CA1057107A - Phototropic ophthalmic glass - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A phototropic glass containing P2O5 as the principal component and, as agents imparting phototropy, silver and halogen in non-crystalline form.

Description

BACKGROUND
~ vnriety o phototropic gla~RcR nre fllrendy known, Enrly work in tho field 18 di~closed in Armi~tent U,S, pntent 3,208,8GO (Gcrmnn 1,421,838) where tha bAsc ~lnss ls n ~illcnte or n boroslllcntc glnss, The nctlvnting ngent, l,e, the componcnt lmpnrtlng phototropy, la sllver hnlldeln the form of .
crystnls or mlcrocrystnl~, ~

.

Rasper 223.1 CIP FMM
FMM/w~p , 1057107 December 27, 1974 1 Later, phototropic gla~ses in which the bs~e glass i8 a

2 borate glass, were developed. Nere ~180 the sctivatlng agent is

3 s$1ver halide ln the form of crystAls. Such glasses are dis-

4 closed in U.S. patent 3,548,060 assigned to Nippon, and Glie-meroth U,S. pntent 3,834,912 (German 1,596,847).
6 Phototropic glasses in which the base glass is a phosphate 7 glass are al80 known. Sakka and Mc~enzie, J. Amer. Ceram. Soc.
55, 1972, 553 disclose such glasses. The activ~ting agent or 9 combination imparting or imparting and affecting phototropy is thallium chloride, or thallium chlorlde and Cu20, or thallium 11 chloride and silver oxide. Those authors, in U.S. 3,615,761, 12 disclose phosphate phototropic glasses in which the activating 13 agent is thallium chloride. A descriptlon of phototropic phos-14 phate glasses that is comprehensive in regard to the breadth of the possible compositions i8 available in German Offenlegungs-16 schrift (DOS) 2,234,283, assigned to Pilkington Brothers, Ltd.
17 Here al80, silver halide crystals dispersed in the glass result in lB phototropy. Thus, the last mentioned phototropic phosphate 19 glasses differ from phototropic gls~ses on a silicate, boro-silicate, or borate basis, such as are described above, only in 21 the composition of the base glass.
28 Glssses of one or more oxides are composed of networks of 23 the prlncipal components. The prlncipal components are bound 24 to the network ln the form of coordination polyhedra Jolned at the apexes. These structure units (prlnclpal components) 26 are S104 tetrahedra ln slllcate glasse~, for ex~mple; similar 27 units of structure exist in borate snd phosphate glasses. These Zd ¦ unlt~ oE ~Lruc ure re bound~d togeLher with dlfEerent ~trengtho . 1057107 sper 223.1 CIP-FMM
De~ember 27, 1974 1 in dlfferent glasse~. The more strongly the central cation~
of these ~tructures polarize the surrounding oxygen~, the ~ore 3 weakly they ~re bonded to ad~acent units of structure (e.g., to the ad~acent tetrahedron~. The polarization of the oxygens by the central cations is, according to Dietzel in Z. Elektrochem.
6 48 (1942) 9, as follows:

8 Table l CentralValency in Distance,a, in Angstroems Field 9 Cation the Glass Between Cation and Oxy- Stren~th z gens,a Z/8 11 Si 4 1.60 1.56 12 B 3 1.36 1.62 13 p 5 1.55 2.08 1~
The greater the field strength of the central cation of 16 such a structural unit i5, the more strongly the surrounding 17 oxygen envelope is polarized and the weaker becomes the bond 18 of this unit of structure (conListing of the central cation and l9 the surrounding oxygen envelope) externally to the ad~acent units of structure.
21 Accordingly, the strength of the bond between the principal 22 components diminlshes, i.e., the overall glaos otructure loosens 23 up, a9 one passes from silicate glasses to borate glasses or even 24 to phosphate glasses.
In Gliemeroth U.S. patent 3,834,912 (German Pat. 1,596,847), 26 phototropic glasses are di~closed whose phototropic properties 27 sre determined by silver halide crystals and in some 2B cases small amounts oi metallic silver, and which are R~aper 223.1 CIP-FMM
1057107 F~w~p December 27, 1974 1 dlstlnguished in that they consist of one or more glass-fonming 2 oxides a8 principsl components whose bond to one ~nother in the 3 glass is weaker than the bontfl in 8 silicate ba~e gla~s with 4 S102 as the glsss-forming component.
In particular Gliemeroth U.S. patent 3,834,912 (German 6 Pat. 1,596,847), describes borate glasses which have better 7 phototropic properties than the silicate glasses which were known 8 at the time, on account of the weaker bond between the principal 9 components.
From the above considerations, which are generally 11 accessible to the technical world, it appears that the phos-12 phate glasses must also be ~uitable for the production of 13 phototropic glass providing a good optical density varistion g under the influence of impinging light rays.
lS If, in accordance with the teaching of Gliemeroth U.S.
16 patent 3,834,912 (German Pat. 1,596,847), the principal 17 component B203 is replaced by P205, the bond between the 18 principsl components will become weaker in the resu~ing glass.
19 On the basis of the examples given in the said patent we will then have the following compositions, which do have phototropic 21 characteristics, but whose phototropy is of poorer quality, To 22 facilitate comparison compositions nre given in parts by 23 welght, as 18 done ln Gllemeroth V.S, patent 3,834,912 (German 24 P~t. 1,596,847).

~ ~

~057107 Table 2 1 P205-Modified Gllemeroth 3,834,912 G1A88 es 2 ~ 2 3 4 5 6 3 P205 14.9 81.4 47.8 71.0 67.5 73.6 4 PbO 69.4 - 35.6 - - -MgO - - - 15.4- 13.5 6 B~O ~ 15.4 7 ZnO 9.90 10.2 - - - 9.20 8 A1203 1.98 - 12.5 9.60 9 N~20 0.10 - _ _ _ _ K20 - 5.08 - - - 7.40 11 KCl - - o 0.48 - 0.92 12 KBr 1.49 1.53 1.44 1.44 1.45 1.38 13 Rl 1.49 1.53 1.44 1.44 1.45 1.38 14 LiF 0.50 - 0.96 0.29 0.29 0.28 Ag20 0.19 ~.30 0.29 0.38 0.38 0.37 16 CuO 0.005 - 0.01 - 0.02 0.02 lq K2Cr27 ~ ~ 0.01 0.005 0.01 13 Zr2 _ _ _ ~ _ 5.52 19 . ~
99.555 100.04100.04 100.04 99.995100.08 2~

2fl ~ 5 - .

lOS7107 sper 223.1 CIP-FMM
.. December 27, 1974 l As it h~s slre~dy been explained in Gliemeroth U.S. patent 2 3,834,912 (German Pat. 1,596,847), the uBe of small amounts of 3 SiO2 contributes toward stabiliza~ion. This m~kes the ~tructure 4 of the glass stronger again. In accordance, ~herefore, with Gllemeroth U.S. patent 3,834~912 (German Pat. 1,596,847), glasses 6 are possibLe, in view of the above consideration, which are 7 listed by way of example in Table 3. The~e glMs~es are listed B ln pflrts by weight to facilltate comprehension, a8 is done in 9 the Ger~an patent.

11 , .

~6 2~

, ``;,',. . i., : 105~107 Table 3 1 SiO2- and P205- Modified GlLemeroth 3,834,912 Glssses 4 SiO2 4,49 3.10 2.99 8.55 7.46 B203 6.00 5.05 4.00 7.05 7.56 6 P2O5 39.33 33.75 32.73 30.19 34.62 7 PbO - 5.79 27.35 - 1.99 8 MgO 6.99 11.57 1.54 3.16 -9 B~O 10.93 12.7B 4.61 6.52 5.17 ZnO 0.44 ~ - - ~
11 C~O - - - 6.92 4.87 18 zro2 1.75 - _ _ 1.99 1~ Ti2 ~ ~ 0.49 o.gg 14 A120317,48 12.05 9.60 21,51 18,90 Na20 4.81 12.15 6.45 5.93 5.97 16 K2O 6,12 1.93 4.07 6.92 8.46 lY RCl 0.70 0.39 2.31 0.99 0.90 18 KBr 0.35 1.16 1.38 0.99 0.40 19 KI 0.09 LiF 0.36 - 0.77 - 0.50 21 Ag20 0.17 0.29 0.19 0.18 0,18 22 CuO 0,02 - ~ 0,02 0,04 23 KHF2 ~ - - -S9 ~4 100.03 100,01 97.99 100.01 100.00 ~ - 7 -~sper 223 1 CIP-FMM
1057107 ~w~p -December 27, 1974 1 Examples 7 to 11 also show that d certnin amount of B203 2 can be incorporated into such phototropic phosphate glas~es.
3 Phosphate glasses sre generslly known for their pareicu-4 larly poor chemical stability. This cBn be counteracted by a high A1203 content, but this will result in a further impair-6 ment of the already poor resistance to devitrification.
7 A glass which is to be used in eyeglass lenses (long-focus 8 portion of bifocal lenses) must satisfy certain requirements 9 in regard to commercial prof~tability. This means that the yield from normal production machinery and equipment must be 11 satisfactory and the product must not be the cause of great 12 numbers of complaints.
13 In the glass making apparaeus commonly used today for 14 production of glass for eyeglass lenses, glasses can be worked which assure a throughput of more than 60 kg/h. To this end the 16 glass must be cut into portions by shearing, at a viscosity of lq: 103 to 104 5 poises, in order then to be pressed into blanks.
18 At this viscosity, which is relatively high for molten glass, 19 many non-silicate glasses devitrify at such high rates that profitable production becomes impossible. To counteract this 21 defic~ency it is possible to resort to viscoslty reduction and 22 to special mo~hods of production (lna~much as portioning by 23 shearing is no longer possible at low viscosities), or to accept 24 8 reduction of output by temporarily interrupting production and clearing up any devitrification thnt may have occurred in 26 production by briefly raising the temperature.
27 All of the gla~ses of the aforementioned Pilkington DOS
8d ¦ 2,234,283 whic dl~clo~e~ phc~ph;te gl~e~ octlvated wlth i057107 s~lver hallde crystals, have Been melted again with this in mind and tested for devitrification. After 60 melts it is apparent that those compositions do not-qualify for normal production, discussed above, on a technical scale.
The qualifications which any glass must meet for use in eyeglasses are established by the following specifications which are generally recognized by the industry:

a) Index of refraction nd between 1.5225 and 1.5238 b) No devitrification between 102 and 104-5 poises.

c) Sufficient chemical stability (characteri~ed by resistance to hydrolysis in accordance with DI~ 12,111 and by the ability to withstand the sweat test).

t) Sufficient chemical hardenability in the standard bath for normal optical crown glass (this requirement normally applles wherever strength improvemen~ is legally prescribed for all eyeglass lenses).

_9_ E
m~p/

--" 10571~7 TH~ ~Ny~NTION
The ob~ec~ of ~he present invention is an ~mproved photo-tropic optical glass for eyeglasses, meeting the above-mentioned requirements, and which comprises one or more glass forming oxides as principal component or components, whose bonding together in the glass is weaker than the bonding in a silicate basic glass containing sio2 as the main glass forming component, and likewise is weaker than it is in a borate base glass con-taining B203 as the principal glass forming component.
Another ob;ect of the invention is an improved phototropic spectacle glass having an index of refraction nd between 1.5225 and 1.5238 as well as the other requirements of (a) to (d) above Another object of the inven~ion is an improved phototropic spectacle glass having little or no devitrification in the viscosity range between 101 and 105 poises, having a chemical stability that is sufficient for use in the long-focus portion of bifocal eyeglass lenses, and preferably having a linear thermal expansion coefficient between 20 and 300C of below 105 X 10-7 per C, e.g. from 99 to 105 x 10-7 per C.
Another ob~ect of the invention is an improved phototropic spectacle glass which can be strengthened by an ion exchan~e below the transformation temperature in a medium containing potassium ions, in which smaller alkali ions are able to diffuse out of the glass.
These ob~ects are achieved by the invention in that, in the phototropic glass of German Pat. 1,596,847 (U.S. 3,834,912), B~09 is replacet by P~0~, and that the phototropic properties m~p/

I 10 7107 Razper 223.1 CIP-F~ l December 27, 1974 1 ln the gla~s of the invention are produced through the crea~ion 2 of ~ilver-rich and halide-rich, noncrystalline, separation 3 phase~ in this glass.
4 In accordance with the invention, the P205 content i8 best between 30.4 and 33.9% by weight.
6 Furthermore, in accordance with the invention, the A1203 7 content is best increa~ed from 14 wt7. to about 23 wt-%, a suit-8 able range being between 22.5 and 25.7 wt-%.
9 The conventional phototropic glasses containing silver snd halogen display good phototropy only when they contain 11 B203; B203-free glas~es of the prior art have either little or 12 unsatisfsctory phototropy. In Table 4 are listed glasses with 13 B203 and which are prlor art glasse~, and the same glasses 14 except without B203, showing their phototropic properties. To facilitate comprehension, this table is given in parts by 16 weight. The compositions given are on a batch basis. The 17 glasses are not according to the invention.

~3 ~7 28 -11-`

Rflsper 223.1-CIP-FMM
10571(~7 F~a8p . ~ October 10, 1974 Table 4 1 Effect of B203 in Prior Art Glasses 2 12 13 14 15 16 17 ~ 18 19 ~ . _.. . _._ SiO2 61.5 61.5 7.5 7.5 9.1 9.1 SO.O 53.0 4 ~23 ~ 16.6 ~ 4.0 ~ 5-0 ~ 18.0 P205 - - 36.5 36.535.0 35.0 2.0 2.0 . . .
6 A1203 10.7 10.7 22.5 22.525.0 25.0 8.0 8.0 7 Zr2 ~ ~ - ~ _ _ 1.0 1.0 B Nfl20 10.7 10.~7 6.5 6.5 6.5 6.5~4.0 4.0 9 R20 - - 6.5 6.5 8.0 8.0 1.0 1.0 MgO - - 4.4 4.4 3.4 3.4 2.0 2.0 11 CeO - - 9.0 9.06.9 9.0 12 B80 - - 6.7 6.76.7 6.7 4.0 4.0 PbO - - - - - - 6.0 6.0 ~12 - - 1.0 2.00.5 0.5 16 Ag20 0.6 0.6 0.14 0.14 0.18 0.18 0.42 0.42 17 CuO 0.01 0.01 0.02 0.02 0.01 0.01 1~ Cl - - 0.6 0.60.6 0.6 1.0 1.0 19 Br 0.8 0.8 0.3 0.30.3 0.3 0.5 0.5 ~1 ~i i~Z 106.76 io2~ 109,5979.92 100.92 21 Anne~llng 1 h 1 h 2 h 2 h1 h 1 h 1 h 1 h . . . .. .. ..
23 Sfltur~tlon 40~. 6% 53% 12%687. No 44%
Trflns- No Darken-24 mlsslon Dsrken- ing after ex- lng posure to 26 llght Regener-27 ~fter 10 20% 2% 24% 4X 277._ 177.
2B mlnutes . . ..... ... .. . . . . . . . .. . ..

0 5 7 1 0 7 R~per 223.1 CIP-F~
~w~p .. December 31, 1974 1 From Table 4, lt appears that the omis~ion of the com-2 ponent B203 impairs phototropy. The effort has slwayY been 3 made, therefore, to lnclude a minimum of B203 in the glass 80 4 ~8 to favor the precipitation of the silver hallde crystals (microcrystals, as they are described, for example in German 6 patent 1,421,838 (Armistead U.S. patent 3,208,860) end German 7 patent 1,596,847 (Gliemeroth U.S. patent 3,834,912))and prevent 8 it from being disturbed by other phase separations.
9 It has now quite surprisingly been found that, even in the complete absence of B203, glasses having phototropic properties 11 csn be obtained, which are at least equal in quality to those of 12 glasses in which the phototropy is produced by silver halide 13 crystals (the ~ust mentioned German patents 1,421,838 and 14 1,596,847, and the aforementioned Pilkington DOS 2,234,283). The glasses of the invention can even display phototropy which is 16 superior to that of the known glasses as regards degree of 17 darkening and speed of regeneration.
18 It has still more surpris~ngly been found that in the 19 glasses of the invention the phototropy i9 brought about, not by silver halide crystals, but by noncrystalline separation phases 21 rich in Hilver halide.
22 To achieve particularly suitable precipitatlon conditions 2S for the noncrystalline silver-rich and halogen-rich separation 24 phase~, the maximum phosphorus pentoxide content should be 33.9% by weight. If the P205 is less than 30.4, phototropy is 26 poor, melting is rendered difficult, and the refrflctive index is 27 too high, Zd Also, to prevent crystallization of the silver-rich and ~ -~ Rasper 223.1 CIP-FMM
1057107 FW/w~p .. Dece~ber 31, 1974 1 halogen-r~ch sepsrstion phases, the halogens must be e~pecislly 2 celected. Flourine is not used ~8 the hslogen even in 8mall 3 concentrstlons it appesrs to promote the crystsllization of the r~e J~ 4 separstion phaseq. It is best lf the glass i8 free of flourlnc It has furthermore been found that, for profitable m~nu-6 facture, a viscosity which i8 relatively high for phosphate 7 glasses, ranging between 101 and 105 poises, is necessary, but 8 that st a silicon dioxide content between 12.1 and 13.9 wt-~ it 9 18 in general simultaneously necessary to keep the MgO content as low as possible, preferably at O wt-% since MgO has a great 11 influrence on the crystallization of the base glass (this base 12 gl~ss crystallization is to be distinguished from the crystal-13 lization of the silver h~logen crystals or of the silver 14 halogen-rich, noncrystalline separation phases, as the case may be, which are the phototropic agents).
16 Devitrification i8 also influenced by other components.
17 Thus, it has been found thst the titanium dioxide concentration lB should best not exceed 0.6 wt-~, but on the other hand thst 19 zirconium dioxide should best be present at least in a concen-tration exceeding 1 wt-%, although both of these components sre 21 known to be nucleating agents which promote devitrification.
22 For optical resson~ the msximum zirconlum dioxide content is 23 best 2.6 wt-%, because otherwise the index of refraction of the 24 glas~ would exceed the required level of 1.5225 to 1.5238. The same applies to the maximum content of lanthsnum oxide nt 2.0 26 wt-%. Both components hsve an additional importance in 27 producing sufficlent ahemical stability in the glass. Further-28 more, zirconium dioxide should b;st be contained in the gla~s .

~ Ra~per 223 1 CIP-FMM
.. lOS7107 December 31j 1974 ~

1 in A mlnimum amount of 1.0 wt-% and lanthanum oxite in a 2 minlmum amount of 0.05 wt-%, since the~e components also are 3 the basia of sn effect on the precipitation of the noncrystal-4 line, silver halogen-rich ~eparation phase~.
The slkali oxide snd a~kali earth oxide (CaO, BaO, SrO) 6 content affect~ not only the kinetics of the precipitation of 7 the noncrystalline, silver halogen-rich separation phases, but 8 also the other propertie~ of the glas~. It has been found 9 that the sum of the alkali oxides should preferably be between 11.2 and 16.2 wt-%, which is e~pecially important also for the 11 possibility of strengthening the glass chemically by ion 12 exchange below 1014 5 poises in a medium containing potassium 13 ion~, e.g., in a melt of KN03, in which smaller alkali ions 14 diffuse out of the glass, It was found that, contrary to expectations, lithiu~ oxide should best be contained in the 16 starting glass in the smAllest possible concentration, and 17. preferably it should not be present at all, prior to the ion 18 exchange, ~lthough in the phototropic glasses known hitherto 19 lithium oxide plays an important part in the crystallization of the silver halide cry~tals.
21 Alkali earth oxides have a great effect not only on phase 22 separation but also on the linear coeficient oE thermal 23 expansion. It has been found that a total alkali earth oxide 24 content of at least 8.6 wt-~, but no more than 12.5 wt-%, is especiAlly favorable.
26 Barium oxide and calcium oxide can be used with good 27 results, but it i8 best if there i~ not more than 5.0 wt-% of 2B calcium oxlde because otherwise the precipitation of the silver ~ Rasper 233.1 CIP-FMM
1057107 F~wjp December 31, 1974 1 halogen-rlch, non-crystalline separation phase~ is disturbed.
2 Small Amounts of strontium oxide have a stsbllizing efect.
3 Lead oxide (PbO) i8 especially desirable at a content between 4 0.4 and 2.5 wt-%, can can be used for correction of the index of refraction the same as titanium dioxide.
6 The content of silver oxide snd halogens in the mixture 7 computed according to synthesis will depend on the melting pro-8 cess and the rest of the batch composition. Approximately 9 O.OS to 0.5 wt-% Ag, calculated a8 Ag20 and 0.2 to 1.0 wt-%
halogen, in particular chlorine and bromine, are desired ln the 11 glass. (The amount stated for the silver oxide and halogen is 12 computed for this purpose on the basis of the analytically 13 determined (as i8 hereinafter described) content of silver ions 14 and halogen ions present in the glass. A percentage of metallic silver is improbsble, but possible. Silver ions and halogen ions 16 do not need to be in a stoichiometric ratio to one another, since 17 other components are also contsined in the noncry6tslline, silver 18 halogen-rich separation phases.) 19 The desirable content of silver oxide and halogen in the batch synthesis, allowing for vaporization losses during the 21 melting is between 0.1 and 1.0 wt-% silvor oxide nnd, for the 22 halogen~ chlorine nnd bromine togethar, between 0.20 and 10 83 wt-%, The weight proportion of bromine and chlorine in the 24 batch may vary from a ratio of bromine to chlorine of 0-5 to 3-4, preferably 0-0.55 to 2.74. Copper oxide (CuO) may be 86 ndded for sensitiZAtion in amounts between O and 0.1 wt-%.
87 The evaporation of the halides depends on the metling unit 28 for the glass preparation. The amount of halide which remnins ~, -- ~ ~L

Rasper 223.1 CIP-FMM
1057107 F~w~p .. December 31, 1974 1 in the glass depends on the procedure followed. So ~ ~ore Z effective method of determinstion is given by a~alyzing the 3 produced glass with RFA (see infra). EspeciAlly the desirable 4 content of chlorlne and bromine ln the gla~8 i8 between 0.2 and 1.0 wt-~ in tot~l, preferably between 0.35 and 0.87 wt-%
6 with ~ preferable weight retio of bromine to chlorine of 1.8 to 7 4Ø Thu8, the correct amounts to be added to the batch can 8 readily be determined by simple testing of products' of trial 9 runs.
The preferred composition of the glasses (batch or snythesis lI ba~is) in accordance with the invention can therefore be 12 characterized by ~e foll~win~ ranges:
13 , Broad Ra~es Optimum Ran~es 14 S10212.1 to 13.9 wt-% 13.0 to 13.9 wt-Z
P2o530.4 to 33.9 wt-% 31.2 to 33.9 wt-%
16 A120322,5 to 25.7 wt-% 22.5 to 25.7 wt-%
17 ZrO21.0 to 2.6 wt-% 1.2 to 2.1 wt-%
lR Na203.0 to 7.S wt-% 4.7 to 7.5 wt-%
19 K205.3 to 10.5 wt-% 6.5 to 10.4 wt-%
CaO3.1 to 5.0 wt-% 3.1 to 4.5 wt-%
21 BaO3.1 to 7.0 wt-% 5.5 to 7.0,wt-~h 22 SrOO to 0.5 wt-% O to 0.3 wt-X
23 PbO0,4 to 2.5 wt-% 0.4 to 2.5 wt-%
24 TiO20.1 to 0.6 wt-% 0.1 to 0.6 wt-%
La2030.05 to 2.0 wt-% 0.05 to 2.0 wt-%
26 Ag200.10 to 1.0 wt-% 0.10 to 1.0 wt-%
27 Cu OO to 0.1 wt-% O to 0.05 wt-%
Cl + Br0.20 to 10.0 parts 0.20 to 10.0 parts per 100 p~rts of per 100 part~ of ' oxide~ ox~de~

na6per 223.1 CIP-FMM
1057107 F~w~p .. December 31, 1974 1 Chlorine and brsmine are used in an amount sufricient to 2 provide 0.2-1.0 wt.% of chlorine plu8 bromine in the gla88. The 3 gl~8 contains 0.05-0.5 wt.% silver.

4 The examples of composition (batch or synthesis basis) given ln Table 5 include the compo~itions of the invention (Examples 6 39 ~nd 54), and also include a lot of compositions which do not 7 fall within the scope of the invention; the property values 8 glven in Tables 6 and 7 show the narrowness of the range of 9 compositions of the invention. In the example~, the compositions given sre in wt.%, and the amount of halogen i9 wt. parts in the 11 batch per 100 wt. parts of the oxides.

12 In 811 of the comparison examples, devitrification 13 occurred, except in Example 32, in which , however, irrever-14 slble d~rkening occurred.
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u.~C G ~ g v ~ c_~,c ~ ~g a ~ ~ a o ~ o ,~ . o oO 3 ~ ,c w ~ O c w a, E~ ; a ~ ~ u~ 5 . 1057107 FMM/ssp . October 11, 1974 "~

8 The phototropic properties of the following base glass ~ compoaition (bstch or synthesis basis) of the invention have 4 been studied:
SiO2 13.17 wt-% introduced as clean ~and

6 P2O5 32.42 wt-~ introduced as phosphorous pentoxide 8 A1203 24.~2 wt-% introduced as aluminum hydrate Zr2 2.03 wt-% introduced as zirconium oxide 9 Na2O 6.08 wt-% introduced as soda, nitrate and chloride 11 K2O 9.12 wt-% introduced as pota~h and bromide 12 CaO 4.06 wt-% introduced as carbonate 13 BaO 6.23 wt-% introduced as carbonate and 14 nitrate SrO 0.05 wt-% introduced as nitrate PbO 1.42 wt-% introduced as minium TiO2 0.30 wt-% introduced as oxide 17 La O 0.20 wt-% introduced as oxide 18 2 39~--40-19 To this base glass various quantities of silver and halogen components were introduced in the form of silver nitrate and 21 sodium chloride or potassium bromide lnto various indlvidual 22 batches but with constant prep~ntion procedure to get approp-23 riate RFA-values of the silver and halide concentration 24 t9ee lnfra) ; from time to time the CuO sensitizing agent, introduced as copper oxide, wn9 vnried nlso. After the batch 26 was mixed the componentR were placed together in a platinum 27 melting crucible, The crucible was of 1 liter si2e; batches 28 were l.5 kg, The atch wa9 he;ted from room temperature to _~ Ra3per 22~ CIp-FMM
~ FMM/ssp . 1057107 October 11, 1974 .~ . . ., 1 1435C in 10 minute~, held at 1435C for 2 hours, 20 minutes; re-8 fined by stirrin8 at 1435C for 10 minutes. The batch was then 3 cooled to 1200C in 10 minutes, poured into steel molds9 ~nd 001ed at about 22 degree~ per hour to room temperature. Then ~ ~pecimens were prepared from each gla88 with the dimensions 40 x 6 40 x 5 mm. These specimens were he&ted at a rate of 50 degrees

7 every 10 minutes up to 615C, held at this ~emperature for 115 ~ mlnutes, and then cooled down at 190 degrees per hour to 300C.
., 9 The rest of the cooling down to room temperature was performed at 50 degrees per hour. On these specimens the relative content 11 of Ag, Cl and Br W88 tested by X-ray fluorescent analysi~ (RFA-12 values, see infra), and the index of refrsction, the phototropy 13 and the degree of crystallinity of the preclpitates which are 14 the agents of the phototropy, as well a8 the ~lze of the preci-~5 pitates were determined. Table 8 shows the results, The 16 addition of only 0.10wt-% F resulted, for example, in crystalli-17 zation of the agents of phototropy.
18 The index of refraction nd was determined with commercial 19 Abbe refractometer~. The vlscosity was measured in relation to temperatura with commercial rotational viscosimeters, The 21 chemical ~tability was identlfied by the titrimetrically 22 determined values of the alkali leachlng 'n ~ccordance with ~3 DlN 12,111. The ion exchange was Judged by preparlng thln 24 seotions of ion-exchanged glass, the section being taken per-pendlcularly to the surface of the glass. The thlckness of the 26 lon exchange layer ~nd the compressive tension prevalling there-27 ln were determlned by optlcal measurements with polarized light.
2~ The realRtance of a glaos to devitrificstion W~8 te~ted by heat ..... . . .. . . . .. . . _ . .. . ....

~ R~3per 223.1-CIP-FMM
FMM/~18p 10571~ Ostober 11, 1974 .~ .
1 treaeing wlth tempernture gradients ln a furnace for a perlod ~ of 60 mlnutes, the glass belng protected against v~porizatinn 3 at the surface. After thi~ treatment the upper de~itriflcation 4 limit (- liquidu~ temperature, OEG), the lower devitrificatlon S limit (UEG), the cry~tallizstion maximum (KS~aX) and the grnwth 6 rate of the crystals were determined microscopically. The 7 thermsl expsn~ion and the trnnsformation tempersture were de-9 termined with the dilstometer.
9 The darkening action and regeneration of phototropic glasses wss measured a8 the monochromatic transm~s~lon measurement at 11 545 nm as 8 functi~n of time. The excitation waa performed 12 with unfiltered xenon light of an intensity of 2 cal cm 2 min 1, 13 Unle~s otherwlse noted, the temperature during the measurement 14 Wa8 20C and the thickness of the glas~ 2 mm. All glasses had more than 86% transmission, most 90% trsnsmis~ion. Difference 16 of transmission was calculated as difference before and after l?~ excitation.

1~ The electron microscope studies were performed on specimens 19 whlch had been thinned by the ion steel etching process. This process has the advantage over other thinning processes that the 21 ~pecimens do not come in contact with liquids and solution~
22 durlng the thlnning process or thereafter, and thus there i0 no 83 contamlnation of the 0peclmens when they are floated and re-24 ~cted, because the specimen is fastened to the ob~ect holder of the electron microscope during the thinnlng process and enters 26 the electron mlcro~cope together with the obJect holder, wlthout 27 any other treatment. The avoldance of liquids and solutlons 2~ also prevent0 hydration of the ~pecimens. Consequently the .. . . . . .. . .......

~ Rasper 223.1-CIP-FMM
1057~07 October 11, 1974 .~
1 ~tructures which are visible in or on the ~pecimen when it ls 2 irradlnted are clearly attributable to the ~pecimen. If, how-3 ever, processes are used in which the specimen come~ in contact with liquids, crystalline re~ction products are often found 5 whlch can be attributed to reactions between the glass surface 6 and the liquid. On the basis of the specimens thinned with 7 ions, when irradiated with electrons, conclusions may be reached

8 from the contrast ns to the presence and the distribution of

9 substances having unequal mass absorption. By means of electron diffraction photographs and fine-range electron diffraction 11 photographs additional information can be obtained on the 12 crystal structure in the matrix and phase separation areas of 13 the phototropic glasses under study.
14 More particularly, crystallinity herein is determined as follows. The glass is annealed 2.5 hours at the temperature 16 which belongs to a viscosity of 1014 poise, so that the diffu-17 sion rate and diffusing amount is the same for all samples.
18 ~fter this precipitation of the phase separations, the crystal-19 linity i9 measured with an electron microscope by electron beam bending using a Slemens Elmiscope Type ElA for selected 21 ares diffraction patterns producing electron diffraction pat-22 terns of the originsl glA~s, thinncd by ion sputtering. To 23 secure that no changes of structure occurred during preparation Z4 and analy9is thin silver chloride sam~ wcra tested.
By heating by increasing ~he electron density in the 26 beam no changes occur. The degree of crystallinity is determ-27 ined from three samples in randomly specified arcAs and calcu-28 lated as percent. The figure in % gives the amount of ~11 0 5 7 1 0 7 Rasper 223.1-CIP-FMM _.
' ' ' PMM/asp October 11, 1974 ~ ~ .
1 sllver- and halide-rich phase separations, which are found 2 to be crystals. The remainder (100% minu~ the % cry~tals) .~ i8 the % of non-crystalline phase-separations. This % crys-tals is referred to herein as % silver halide crystals.
The data is reported in Table 8, wherein the composition 6 figures are parts by weight per 100 parts by weight of the 7 base glass batch. RFA means X-ray fluorescent analysis with the Siemens SRS 1 instrument. The analy~ing crystals were LiF
9 for Br and Ag and PET-crystals for Cl. Wet chemical analysis seandards were used to correlate the RFA values.
t - 11 Examples 57, 59, 61, 62 to 66 are according to the12 invention; the remaining examples, 58, 60 and 67, are for 13 comparison. In 58 the phase separations are crystalline;
14 in 60 the Ag is too high; and in 67 the Ag i8 too low. In - 15 the examples of the invention the amount of silver halide 16 crystals is less than 10%.
17 The RFA-values given in Tsble 8 show the relatlve con-lB centrations with reference to the standard ~amples. The wet 19 chemical analyses showed, that the relative concentration 1 (RFA-value) for silver oxide corresponds to 0,22 wt-% Ag~O, 21 for bromlne to 0.26 wt-% Br, and or chlorine to 0.24 wt-% Cl.
22 The calculated wt-% of silver, chlorine and bromine are in-23 cludet.
24 From the analytical evaluation and from the desired and achieved phototropic properties of the glass, the limit of 86 concentrations, ln the glass, of Ag2O ant the halides chloride 27 and brom~ne were determ~ned as 0.05 to 0.5 weight % Ag20 and 0.2 to 1.0, preferably 0.35 to 0.87 weight %,of bromine and . . .
A~^` ;` ~ -29-. _ . . . ~ . . . . . _ . .

~ ~a~per 223,1 CIP-FMM
1~)57107 P~w~p .~ December 31, 1974 l chlorine, and a preferred we~ght ratio of bromine to chlorine 21ofl~oto4~o~ l ' .' 13 .

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1057107 R~sper 223.1-CIP-FMM
FMM/asp December 31, 1974 1 The linesr thermal expan~ion coefficient of phosphate 2 gl~sses or glasses which contain phosphate as main component, 3 often is higher than 105xlO 7 per C between 20 and 300C. The 4 combination of the components slkali oxide, earth-alkali oxide ~nd lead oxide normally brings a magnification of the expan~ion coefficient. A usual way to lower the expansion coefficient 7 18 by u~ing boric oxide or fluorine. In the glass of the 8 invention, avoiding such use of boric oxide and fluorine, the 9 lowering of the expansion into an appropriate preferred area from 99 to 105 x 10 7 per C for better technical properties is 11 achieved by the special combination of the components SiO2, 12 P2O5, A12O3, ZrO2, Na2O, K2O, CaO, BaO, SrO, PbO TiO2, La2O3, 13 Ag2O, CuO and the halides Cl and Br. Higher and lower expansion r 14 coefficlent can be achieved by varying the compositions within the scope of the invention.
16 Examples 68-71 according to the invention are reported 17 ln Table 9. The data for Examples 39 and 40, whlch appear above wieh comparison examples in Table 5, are also reported 19 in Table 9. In Examples 39, 40 and 68-71 the % silver halide crystals is less than 10%.

2~

1057107 ~er 223,l-CIP-FMM
December 31, 1974 Table 9 wt-% in ~ynthe 8i 8 39 54 68 69 7o 71 . . _ SiO2 12.65 13.17 13.0013.85 12,15 13~76 P205 32.39 ~2.42 32.333~45 33,B1 32,60 Al20~ 24.29 24.32 22.5625,68 25,59 23.66 ZrO2 1,92 2,02 1,25 1,~5 2,56 1~05 Na20 6.07 6.08 4-72 7.45 3.54 4.95 O 9.11 9.12 10.375~5o 9.25 9.75 CaO 4.05 4.06 4.72 3.26 4.85 3.92 BaO 6.18 6.23 5.57 6.95 6.20 6.20 SrO _ O,05 _ O.02 o.on . . . . . .
PbO 1.82 1.42 2.42 2,49 0~79 2~42 ~i2 0,41 o.3o 0,15 o.59 0.18 0.40 La20~ 0,50 0,20 1.98 1,90 0-10 1.00 A~20 0.61 0.56 0.89 0.18 0.95 0.29 CuO _ 0,04 0,05 0.03 0.02 al 100 o~J 2.02 2,23 o~7o 0.97 1.42 1.05 Br 2,02 7,60 0,90 1.80 1.88 1.76 :,.
Phototropy and other essential properties ~ransmission 92 % 93 % 92 % 92 % 92 % 91 %
Unilluminated .
_ _ _ _ Saturation 28 % 22 % 25 % 33 % 26 ~ 29 Q/o transmis~ion _ _ % Regeneration 20 % 28 % 23 ~ 28 % 32 % 32,5 %
after 10 mi~
. I .
Size and 160 A 170 X250 ~200 ~ 100 ~ 7 structure of phase-separation glassy glassy glassy glassy gla88y glas~y crystalli~ation All glt 88 show lo cry3t _ _ ~n i~ bh ~ _ _ ~i8co~ity ran~o betwoen 10 and 10 poises Rorraotive index ~d 1. 52291.52311.5229 1.52371,5226 1.5230 sion x70~ C 103.5 103.5101,699,1 100.6 100.8 (20- _0 C) I _ _ _ li~ty in ml IICl 0.12 0,090.03 0.08 0,12 0.05 accordinG DIN _ Layer thiclcnoss and 65/um ~ 80 ~m 85 ~m 70 ~m 58 ~m 90 ~m compression after , chemicQl t;tren~thening 3200 8900 620,0 650,0~600,0 '720,0 in ~N03 -Saltbath nm/cm nm/cm nm/cm nm/cm nm/cm nm/cm . -34-~ 1057107 Rssper 223.l-CIP-FMM L
F~y~P
, December 31, 1974 1 A~ nentloned above the glasses of the invention nre best 2 free of boric oxide. In T~ble lO three known glssses fre~ of 3 boric oxide are reported. In these glasses the silver h~lide i8 4 mo~tly crystalline. Apsrt from boric oxide, the glasses fail to come within the composition according to the invention, e.g. in 6 Example 72, the P205 content; in Examples 73 and 74, the SiO2 7 content.

9Tuble_lO - Comparison Example~
wei6ht-C~ 72 73 74 . _ ll P205 41.7 37. 34.6 12 Al23 25.4 21.5 24.8 SiO2 14.8 7.5 8.7 13 ~IgO 4.2 4.3 ~.2 14 K20 , 12.8 7.0 7.9 , Na20 0.7 6.0 6.6 F , 0.2 0.1 0.4 16 CaO _ 9.0 7.o BaO ~ 6.7 6.6 17 TiO2 _ 0.9 0.47 ,, .. . __ 18 __ 1.4785 1.5375 1.5285 crystallization OEG 1142 C 1025 C 10~8 C
20UEG 615 C 600 C ' 685 C
21KGmax 932 C 910 C 917 C
Growth xate ~5 ~m/min > 25 ~m~min 32 ~m/min 22 _ structure of carroers fully more than ~ 70 %
23 of photobropy (% sil- crystalline 80 7v crystalline ~er ~ d~ cry~t~ _ _ crystalline 24 _ _ chemical stability , , in ml IICl 0.68 1.02 o.97 accordinG DIN 12111 26 , _ - _ . .
~7 28 ' , -35-Rasper 223.1-CIP-FMM
'~ ~1.07 FM~/asp ~L~J5 ~ December 31, 1974 1 In Table 11, effects of composition on crystallinity 2 are indicated. Example 75 i~ substantially the same as Example 3 54 (lnventio~) in Tables 5 and 9; Examples 76 and 77 tcomparison) 4 demonstrate a change in crystsllinity with increase in P205 content and decrease of A1203, ZrO2, SiO2 and CaO; Example 78 6 iB according to the invention; Example 79 (comparison) con-7 tains no ~ilica; and Example 80 (comparison) contains only B.5% silics and 4% B203. Examples 79 and 80 are known glas~es.

19 .

~3 ~4 ~7 ~8 '\ .

Raaper 223.1-CIP-FMM
057107 ~ ~8p De~ember 31, 1974 Table ll - Ex~mples 75 and 78,the Invention ;
_ Examplea 76,77,79 snd 80, Comparisons.
.1 i~ wei~ht-% 75 76 77 78 79 80 . ~ . .
~i2 13.17 11.67 11,1713.47 _ 8.50 P205 32.42 35.42 38.4233.00 34.20 35.5 Al23 24.32 24.32 22.3222.70 24.20 21.00 ~2 _ _ _ _ 16.70 4.00 ZrO3 2.02 0.52 0.52 ,2.00 2.94 0.91 ~ . . ~ __ N~20 6.08 6.08 6.08 6.37 , 6,38 6.00 K20 9.12 9.12 9.12 9.22 8,14 9.00 CaO 4.06 4.06 3.o~ 4.45 4.42 7.7 BaO 6,23 6,23 6.23 6.80 _ 4.30 MgO , _ _ _ _ 2~00 8rO 0.05 0.05' 0.05 0.08 _ _ I
.
~bO 1.42 1.42 1.42 0.50 _ _ ;
~i2 0,30 0,30 0.30 0.60 2,9~ 1.00 Ls20 0.2,0 0.20 0.20 _ _ . 'i . . .
Ag2o ' 0,32 0.32 0.32 0.32 0.32 0.32 CuO 0.04 0.04 0.04 0.04 0.04 0.04 Cl + B 0.45 0,45 0.45 0.45 0.45 0.45 F _ _ _ _ 0.47 0.13 . .
time of 2 5 h 2 5 h 2 5 h2.5 h 2 5 h 2.5 h annealing . .
annealing 1o14~5p 1o14.5p 1o14-5p 1o14.5p 1o14.5p 1o14.5p .
amount of allver- about halide cry~taln in oompar~on bo < 10 % ~ 25 % 80 % ' 5 % ~ 95 % >7 %
ull oilverhalide-rich phaae-separatione __ . .

-- 1057107 Rasper 223.1-CIP-FMM
~ FMM/s8p .~ December 31, 1974 1 More detniled studies of che~ical strengthenlng were per-2 formed, for example on the compositions of compari~on Example~
3 22 and 25 and on the composition of Example 54 of Table 5.
Table 9 shows the results in relation to the duration of the lon exchange and the temperature involved. The strengthening 6 was performed in a KN03 molten salt bath. It can be seem q from Table 9 that the composition of the invention (Ex~mple 54) has superior properties.

~3 1057107 R~8pèr 223.1-CIP-FMM
, ~ esp December 31, 1974 Table 12 - Che~ic~l Stren~thenin~

TemPer~ture Time L~yer Compre~ve Th~cknesa Ten~on - Cl ~hl [ ~ 1 ~nm/cml , _ . .. ... . . . .. ~ _ .

ComPosition 22:

Composltion 25:

4~0 16 30 3300 4~0 16 35 3200 420 ' 16 35 2340 _ `
' ~ ~ . r Ru8per 223~1-CIP-FMM
. 1057107 Dece ber 31~ 1974 Table 12 (continued) ComPo~itlon 54:

Te~p~rffture Ti~e Leyer Compres~ive Thi~kneasTensi~n 1- cl . lhl [~ lnm/cm~

400 16 Ten~iom-free 430 2 25 1300 .

430 16 59 69~0 4S0 16 60 . 5800 470 4 . 49 6400 r ~40 ~
!

\ - Rasper 223.1-CIP-FMM
1057107 F~a8p December 31, 1974 ~SU~ARY
6 Thu~, the invention provides a phototroplc glass com-7 prising at least one glass forming oxide whose inter-unit bond O i~ weaker than the inter-unit bond of SiO4 units in silicate 9 glass, containing P205 as the pr~ncipal glass forming com-ponent, and deposits rich in silver and halogen imparting 11 phototropy to the glass. Desirably, the glass has a tarkening 12 of ~t least 50% transmission difference and a regeneration of 13 the transmission after about 10 minutes of at least 20X after 14 the end of the illumination, on the basis of the tests which are the sub~ect o Table 8.
16 The glass can contain non-crystalline silver-rich and 17 halogen-rich phase deposits in the size range of 40-350 A
1~ ~Angstroms). Additionally, the glass desir~bly has a suffic-19 ient chemical stability, charac~erized by a titration value in accordance with DIN 12,111 of less than 0,2 ml HCl.
~1 Desir~bly the glass is fluorine free, nnd ~lso free of ~8 boric oxide, and magnes~um oxide.
2$ The glasses are silicate-alumino-phosphate glasses con- ~-24 tainlng:
wt.%
26 S102 12_14 2~ B203 0-2 Alkall oxide; 8 105710 Ra~per 223.1 CIP-FMM
7 F~wJp ... Dec¢mber 31, 1974 l and deposits of non-crystalline silver-halide phase imparting 2 phototropy to the gl~ss, the amount of silver in the glass 3 being greater than 0.05 wt.%, and the amount of silver ~U~r 4 cryst~lc in the glass being less than about lOq.. The compo~i-tions of the invention are d~stinctive in that opthalmic 6 pressings thereof can be made by normal product~on procedures 7 for such pressings.
:~ 8 ~0 2l .,

Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A phototropic glass comprising at least one glass forming oxide whose inter-unit bond is weaker than the inter-unit bond of SiO4 units in silicate glass, containing P2O5 as the principal glass forming component, and deposits of a non-crystalline phase which in rich in silver and halogen and imparts phototropy to the glass, said phototropic glass containing:
the amount of silver therein being greater than 0.05 wt.% and the amount of silver halide crystals therein being less than about 10%.

2. A phototropic glass containing P2O5 as the principal glass forming component, and deposits of a non-crystalline phase which is rich in silver and halogen and imparts photogropy to the glass, the amount of silver halide crystals in the glass being less than about 10%, said phototropic glass containing:
the amount of silver therein being greater than 0.05 wt.

43 A phototropic glass according to claim 2, the halogen being at least one of chloride and bromide.

4. A phototropic glass according to claim 2, which:
contains silver-rich and halogen-rich, non-crystalline phase separations with a diameter between 40 and 350 A;
shows no crystallization in the range between 101 and 105 poises;
has a refractive index nd between 1.5225 and 1.5238;
can be strengthened chemically by ion exchange in a medium containing potassium ions at a temperature below 1014.5 poises, whereupon alkali ions smaller than potassium diffuse out of the glass;
has a sufficient chemical stability, characterized by a titration value in accordance with DIN 12,111 of less than 0.2 ml HCl.

5. A phototropic glass according to claim 4, having the following phototropic properties, as determined by monochromatic transmissions at 545 mm, with excitation by an unfiltered xenon light of an intensity of 2 cal cm-2 min-1, at a temperature of 20°C, on 2 mm thick specimens of the glass: a darkening of at least 50% transmission difference and a regeneration of the trans-mission after 10 minutes of at least 20% after the end of the illumination.

6. A phototropic glass according to claim 2, said glass being free of magnesium oxide.

7. A phototropic glass according to claim 2, said glass being free of boric oxide.

8. A phototropic glass according to claim 6, said glass being free of boric oxide.

9. A phototropic glass according to claim 2, the glass containing 0.05-0.5 wt-%Ag calculated as Ag20, and 0.2-1.0 wt-%
halogen, which halogen is at least one of Cl and Br.

10. A phototropic glass according to claim 2, having the following composition on a batch basis in weight percent:

and at least one of bromine and chlorine in an amount suffic-ient to provide 0.2-1.0 wt.% of bromine plus chlorine in the glass, said glass containing 0.05-0.5 wt. % Ag, said com-ponents further being in relative proportions to provide said phototropy.

11. A phototropic glass according to claim 2, having the following composition on a batch basis in weight percent:

and at least one of bromine and chlorine in an amount suffic-ient to provide 0.2-1.0 wt.% of bromine plus chlorine in the glass, said glass containing 0.05-0.5 wt. % Ag, said com-ponents further being in relative proportions to provide said phototropy.

12. A phototropic glass according to claim 10, which:
contains silver-rich and halogen-rich, non-crystalline phase separations with a dia-meter between 50 and 350 A;

shows no crystallization in the range between 101 and 105 poises;

has a refractive index nd between 1.5225 and 1.5238;

can be strengthened chemically by ion exchange in a medium containing potassium ions at a temperature below 1014.5 poises, whereupon alkali ions smaller than potassium diffuse out of the glass;

has a sufficient chemical stability, characterized by a titration value in accordance with DIN
12,111 of less than 0.2 ml HCI.

13. A phototropic glass according to claim 12, having the following phototropic properties, as determined by mono-chromatic transmissions at 545 nm, with excitation by an un-filtered xenon light of an intensity of 2 cal cm-2 min-1, at a temperature of 20°C, on 2 mm thick specimens of the glass:
a darkening of at least 50% transmission difference and a regeneration of the transmission after 10 minutes of at least 20% after the end of the illumination.

14. A phototropic glass according to claim 10, the sum of the alkali earth oxides being 8.6 - 12.5 wt. %.

15. A phototropic glass according to claim 10, the sum of the alkali oxides being 11.2-16.2 wt. %.

16. A phototropic glass according to claim 2, containing on a batch basis:
a) 30.4 to 33.9 wt. % P2O5;

b) Ag2O in an amount sufficient to provide Ag in the glass in amount of 0.05-0.5 wt. %, calculated as Ag2O;

c) molecularly combined halogen in an amount sufficient to provide halogen in the glass in amount of 0.2-1 wt. %;

d) balance, oxides compatible with the afore-mentioned components to provide said photo-tropic glass.

17. A phototropic glass according to claim 16, further containing, in weight percent:

18. A phototropic glass according to claim 2, said glass being free of fluorine.

19. A phototropic glass according to claim 18, said glass being free of B2O3.

20. A phototropic glass according to claim 19, said glass being free of magnesium.