Patent GB-684134-A (GB)
Title: Photosensitive opacifiable glass
IPC: C03C3/112
Filed: 1951-02-07
Abstract: 684,134. Photosensitive glass compositions. CORNING GLASS WORKS. Feb. 7, 1951 [July 7, 1950], No. 3018/51. Class 56. A transparent photosensitive glass contains the following essential constituents in the given weight percentage ranges, 55-75, SiO 2 , 12-18 alkali metal oxide consisting of 5-18 Na 2 O, 0-1 Li 2 O, and 0-13 K 2 O, 2-12 Al 2 O 3 , 0.0001- 0.3 silver calculated as AgCl, 0.005-0.05 CeO 2 , 1.8-3.0 analytically determined fluorine and 0.01-0.2 analytically determined chlorine or 0.02-0.4 analytically determined bromine or 0.03-0.6 analytically determined iodine or a mixture of two or three of Cl, Br, and I in which their total content calculated as grammolecules is within the limits set forth for any one of them calculated as gram-molecules, the essential constituents totalling at least 85 per cent and the glass being capable of heat developed opacification by light diffusion fluorinecontaining crystallites only in such areas as have previously been exposed to short wave radiations. Preferably the alkali metal oxide comprises Na 2 O alone. Other ingredients may be added in the following weight percentage ranges, 0-5 B 2 O 3 , 0-3 BeO, 0-3 MgO, 0-3 CaO, or 0-3 of a mixture of two or more of BeO, MgO, CaO, 0-12 ZnO, 0-12 SrO, 0-5 CdO, 0-12 BaO, the total divalent metal oxide content being not over 12 per cent, 0-0.2 Sb 2 O 3 , 0-0.1 SnO 2 , the Sb 2 O, and SnO 2  together not exceeding 0.2 per cent, and 0-0.01 gold calculated as Au. Materials which absorb UV radiations must be avoided, e.g. most glass colourants particularly Se and its compounds, oxides of Fe, Cu, U, and V, and oxides of As, Tl, and Pb. The silver is preferably introduced as a water solution of AgNO 3 . The amount of fluorine must be insufficient to cause opacification on reheating. The compositions of several batches are given, and the weight percentage compositions of two resulting glasses are, (1) 69.6 SiO 2 , 16.8 Na 2 O, 10.1 Al 2 O 3 , 0.6 B 2 O 3 , 0.0017 Ag as AgCl, 0.01 CeO 2 , 0.09 Sb 2 O 3 , 2.6 F, and 0.2 Bt, and (2) 69.1 SiO 2 , 16.5 Na 2 O, 6.6 Al 2 O 3 , 4.8 ZnO, 0.002 AgCl, 0.02 CeO 2 , 0.1 Sb 2 O 3 , 2.6 F, and 0.2 Br. The batches contain a mild reducing agent, e.g, NaBr, NaI, but with chlorides other than NH 4 Cl another material such as corn starch must be added. The addition of a minor amount of a reducing agent other than a halide, e.g. starch, sugar or urea, increases the photographic speed of the glass by as much as five times, in general up to about 0.2 per cent of corn starch or its equivalent is sufficient. An oxidising agent such as NaNO 3  is desirably used with larger proportions of Ag, and when Au is present. An invisible latent image is formed on the glass by exposure to short wave radiations, preferably between 3000-3500 Angstroms, the image being later formed into an opaque image by heating. The exposure varies from ten seconds to one hour, e.g. 30 seconds at 1 foot distance from a 60 amp carbon arc. The irradiated glass is first heated for between 1 minute at 50‹ C. above the softening temperature and 1 hour at 150‹ C. below the softening temperature, preferably at 50-100‹ C. below the softening point and not too rapidly above 500‹ C. This heating does not cause opacification, but if the Ag content is above about 0.002 per cent a real transparent yellowish image may develop. The glass is then cooled below about 500‹ C. and again heated to a temperature not lower than about 100‹ C. below the softening point, when a final opacified white image appears, unless tinted yellow as above. The density of the image depends upon the temperature and the time of heating, usually 3-15 minutes. In some glasses containing fluorine in the upper half of the range an opaque image may be developed by a single heating. Various hues may be obtained in the same article by varying the intensity and time of irradiation and time of heating of different areas of the glass, thus yellow, brown, orange, rose, purple, blue and green hues may be obtained in that order. Sb 2 O 3  and SnO 2  also impart a greyish or pinkish tint, and Au a pinkish colouration, and a coloured background may be obtained by including small amounts of colour oxides such as cobalt oxide or manganese oxide in the glass. If the glass is further irradiated between the two heating treatments the re-exposed areas remain clear after the second heating, so that a clear image may be formed on an opaque background. Glass sheets may be formed with a series of opaque stripes for use as louvres, and the glass may also be used to form tableware and incandescent lamp bulbs. The glass may be tempered, and may be inspected for weakening beads &c. before opacification. Specifications 597,089, 636,151, 654,740 and 668,767 are referred to.