarily ultraviolet radiation, and which return to their origin~l color when removed from the source.of actinic radiation. Such reversible optical prop-erties are achieved via the incor?oration of effective amounts of silver and at least one halide of the group chloride, bromide, and iodide into the glass composition which combir.e to form silver halide crystallites in the glass. The crys~allites are so small as to be invisible to the unaided eye, vet are darkenable unde~ t~e action of actinic radiation to reduce the optical transmittance of the glass. ~nen t,he actinic radiation is extinguished, the crystallites fade to their original state, thereby restor.ng the optical transmlt,ance to its initial level. This cycle of darkening and fading can be repeated indefinitely without fatigue in photochromic glasses.
By far the most prevalent use for photochromic glasses has been in the fabrication of ophthal~ic lenses. One example of that application is provided in United States Patent No. 3,197,296 which describes a family of refractive index-corrected silicate glasses containing silver halide crystals to provide the desired photochromic behavior.
Those glasses demor.strated, in conventional 2 mm thickness, photochromic properties sufficiently developed for prescription ophtha~mic applications along with the necessary refractive index to be compatible with conventional lens grinding practices.
The manufacture of ophthalmic lenses commonly involves the pressing of glass lens blanks of optical quality from a melt followed by the grinding and ?ollshir.g of ~he blan~s to predetermined prescriptions. It is believed sel~-evident that the production of non-prescription photochromic glass lenses, for exa~ple, sunglass lenses, in large quantities by processes demanding grinding and polishing is not only expensive and time consuming, but is also wasteful of material.
Consequently, less costly mear.s for producing photochromic glass sheet for lenses or other applications would be highly desirable. Assu.~ing that the sheet could be produced in optical quality, the sheet could be inexpensively thermally sagged to the curvatures required for lenses, windshields, and other sheet glass configurations.
The commercial sheet glass forming processes practiced today contemplate maintaining substantial volumes of molten glass at temperatures ~herein the glass has the necessary ~ 62 ~
viscosity for sheet Corming, vi~ , at a viscosity between about 104-lC6 poises By the ve-y nature of the drawing process, .hose vol~mes of glass are in prolonged contact with ref~actory me~als or ceramics which serve as the mear.s for formirg dr~n sheet. Thus, the sheet drawi~g processes impose severe constrair.ts upon glass composition because of the formidable liquidus and glass stability problems associated with the handling and processing of glass at relatively low temperatures and high viscosities.
In addition to good formability proper.ies, suitable glass sheet for ophthalmic purposes will e~hibit high optical quality, good chemical durability, high strength, and good photochromic darkening even in sheet of moderate thic~ness Where the sheet is scheduled for use as light--~eight sunglass lenses, the glass must also be chemically strengthened such as ,o meet the Food and Drug Acministra--tion (F~A) reaui-emer.ts for eyeglass lens safet~,~. Federal safety -equi-ements cannot be rout;nel-~ met in lightwei~ht glass of mode~ate thicknesses (1.3-1.7 millimeters) ir. the ~0 absence of chemicsl strengthening, or by utilizing an air tem?ering procedure. Ur.ited Sta_es Patent No 4,31~,965 describes a group of glass com~ositions which de~onstrates the properties necessary for photochromic sheet glass appli-cations.
~ e have unexpectedly been able to provide a glass, which, in addition to the necessary melting, forming and chemical strengthen-ing capabilities, as well as the physical characteristics ~ 8~ ~ ~
conventi3nally demanded in non-photochromic ophthalmic ware is characterized by a number of further highly advantageous and surprising features and properties.
First, the glass when having a 1.5 mm thickness at room temperatures (25-30C.) will exhibit an optical transmittance in the range of 60-90% before exposure to actinic radiation but when irradiated with actinic radiation, e.g., bright outdoor sunlight, it will darken to a transmittance of less than 30%.
Second, the glass, when having a 1.5 mm thickness and at 25-30C.

will fade very rapidly when removed from the incident actinic radlation; i.e., the glass within five minutes will fade to a transmittance of about two times its darkened transmittance and within an hour will fade to a trans~it~ance of a; least 80% of its origir~al t-ans~ittance.
One circumstance which must be kept in mind wnen conducting research involving photochromic glass is the fact that the dynamics of photochromic behavior exhibited by glasses are directly related to the in.ensitv or the actinic radlation impinging thereon and the temperature of the glass while beir,g irradi2ted. Accordingly, ~he-e other parameters are held constant, a photochromic glass will customarily darken to a lower trans~i,tar.ce wnen exposed to actinic radiation while at a lowe ,em?erature. ~oreover, the intensi.y of solar radiation can obviously vary greatly depending upon the season of the year, the location of the exposure (angle of declir.ation of the sun), cloud cover, snow cover, ai- mass value, etc.
With respect to temperature dependence, i.e., the degree of darkening demonstrated by a photocnromic glass over a range of ambier.~ temperatures, some photochromic glasses in 1.5 mm thickness may darken to a trar.smittance of less than 5% when subjected to solar radiation at a temperature of -18C. (0F.). Such glasses would not comply with the specifications of the American National Standards Institute (ANSI) which specify lenses for general use as fixed tint sunglasses to exhibit an optical transmittance of at least 5~.
Consequently, a third feature of the proposed photochromic glasses which are to be used for ophthalmic applications is that in 1.5 mm thickness the glass will not darken to a tranmittance of less than 5% at -18C.
The converse of the above-stated rule regarding temperature dependence also holds true; viz., where other parameters are maintained constant; a photochromic glass will carken to a lesser degree, i.e., the final darkened optical transmittance will be higher, when the glass is at a higher temperature when exposed to actinic radiation. To have practical utility as a sunglass, it has been deemed that a photochromic glass should darken to an optical transmittance of less than 50% when exposed to outdoor sunlight at temperatures encountered during summer.
Accordingly, a fourth feature of the proposed glass is that the photochromic glass in 1.5 mm thickness will darken to a transmittance less than 50~ when exposed to actinic radiation of 40C. (104F.).
To simplify manufacturing techniques, while concomitantly maintaining the optical properties of the pristine glass surface, the ideal glass compositions would permit the desired photochromic properties to be developed concurrently with the required lens curvature during a thermal sagging operation. Canadian Application Serial No. 274,855 filed March 28, 1977 in the names of ~ - 5 -Bourg, Hazart, and Joure~, discloses such a technique for simultaneously heat treating and sagging shcet of photochromic glass into lens blar.ks of a desired cur~ature.
It is believed apparent from the prior a.t that the photochromic properties exh bited by a ?articular glass are dependent upon both composition and the heat treatment to which it is subjec~ed. The cur~ature secured in a thermal sag cycle is also a func~ion of such parameters as glass composition and incident thermal c.ycle resulting through the combined effects of sur ace energy, density, and viscosity, this latter factor being strongly dependent upon temperature.
A most fortuitous circumstance would exist where the desired photochromic behavior could be achieved through the same heat treatment as that giving rise to the necessary lens curvature.
Therefore, a fifth criterion proposed is a glass capable of beir.g concu,rer.tly heat treated and sagged to simultaneously yield the desired lens curvature and photo-cnromic properties.
United States Patent No. 4,190,451, filed March 17, 1978 by G. B. Ha_es, D. L. Morse, D. W. Smith, and T. P. Seward, III, discloses a silve- halide-containing, silicate photochromic glasses exhibiting quite rapid fading characteristics and relatively low te~perature dependence of darkening. Several of the co~positions recited in that application are operable for sheet dra~ing processes but are not suitable for a simultaneous heat t eating-sagging procedure, such as has been described above. The inapplicability of those glasses for such a process resides in the fac~ that the times and temperatures demanded to sag the glass sheet are such as to cause the glass to sag into contact ~ith 1~1186Z~
fo~mers which ?roduce ,he necessa y lens cu~va~u.e, this contact causing the destruction of ~he good optical proper;ies of the pristine su~face. Yet, without such formers, those glasses would sag tO a much higher curvature thar. desired.
Thus, the lens blanks fabricated from those glass compositions via a heat ~reatir.g-sagging technique would require grinding and polishir.g to provide the re~uired optical quality surface.

The principal feature of the instant invention is the manufacture of transpa~ent photochr~mic glass which, in sheet form, will be suitable for the fab~ication of sunglasses through a heat treating-sagging process and which, in 1.3-1.7 mm thicknesses, manifests the following photochromic and pnysical properties:
(a) at about 25-30C., the glasses will dar~en to 2 luminous transmittance below 30% in the presence of actinic radiation, e.g., bright outdoor sunshine; the glasses will fade to a luminous transmittance at least 1.75 and, prefer-ably, two times the dar~ened transmittance after five minutes' removal from the actinic radiation; and the glasses will fade to a luminous t~ansmittance in excess of 80% of their original undarkened transmittance in no more than one hour _ after being removed from the actinic radiation;
(b) a~ about 40C., the glasses will tarken to a luminous transmittance below 50~lO in the presence OC actinic radiation, e.g., bright outdoor sunshine, and will fade to a luminous transmittance in excess of 80% of their original undarkened transmittance in no more than one hour after being removed from the actinic radiation;

~11 86 2 ~

(c) in the undarkered state, the glasses will exhibit a luminous transmittance (clear luminous transmittance) of at least 60%, corveniently obtained by incor~o~ating a fixed tint in the composition, but more typicall-; within the range of 85- 92~1o;
(d) at about -18C , the glasses will not darken to a luminous transmittance below 5% in the presence of actinic radiation, e g , bright sunlight;
(e) the glasses are capable of being strengthened via either thermal tempering or chemical strengthening while maintaining the desired photochr~mic properties; and (f) the glasses in sheet form have the capability of being simultaneously heat treated and sagged to produce lens blanks of the proper curva;ure with the desired phstochromic properties A suitable glass composition in the present invention consists essentially in weight percent on the oxide basis as calculated from the batch, or about 54-66% SiO~, 7-15~o A1203~ 10-2570 B203, 0 5-4 0~,' Li2O, 3 5-lS% Na2O, 0-10% K2O, 6-16r/D total of Li2O
+ ~a2O + K2O, 0-1 2~Z PbO, 0 10-0 3% Ag, 0 2-1 0C,~o Cl, ~-0 3% Br, 0 002-0. 02% CuO, and 0-~ 5~io F . The glass mav optionally additionally contain colo.ant oxides selec~ed ir, the indicated propo-tions from the ~roup corsisting of 0-1~, total or transition metal oxide colo.ants and 0-5% total of rare earth oxide colorants Glasses produced frGm the above-described compositions exhibit viscosities of at least about 104 poises at the liquidus temperature, thereby providin~ a liquidus-viscosiey ~118~29~
relationship permit~ing formir.g via direct sheet drawing from the melt. The glasses also demonstrate long term stability against devitrification in contact ~ith platinum at temperatures corresponding to glass viscosities in the range of 104-105 poises, and, hence, can be drawn from a melt at those viscosities utilizing platinum or platinum-clad drawbars, downdraw pipes, or other sheet forming means to t,~ield glass sheet of optical quality. As defined herein, long term stability against devitrification comprehends good resistance to surface crystal growth in contact with platinum at temperatures corresponding to glass viscosities in the 104-106 poise range. The growth of a crystalline layer not exceeding 10 microns in thic~ness at the glass-platinum interface over a contact period of 30 days at those viscosities is considered good resistance to crystal growth.
The inventive glasses also display excellent chemical durability, by which is meant that the glasses manifest no visible surface film formation or iridescence ~ollowing a 10-minute exposure at 25C. to 10% by weight aqueous HCl.
Glasses within the above-recited composition area are capable of being chemically strer.gthened to modulus of rupture values in excess of about 45,000 psi with a depth of ion-eYchanged layer of at least 0.0035", as determined by conventional stress layer examination techniques employing, for example, a polarizing microscope equipped with a Babinet compensato,. Such strength and compressior. layer characteristics can be secured through conventional sodium-for-lithium salt bath ion exchange processes at normal ion exchange temperatures (300-450C.), the surface compression being generated by the replacement of Li+ ions in the glass surface ~ith the larger Na+ ions or the molten salt. Such physical properties ~llB~;~4 permit glass sheet of 1.3-1.7 mm thic~ness to readily pass the Food and Drug Administration i~pact test for ophtha~mic lenses (the drop of a 5/8" steel ball from a height of 50").
Finally, glasses within the inventive composition region exhibit an excellent combination of photochromic properties following heat trea~ment in accordance with conventional practice. These properties include, in glass sheet not e~ceeding about 1.7 mm thickness, a darkened luminous transmittance of less than 30% at 2S-30C., a luminous trans~ittance after five minutes' removal from actinic radiation of at least 1.75 and, preferably, two times that of the darkened state, and a luminous trans-mittance after one hour's removal from actinic radiation of at least 80% of their original undarkened transmittance.
Upon exposure to actinic radiation at -18 C., the luminous transmittance of the glasses will not fall below 5Z. At 40C., e~posure to actinic radiation will darken the glasses to below 50% transmittance and the glasses ~ill fade to a luminous transmittance in excess of ~0~' o, their origir,al undarkened transmittance.
For the purposes of the presen~ descrlption, the luminous transmittance of a glass is de~ined as the value Y
delineated in terms of the 1931 C.I.~. trichromatic colori-metric system ut11izing the light source Illuminant C. This colorimetric system and light source a-e described by A. C.
Hardy in the Handbook of Colorimetr7, Technology Press, .I.T., Cambridge, ~assachusetts (1936). Also, as employed in this disclosure, the clear or undarkened state is obtained via an overnight (at least 8 hours) fadir.g of the glass in the absence of light. A slightly clearer glass (2-3 per-centage transmittance units higher) can be secured by sub-merging the glass in boilir.g water for five minutes.

Glass designed for sunglass lens applications will preferably exhibit a clear luminous transmittance of at least about 60%, this value being readily obtainable in the inventive glasses in combination with the other desired photochromic properties.
Darker glasses having clear luminous transmittances of less than 60%, however, can be achieved within the inventive glass compo-sition interval where maximum or near maximum concentrations of the cited col.orants are included.
Thus in one embodiment the present invention provides a method for simultaneously shaping articles from glass sheet and developing photochromic properties therein which comprises the steps of:
(a) melting a batch consisting essentially, in weight per-cent on the oxide basis, of about 57.1-65.3% SiO2, 9.6-13.9%
A12O3, 12-22% B2O3, 1-3.5% Li2O, 3.7-12% Na2O, Q-5.8% K2O, 6-15% total of Li2O + Na2O + K2O, the molar ratio Li2O:Na2O
+ K2O not exceeding about 2:3, 0-1.25% PbO, 0.12-0.24% Ag, 0.2-1% Cl, 0.06-0.25~ Br, 0-2.5% F, and 0.002-0.02~ CuO;
(b) adjusting the temperature of at least one region of the glass melt to provide a viscosity therein of about 104- 106 poises;
(c) drawing the glass melt at a viscosity of about 104-106 poises directly past refractory forming means to produce potentially photochromic drawn glass sheet;
(d) cooling the glass sheet below the softening point of the glass and cutting articles of desired geometries therefrom;
(e) edge supporting said articles on alveolated molds; and then (f~ heating said articles at a temperature between about 610-660C. for a period of time sufficient to simultaneously sag the glass into the concave portions of the alveolated molds and develop photochromic properties in the glass.

.- ~

11~8~24 Preferably such a method is provided wherein said time sufficient to simultaneously sag the glass and de~elop photo-chromic properties therein ranges from about 6-15 minutes at temperatures between 610-640C. or from about 5-12 minutes at temperatures between 640-660C.
Also preferably said batch may contain up to 1% total of transition metal oxides and/or up to 5% total of rare earth metal oxides as colorants, and more preferably said transition metal oxides are selected in the indicated proportions from the group consisting of 0-0.5% CoO, 0-1.0% NiO, and 0-1.0% Cr2O3, and said rare earth metal oxides are selected from the group consisting of Er2O3, Ho2O3, Nd2O3, and Pr2O3.
Also preferably the method may be provided wherein the content of PbO in said batch ranges between 0.15-0.7%, and more preferably said batch also contains up to 1% total of transition metal oxides and/or up to 5% total of rare earth metal oxides as colorants. Preferably in such a method said transition metal oxides are selected in the indicated proportions from the group consisting of 0-0.5% CoO, 0-1.0% NiO, and 0-1.0% Cr2O3, and said rare earth metal oxides are selected from the group consisting of Er2O3, Ho2O3, Nd2O3, and Pr2O3.
Also preferably the method may be provided wherein the glass is sagged into the concave portions of the alveolated molds without establishing contact with the mold surface, thereby achieving an optical quality surface without grinding and polishing.
The method of the instant invention comprises an improved process for the production of drawn photochromic glass sheet wherein a glass-forming batch is melted, the melt adjusted in temperature to provide a viscosity of 104-106 poises, and then drawn past refractory forming means within that range of vis-cosities to yield potentially photochromic glass sheet. As used herein, potentially photochromic glass sheet is defined -11 ~)-as glass sheet including s~lver halides and sensitizing agents or activators such as copper oxide wh~ch can be rendered photochromic via an appropriate heat treatment after the forming step. The glass sheet can be formed utilizing conventional up-draw or downdraw processes.
Observance of the inventive compositional and process para-meters, coupled with supplemental heat treatments and strengthening procedures involving conventional time-temperature schedules, permits the production of chemically strengthened photochromic drawn sheet glass articles which can be especially suitable for the fabrication of thin, lightweight photochromic ophthalmic or sunglass lenses. Most importantly, the above-described compositional and process parameters enable the potentially photochromic sheet ~ 3 ~ 6 2 ~
to be rendered photochromic during a heat treatment schedule deslgned to produce sagged lenses for ophthalmic or sunglass applications. ~hus, the imparting of photochromic behavior to the glass and the sagging thereof to the proper curvature are accomplished in the same heat treatment.

Description of the Preferred Embodiments Inasmuch as the chemical, photochromic, and physical properties, along with sagged lens curvatures (when required), are complex functions of the several constituents of the glass composition, strict adherence to the compositional limitations of the inventive glasses is vital ~o achieving the desired combination of properties.
As was observed in United States Patent Mo. 4,018,965, supra, the presence of Li20 is demanded in the glass composi-tion to secure the capability of being chemically strengthened.
Hence, where less than about 0.5% by weight Li20 is present in the composition, modulus of rupture values in excess of about 4j,000 psi and depth of compression layers of 0.0035"
cannot be consistently obtained. On the other hand, Li20 contents in e~cess of 4~,' by weight give rise to dec-eased glass stability against platin~m metal when the molten glass has a viscosity within the 104-106 poise interval, and hazards the development of haze in t~e glass. The desired mechanical strength and depth of compression layer cannot be attained in the absence or near-absence of Li20 emploving, for e~ample, a K~-for-Na+ ion exchange t~eatment to strengthen the glass.
Control must be maintained over the levels of the other alkali metal oxides because of their effect upon both photo-chromic and chemical strengthening characteristics. For ~ L1862~example, where less than the stated concentrations of ~.la20 and K20 are present, photochromic darkenability and the capacity for chemical strengthening are impaired. ~uantities of alkali metal oxide greater than the total specified act to reduce the fading rate of the glass and K20 in excess of the stated limit seems to reduce the chemical strengthening potential of the glass.
The presence of A1203 and B203 in the composition appears to counter the adverse effect upon fade rate exercised by the alkali metal oxides. Hence, glasses containing less than the recited amounts of those components will generally demonstrate inferior photochromic behavior. The inclusion of more than about 25% by weight B203, howe~er, tends to decrease the chemical durability of the glass. T~here more than about 15% by weight A1203 is employed, the glass stabilitv against devitrification is substantially degraded, the excess A1203 being prone to combine with the Li20 of .he composition to produce spodumene solid solution crvstals.
The presence of lead oxide in the specified range can be of great significance in providing the desired combination of photochromic properties in the glass, particularly with regard to the amount of darkening ard the rade rate, as will be discussed in more detail infra.
The addition of minor amounts of compatible constltuents to the glass composition is permissible but is generally avoided because of the possibility of adversely affecting the desired combination of photochromic and physical charac-teristics. Accordingly, whereas alXaline ea_th and other multivalent metal oxides may be included, no substantive property advantages have been perceived in so doing and, frequently, such additions tend to increase the liquidus 1~186 ~ 4 tem?e_ a~u~e 2nd d~c~ease _he long ,erm stability of the glass. ~inor amounts of the alkali metal oxides Rb20 and Cs20 may be added, but such appear to :~mpai~ the chemical s~-engther.inc potential of th~ class.
TiO2 and ZrO2 will preferzbly be entirely absent due to their known function as a nucleating agent for crystal crowth. As little as 0.8% ZrO2 can promote zircon crystal-li~atior. at temperatures in the glass forming range.
of SnO2, Sb2O3, and/or As2O3 may be useful in modifying the characteristics of the glass melt, particularly with regard to the oxidation state thereof.
As has been disclosed above, lead oxide can ~lay an ~mportant role in controlling photochromic properties.
Improved darkening of the g~ass is secured when PbO is present in an ~mo~nt of at least 0.15%. The fastest fadir.g glasses contai~ PbO in levels less than about 0.7l~ by weight.
A very signific2nt facet of the instar.t ;nvention ls tne d~scovery that copper at concentrations of 0.002-0.020;, by weight CuO can pl2y 2 mea~ingful part in achieving low temperature depe~dence of darkeni..g without degr2datior. in fade rate. Consequer.tly, when ,he composition of the base glass is changed to modify the physical proper~ies thereo', and, in so doing, the content of PbO or alkali metal oxide is increased, an ihcreased amount of coppe- -~ill be requi-ed to achieve the optimum combination of darker.ing, fadin~, ar.d low temperature dependence.
Where a simultaneous heat treat-sag processing step f constitutes an elemen; in the line of production, silver and bromide, as analyzed in the glass, should fall within the indicated ranges of 0.12-0.18% and 0.060-0.13%, respectively. Smaller amounts of Ag and Br do not provide sufficient nucleation and, as a result, the glasses tend to be hazy and darken poorly. With greater quantities of Ag and Br, nucleation is excessive and the glasses do not da ken well when heat t-eated for only the short periods of time required to give good sagged lens curvature.
Good darkening character has been found consistent with high chloride concentrations Thus, Cl levels greater than 0.2% and, preferably, in excess of 0.3% by weight are required.
Nevertheless, because high chloride contents appear prone to increase the temperature dependence of the glass somewhat, discretion dictates that the chloride concentrations be kept at such low levels as is practically consistent with good darkenability.
The inventive glass compositions can be compounded from conventional glass batch constituents in proportior.s that will yield the desired oxide components in the proper amounts at temperatures utilized for melting the glass. The melting may be undertaken ir. accordance~wi;h conventional

2~ optical glass melting practice in cruc bles, pots, tanks, or other melting units at temperatures w.thin the 1~00-1550C.
interval.
The molten glass may be formed utilizing any of the techniques kno~N~ to the glassma~ir.g art such as blowing, casting, p-essing, rolling, and spinr.ing. ~oreover, the glass is sufficiently stable that it may also be formed ir.to sheet by direct drawing fr~m the melt, at leas. where platinum or other refractory metal-lined drawbars, do~N~draw troughs, or other forming means a_e utilized.
The glass sheet or other articles ~ay then be heat treated in accordance wlth thermal schedules conventional 1~1 8~ 2 ~
for photochromic glasses in order to develop the desired photochromic behavior therein. Thus, operable heat treat-ments contemplate exposure of the glass sheet to temperatures within the range of 580-750C. for times ranging from a few seconds to a few hours. To insure the required optical surface quality, the glass will be supported in a manner calculated to preclude surface marking as, for example, via edge support means.
Where the glass will be sagged to the desired lens curvature and the photochromic properties developed simul-taneously during the same heat treatment, temperatures between about 610-64QC. for periods of time between about 6-15 minutes or about 640-660C. for about 5-12 mir.utes have been found suitable. Lens curvatures of about 4-6 diopters in 60-80 mm diameter ler.ses have been developed.
Finally, after the photochromic properties have been generated, the glass articles can be subjected to conver.tior.al chemical strengthening treatments; for example, immersion ln a bath of molten ~aN03 or a bath of molten Na~03 + K~03 containing at least 30% by weight ~1a~03. ,he desired strength and depth of compression layer car. be attained where the immersion is conducted for about 4-24 hours in baths at temperatures bet-~een about 300-450C.
The most optimum combination of photochromic and physical properties, wherein lenses a_e simultaneously sagged tO the required curvatures and photochromlc prop-erties are developed therein, and those lenses are sub-sequently chemically strengthened without substantial impairment of the photochromic properties, is produced within a preferred ~roup of glasses having compositions consisting essentially, in weight percent on the oxide basis 1~18~Z~

2S calculated from the batch, of about 57.1-65.3% SiO2, 9.6-13.9% A12O3, 12.0-22.0% B2O3, 1.0-3.5% Li2O, 3.7-12.0% ~a2O, 0 5.870 K2O, 6-15% total of Li2O + Na2O + K2O, a molar ratio of Li2O:Na2O + K2O not exceeding about 2:3, 0-1.25% PbO, 0.12-0.24% Ag, 0.2-1.0% Cl, 0.06-0.25% Br, 0-2.5V/o F, 0.002-0.020% CuO, 0-1.0% total of transition metal oxides selected in the indicated proportions from the group consisting of 0-0.5% CoO, O-1.0% ~iO, and 0-1.0% Cr2O3, and 0-5.0% total of rare earth metal oxides selected from the group consisting f Er23~ Pr2O3~ H2O3, and Nd2O3, Specific examples of preferred glass compositions falling within the above ranges are reported in Table I
below. The individual components are expressed in parts by weight on the oxide basis as calculated from the batch, except that the halides and silver are tabulated on an elemental basis in accordance with customary glass analysis practice. Analyzed values are also recorded for Ag, Br, and Cl since it is the retained concentrations of those ingredients which are critical to the invention. The values to the left of the slash mark represent batch content and those to the right of the slash mark analyzed levels. Wet chemical and ~-ray emission techniques were employed in those analysis.
Inasmuch as the sum of the several ingredients approximates 100, for all practical pur?oses the values reported can be deemed to represent weight percent.
Each of the glasses within the above range of prefe-red compositions, including the s?ecific examples of Table I, has a viscosity at the liquidus of at least 10~ poises, as well as excellent chemical durability as characterized by essential inertness in the above-described acidic solutions.
The glass also demonstrates long term stability against ~118624 devitrification in that it manifests good resistance to crystallization when in contact with platinum at viscosities within the 104-106 poise range. Furthermore, all of the preferred glasses are capable of being chemically strengthened to modulus of rupture values of at least 45,000 psi with a depth of compression layer of at least 0.0035", utilizing conventional ion exchange strengthening techniques.
Table I also includes photochromic property data measured on individual samples at 27C., 40C., and -18C., wherein YO indicates the clear luminous transmittance of the glass, Ylo and Y20 represent the darkened luminous transmittances of the glass after 10 and 20 minutes' exposure, respectively, to actinic radiation, and YF5 reports the luminous transmittance of the darkened glass after a five-minute removal from the actinic radiation.
In the past an ultraviolet lamp has been used as a convenient source of actinic radiation to test the photochromic characteristics of glass samples, since it was recognized that photochromic glasses were primarily activated by radiations in the ultraviolet and low visible portions of the spectrum. It has been found, however, that frequently the correlation between the data obtained with the ultraviolet lamp and the values measured from solar radiation outdoors was poor. Consequently, in order to secure better correlation with outdoor solar exposure, a "solar simulator" was devised for the determination of the luminous transmittance Y in Table I.
The solar simulator apparatus, as described in United States patent No. 4,125,775, filed October 5, 1977 in the name of Chodak, is based around a 150 watt xenon arc source fitted with a filter to modify the spectral output thereof so as tO closely approach the solar spectrum, particularly in the ultraviolet, blue, and red portions.
The infrared region of the spectrum is attenuated with a layer of water of sufficient thic~ness to provide equal irradiance to that of the sun, but with no special regard for the spectral distribution within that region.
The intensity of the arc source was adjusted such that the amount of darkening resulting from exposure to the light source was essentially identical to that of a number of commercially available photochromic glasses, including PHOTOGRAY~ lenses, darkened outdoors at noon during a cloudless early summer day in Corning, New Yor~ (air mass value of about 1.06). ~umerous e~perimental photochromic glasses of widely-variant compositions were also subjected to the solar simulator and to outdoor sunlight. Good overall agreement was observed in comparisons between the two types of measure-ments.
In order to continuously monitor the darkened trans-mittance of the specimens, each sample was interrogated with a chopped beam of light from a tungsten-halogen lamp detected by a PI~ silicon photodiode whose out?ut was demodulated via a loc~-in amplifier. A composite color filter was placed into the beam so that the product of the light's spectral output, the silicon detector spectral sensitivity, and the filter transmittance would closely a?proximate the s~ectral sensitivity of the human eye.
This apparatus was interfaced to a PDP-11/04 com?uter (mar~eted by Digital Equipment Corporation, ~aynard, ~assa-c'nusetts) to enable automatic sample change, temperatu~e selection, event sequencing, and data collection, storage, reduction, and retrieval with a minimum of operator's involvement.

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The exposure of three commercially-available photo- ~
chromic glass samples to the solar simulator gave the following average values recited below. Approximate analyses in weight percent for each glass are also reported. The glasses marketed under the names PHOTOGRAY~*and PHOTOVITAR *
were measured in 2 mm thickness and exhibited clear luminous transmittance of about 90-92%, whe_eas the glass marketed under the name SU~SITI~ETM*is a sunglass product p.oGuced from 1.5 mm thick sheet. That glass demonstrated a clear I luminous transmittance of about 70-72%. YD designates the darkened transmittance and YF5 represents the transmittance of the sample fi~e minutes after removal from exposure to the solar simulator source.
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~ ~ 86 ~ 4 Several general conclusions can be reached from a study of the above data. First, each glass darkens to a lower transm'ttance when the temperature of exposure is lower. The PHOTOVITAR glass does not darken to a very great extent at high ambient temperatures, but darkens to low levels at low temperatures. The PHOTOVITAR glass displays more rapid fading than either of the other two specimens, but none of the glasses fades rapidly at low temperatures.
This sluggishness in fade rate at low temperatures, however, may not be of substantial ~ractical significance since, in many instances, the glass will be warming up during the fading process. For example, the wearer of ophthalmic lenses will be coming indoors from being outside on a cold day and, as can be observed from the above data, the fade rate increases as the te~perature rises.

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~ 2 4 Table II illustrates the completeness of fade or the long ~erm fading characteristics of Examples 5 and 7 of Table I when measured at 27C. in 1.5 mm thickness. YO
represents the clear luminous transmittance, YD20 and YD60 designate the darkened transmittance after exposures of 20 minutes and 60 minutes, respectively, to the solar simulator source, YFs, YF60~ and YF overnight indicate the trans~ittance after five minutes, 60 minutes, and an over- ;
night (-8 hours), respectively, removal from the solar simulator source, and YF60lYo reflects the percentage to t which the glass has faded after 60 minutes with respect to the original luminous transmittance.

TABI E II

Example Y ~20 ~ 60 ~ 5 YF60 YF overnight ~6~o 69~ 22% 20% 45% 65X 67.3% 94Z
7 89% 28% 26% 60% 83% ~6.5~ 93%

The best possible fade rates can be achieved in those compositions where PbO is present but at low values. Thus, glasses displaying the most rapid fading rates, i.e., glasses wherein the luminous transmittance after five minutes of fading at 25-30C. can exceed 2.25 times the transmittance in the darkened state, have compositions consisting essen-tially, in weight percer.t on the oxide basis as calculated from the batch, of about 57.1-65.3% SiO2, 9~6-13~9~/o A12O3, 12.0-22.0% B2O3, 1.0-3.5% Li2O, 3.7-12.0% Na2O, 0-5.8,' K2O, 6-15~/o total of Li2O + Na2O + K2O, the molar ratio Li2O:Na2O
+ K2O not exceeding about 2:3, 0.15-0 . 7% PbO, 0.10-0.30,' Ag, 0.2-1.0% Cl, 0-0.30% Br, 0.002-0.02% CuO, 0-2.5% F, 0-1.0%
total of transition metal oxides selected in the indicated Z~
proportions from the g-oup consisting of 0-0.5~O CoO, 0-1.0%
NiO, and 0-1.0~,' Cr2O3, and 0~5.02 total of rare earth metal oxides selected from the group consisting of ErzO3, Pr2O3, Ho203, and Nd203.
Glasses manifesting similar excellent fading rates which can be drawn as sheet and the sheet then simultaneously sagged to yield lenses of desired curvatures and photochromic properties developed therein, as has been described above, have compositions falling within the same ranges set OUt immediately above except for the Ag and Br contents. Those constituents, as analyzed in the glass, will vary as 0.12-0.18 Ag and 0.06-0.13% sr.
The photochromic properties of the drawn sheet a-e self-evidently affected to some degree by the heat t~eatment employed to develop those properties. This situation is particularly true when the temperature range of treatment is strictly Limited because of the requirements of the s~multaneous heat treating-sagging process. However, those properties are also critically depender.t upon the composition of the glass. Thus, changes in esser.tially any of the glass com-ponents will result in modifications of the photoch-omic behavior. For example, r,ot only will variations in the "photochromic elements", i.e., silve_, the halides, and copper oxide, alter the photochromic characteristics of a glass, but also, albeit to a lesser e~tent, wnll changes in the levels of alkali metal oxide, SiO~, B203, and PbO.
Table III lists several exemplary glass compos tions in parts by ~eigh. within the scope of Ur.ited States Pa~er.t ~o.

4,018,965, but outside the scope of the instant ir.vention, which demor.strate poor fading characteristics. ~his failure is attributed to compositional differer,ces. In addition, sheet of ~xample B cannot be simultaneously heat treated to 36'Z~

develop desired photochr~mic properties while being sagged into lenses ha~ing curvatures of 4-6 diopters. The concen-trations of the glass ingredients are delineated in parts by weight on the oxide basis as calculated from the batch for each glass, except for silver and the halides which are recorded on the elemental basis. Batch amounts of Ag, Cl, and Br are recited to the left of the slash marks and analyzed values to the right. The glasses can be compounded and melted in like manner to the description underlying Table I.
The working examples 1-6 reported in Table I of United States Patent No. 4,018,965 can ser~e as additional glass compositions outside the scope of the instant inventive glasses, again displaying poor ~ading characteristics.

TABLE III

A B C
__ SiO2 59.1 59.1 58.2 B2O3 17.5 - 17.5 17.5 A123 11.5 11.5 ~ 11.5 Li2O 2.0 2.0 2.0 Na2O 7.7 7.7 6.7 K2O - - 1.5 PbO 2.2 2.2 2.2 Ag 0.23/0.18 0.27/0.22 0.23/0.18 Cl 0.37/0.35 0.37/0.35 0.26/0.24 ~r 0.15/0.12 0.22/0.19 0.14/0.10 CuO 0.023 0.023 0.018 F 0.23 0.23 0.23 Table IV reports further e~emplary glass compositior.s in parts by weight, the glasses being within the broad 1~18624 purview of the instant invention but outside of the pre-ferred ranges of compositions. That is, the glasses demon-strate the desired photochr~mic properties, but cannot utilize the same heat treatment to develop photoch~mic behavior while sagging sheet to desired lens curvatures.
Again, batch quantities of Ag, Cl, and Br are recited to the left of the slash mar~ and analyzed values to the right.

TABLE IV

12 l3 l4 15 l6 17 SiO2 60.4 60.4 60.4 60.4 62.0 59.3 B203 17.7 17.7 17.7 17.7 16.7 17.8 Al2O3 11.8 ll .8 11.8 11.8 9.4 11.4 Li20 2.1 2.1 2.1 2.1 1.9 2.1 Na2O 5.9 5.9 5.9 5.9 3.8 5.8 K2O 1.6 1.6 1.6 1.6 4.9 1.6 PbO 0.25 0.25 0.25 0.25 0.5 1.0 Ag 0.11/0.08 0.31/0.20 0.31/0.20 0.25/0.22 0.30/0.20 0.25/0.21 Cl 0.56/0.39 0.67/0.39 0.37/0.22 0.35/0.31 0.30/0.19 0.35/0.31 Br 0.20/0.11 0.20/0.09 0.19/0.11 0.15/0.11 0.20/0.12 0.20/0.12 CuO 0.006 0.006 0.006 0.005 0.012 0.010 F 0.23 0.23 0.23 0.22 - 0.22 It is believed that the amo~nt of Ag is too low in E~ample 12 and too high in Examples 13-17.
The optional addition of the above-described transitior.
metal oxide and rare earth metal oxide colorants to the glass compositions of the instant ir.vention can be useful in securing some light atter.uation and coloration in the faded state, customarily for cosmetic purposes, and also to provide some coloration and attenuation in the darkened state.

~ 6 2 4 Nevertheless, caution must be exercised in selecting colorants for these photochromic glasses because the effecti~eness of multivalent colorant ions is frequently strongly dependent upon the oxidation state of the glass. Furthermore, some colorants absorb ultraviolet radiation, thereby reducing the darkening potential of the glass. For these reasons the foregoing recited transition metal and rare earth me~al colorants are preferred. Nonetheless, minor amounts of additional colloidal or ionic colorants such as uranium, cadmium sulfide, cadmium selenide, metallic gold, or the like can be included provided such additions do not dele- j teriously affect the photochromic properties of the glass. I
Table ~ records specific e~amples of tinted glass compositions falling within the scope of the instant inven- ;
tion illustrating the use of several of the preferred colorants and the colors induced thereby. The base com-position for each example was Example 8 of Table I such that only the concentrations of the colorants, in parts b~ weight, are tabulated. The correspor.ding and melting practices utilized with the glasses of Table I were also employed here.

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To aid in fur,he- underst~ndi~g the psDduct~on praction for fa~ric~ .g d awr. sheet glass c~-ti_~ es ~n accor~anc~
with the instar.t in~ention, the following w~rking example is provided EXA~PLE

A glass batch was compounded and mel,ed a~ a tempera-ture of a~out 1400C , the batch ha~ing a composition, in parts by weight, of about 60.4 SiO2, 17.7 B~03, 11.8 A1203,

5.9 Na2O, 1.6 R2O, 2.1 Li2O, 0.28 PbO, 0.25 Ag, 0.66 Cl, 0.20 8r, 0.23 F, and 0.~05 Cu0 The molten glass was fed into a refractory ~verflow dow~draw fusion pipe at a ~iscosity of about 104 poises and delive ed fro~ the pipe as drawr. glass shect about 1.5 mm in thickness. The trawr. sheet was cooled below the glass softer,ing point and sep2rated into sections of sheet glass from which small samples of desised geometries were cut. (Analyzed Ag ~ 0.16~, sr = 0.10%.) The sheet glzss s~mples we-e then exposed to a heat treatmen; to de~elop photochromic propert~es ;he-e~r,, the heat treat~ent comprising heating the sam?les in a lehr, i"
a marner such as is desc-ibe~ in Canadian Applicatior.
Serial ~o. 274,855, supra, th2t is, edge suppo-red on alveo-lated ~olds to prevent s~rIace damage thereto, at a rate of a~out 600C./hour to 640C. holding tha; te~?erature _or 10 ~inutes ;o sag the Elass into the conca~e portions of the alveola;ed molds, cooling the samples at 600C /hour to at least below 4~0C., and ther, removihg the samplcs rom the lehr.
The photochr~mic glzss ss~ples were then s~bjected to a che~ical st-engthenir,g treatment which invol~et immersing the samples into a bath of ~ol~en NaN~3 operating at 410C

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~ 11862~
for 16 hours. The samples were thereafter removed from the bath, cooled, the excess salt washed off with tap water, and tested for strength and photochromic properties.
Modulus of rupture values in excess of 45,000 psi were determined and the depth of the surface compression layers was observed to vary between about 0.0035-0.004".
The fully faded luminous transmittance of a typical 1.5 mm thick photochromic drawn sheet glass article produced in the manner described above is about 90~/O. After exposure for 60 minutes to the solar simulator source at 25 C., a darkened luminous transmittance of about 26~/o is measured. After a five-minute withdrawal from the solar simulator source, the glass commonly fades about 34 luminous percentage units to a transmittance of about 60%. The glass will fade to a luminous transmittance of about 83~/o after one hour, this value being about 92% of the original transmittance.
Upon exposure to the solar simulator source for 60 minutes at 40C., a darkened luminous transmittance of about 45% is read. At -18C., a darkened luminous transmit.ance of about 22% is measured after a 60-minute exposure.
The foregoing example, which is merely illustrative and r,ot limitative of the various compositions and procedures operable in the instant invention, clearly demonstrates the effectiveness of the inventive compositions in producing strengthened photochromic dra~ sheet glass articles exhibi.ing the r.ecessary properties for ophthalmic and other applications.

Claims (20)

WE CLAIM:

1. A glass composition consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 54-66% SiO2, 7-15% Al2O3, 10-25% B2O3, 0.5-4% Li2O, 3.5-15% Na2O, 0-10% K2O, 6-16% total of Li2O + Na2O + K2O, 0-1.25% PbO, 0.1-0.3% Ag, 0.2-1% Cl, 0-0.3% Br, 0.002-0.02%
CuO, and 0-2.5% F, having a viscosity at the liquidus of at least 104 poises, long term stability against devitrification in contact with platinum at temperatures corresponding to glass viscosities in the range of 104-106 poises, excellent chemical durability, and being chemically strengthenable to modulus of rupture values in excess of 45,000 psi with a depth of compression layer between about 0.0035-0.004", said glass, in bodies of about 1.3-1.7 mm cross section, exhibiting the following photochromic properties:
(a) at about 25°-30°C. will darken to a luminous transmittance below 30% in the presence of actinic radia-tion; will fade to a luminous transmittance at least 1.75 times the darkened transmittance after five minutes' removal from the actinic radiation; and will fade to a luminous transmittance in excess of 80% of its clear luminous trans-mittance in no more than one hour after being removed from the actinic radiation;
(b) at about 40°C. will darken to a luminous trans-mittance below 50% in the presence of actinic radiation and will fade to a luminous transmittance in excess of 80% of its clear luminous transmittance in no more than one hour after being removed from the actinic radiation;
(c) at about -18°C. will not darken to a luminous transmittance below 5% in the presence of actinic radiation;
and (d) in the undarkened state will exhibit a luminous transmittance of at least 60%.

2. A glass composition according to claim 1 which also contains up to 1% total of transition metal oxides and/or up to 5% total of rare earth metal oxides as colorants.

3. A glass composition according to claim 2 wherein said transition metal oxides are selected in the indicated pro-portions from the group consisting of 0-0.5% CoO, 0-1.0%
NiO, and 0-1.0% Cr2O3, and said rare earth metal oxides are selected from the group consisting of Er2O3, Ho2O3, Nd2O3, and Pr2O3.

4. A glass composition according to claim 1 capable of being sagged into lenses of a desired curvature while simultaneously developing the recited photochromic prop-erties consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 57.1-65.3% SiO2, 9.6-13.9% Al2O3, 12-22% B2O3, 1-3.5% Li2O, 3.7-12% Na2O, 0-5.8% K2O, 6-15% total of Li2O + Na2O + K2O, the molar ratio Li2O: Na2O + K2O not exceeding about 2:3, 0-1.25%
PbO, 0.12-0.24% Ag, 0.2-1% Cl, 0.06-0.25% Br, 0-2.5% F, and 0.002-0.02% CuO.

5. A glass composition according to claim 4 which also contains up to 1% total of transition metal oxides and/or up to 5% total of rare earth metal oxides as colorants.

6. A glass composition according to claim 5 where said transition metal oxides are selected in the indicated proportions from the group consisting of 0-0.5% CoO, 0-1.0%
NiO, and 0-1.0% Cr2O3, and said rare earth metal oxides are selected from the group consisting of Er2O3, Ho2O3, Nd2O3, and Pr2O3.

7. A glass composition according to claim 1 which at 25°-30°C. will fade to a luminous transmittance at least 2.25 times the darkened transmittance after five minutes' removal from the actinic radiation consisting essentially, in weight percent on the oxide basis as calculated from the batch, of about 57.1-65.3% SiO2, 9.6-13.9% Al2O3, 12-22% B2O3, 1-3.5%
Li2O, 3.7-12% Na2O, 0-5.8% K2O, 6-15% total of Li2O + Na2O
K2O, the molar ratio Li2O:Na2O + K2O not exceeding about 2:3, 0.15-0.7% PbO, 0.1-0.3% Ag, 0.2-1% Cl, 0-0.3% Br, 0.002-0.02% CuO, and 0-2.5% F.

8. A glass composition according to claim 7 which also contains up to 1% total of transition metal oxides and/or up to 5% total of rare earth metal oxides as colorants.

9. A glass composition according to claim 8 wherein said transition metal oxides are selected in the indicated pro-portions from the group consisting of 0-0.5% CoO, 0-1.0%
NiO, and 0-1.0% Cr2O3, and said rare earth metal oxides are selected from the group consisting of Er2O3, Ho2O3, Nd2O3, and Pr2O3.

10. A glass composition according to claim 7 capable of being sagged into lenses of a desired curvature while simultaneously developing the recited photochromic properties wherein, as analyzed, said Ag content ranges between 0.12-0.18% and said Br ranges between 0.06-0.13%.

11. A glass composition according to claim 10 which also contains up to 1% total of transition metal oxides and/or up to 5% total of rare earth metal oxides as colorants.

12. A glass composition according to claim 11 wherein said transition metal oxides are selected in the indicated proportions from the group consisting of 0-0.5% CoO, 0-1.0%
NiO2 and 0-1.0% Cr2O3, and said rare earth metal oxides are selected from the group consisting of Er2O3, Ho2O3, Nd2O3, and Pr2O3.

13. A method for simultaneously shaping articles from glass sheet and developing photochromic properties therein which comprises the steps:
(a) melting a batch consisting essentially, in weight percent on the oxide basis, of about 57 .1-65.3% SiO2) 9.6-13.9% Al2O3, 12-22% B2O3, 1-3.5% Li2O, 3.7-12% Na2O, 0-5.8%
K2O, 6-15% total of Li2O +Na2O +K2O, the molar ratio Li2O:Na2O
+ K2O not exceeding about 2:3, 0-1.25% PbO, 0.12-0.24% Ag, 0.2-1% Cl, 0.06-0.25% Br, 0-2.5% F, and 0.002-0.02% CuO;
(b) adjusting the temperature of at least one region of the glass melt to provide a viscosity therein of about 104-106 poises;
(c) drawing the glass melt at a viscosity of about 104-106 poises directly past refractory forming means to produce potentially photochromic drawn glass sheet;
(d) cooling the glass sheet below the softening point of the glass and cutting articles of desired geometries therefrom;
(e) edge supporting said articles on alveolated molds; and then (f) heating said articles at a temperature between about 610°-660°C. for a period of time sufficient to simul-taneously sag the glass into the concave portions of the alveolated molds and develop photochromic properties in the glass.

14. A method according to claim 13 wherein said time sufficient to simultaneously sag the glass and develop photochromic properties therein ranges from about 6-15 minutes at temperatures between 610°-640°C. or from about 5-12 minutes at temperatures between 640°-660°C

15. A method according to claim 13 wherein said batch also contains up to 1% total of transition metal oxides and/or up to 5% total of rare earth metal oxides as colorants.

16. A method according to claim 15 wherein said transition metal oxides are selected in the indicated proportions from the group consisting of 0-0.5% CoO, 0-1.0% NiO, and 0-1.0%
Cr2O3, and said rare earth metal oxides are selected from the group consisting of Er2O3, Ho2O3, Nd2O3, and Pr2O3.

17. A method according to claim 13 wherein the content of PbO in said batch ranges between 0.15-0.7%.

18. A method according to claim 17 wherein said batch also contains up to 1% total of transition metal oxides and/or up to 5% total of rare earth metal oxides as colorants.

19. A method according to claim 18 wherein said transition metal oxides are selected in the indicated proportions from the group consisting of 0-0.5% CoO, 0-1.0% NiO, and 0-1.0%
Cr2O3, and said rare earth metal oxides are selected from the group consisting of Er2O3, Ho2O3, Nd2O3, and Pr2O3.

20. Method in accordance with claim 13, wherein the glass is sagged into the concave portions of the alveolated molds without establishing contact with the mold surface, thereby achieving an optical quality surface without grinding and polishing.