Patent GB-668767-A (GB)
Title: Photosensitive glass article and composition and process for making it
IPC: C03C3/076
Filed: 1950-01-06
Abstract: 668,767. Photosensitive glasses. CORNING GLASS WORKS. Jan. 6, 1950 [Jan. 7, 1949], No. 422/50. Class 56. [Also in Group XX] A transparent photosensitive alkali-silicate glass contains, 55-75 per cent SiO 2 , 12-18 per cent Na 2 O with or without K 2 O or Li 2 O, 2-12 per cent Al 2 O 3 , 1.8-2.4 per cent of analytically determined fluorine, at least 0.001 per cent but less than 0.01 per cent of gold calculated as Au, 0.005-0.05 per cent CeO 2 , 0-0.2 per cent Sb 2 O 3 , 0-5 per cent B 2 O 3 , 0-3 per cent BeO or MgO or CaO or a mixture thereof, and 0-10 per cent ZnO or BaO or a mixture thereof, the balance, if any, consisting of other compatible constituents, and is capable of heat-developed opacification by crystallites of alkali metal fluoride only when and where it is irradiated with short-wave radiations. Elements and oxides which cause the glass to absorb ultra-violet radiations, e.g. those which colour glass, particularly Se and Te and their compounds and oxides of Fe, Cu, U, V, and Mn, and oxides of As, Tl, and Pb, should be avoided. The gold is preferably introduced by dissolving metallic gold in aqua regia, e.g. 10 gms. per 100 ccs., and adding the solution to the batch. The cerium component should be substantially free from other rare earths, and cerium hydrate containing the equivalent of 75 per cent CeO 2  is satisfactory. The Sb 2 O 3 , increases the heat sensitivity of the irradiated glass and exerts a fining action. The fluorine content must be insufficient to cause opacification on reheating, and is introduced, for example, as sodium or potassium silico fluoride, cryolite, or alkali metal fluoride. The composition of eight batches is given, example 1 containing sand 1000, Na 2 CO 3  346, Na 2 NO 3  39, Al(OH) 3  225, borax 22, 10 per cent Au solution 0.3 cc., cerium hydrate (75 per cent CeO 2 ) 0.22, Sb 2 O 3  1.37, NaCl 3, Na 2 SiF 6  72. The NaCl is believed to facilitate the crystallisation of NaF. The composition and physical properties of the finished glass are given. The glasses are colourless and transparent and remain so when merely reheated, but when irradiated with short wave radiations and heat treated the irradiated areas become opacified while the other areas remain colourless and transparent. By interposing a photographic negative or a stencil between the glass and the source of radiation an opacified white photographic image or design is formed on reheating. The time of irradiation is 1 or 2 minutes to an hour or more, thus with a 60 amp. carbon arc at 12 inches the time was 15 minutes. The glass is reheated for 5 minutes to 6 hours at a temperature within the range from 100‹ C. below to 50‹ C. above the softening point, but should not be heated above its softening point unless it is also to be bent. The time and temperature of heating are interdependent, but too rapid heating above 500‹ C. is harmful. This heat treatment does not cause opacification unless the Au and Sb 2 O 3  are at or near their maxima when a pink image may be formed. The glass is now cooled below 520‹ C., again heated, preferably between 540‹ C.-700‹ C., e.g. for one hour at 540‹ C. or a few seconds at 700‹ C., and finally cooled, the image being formed during the second heating. A glass sheet may thus be formed with a series of narrow opacified linear strips extending from face to face of the sheet at any required angle which act as louvres. Specifications 597,089, 636,151, and 654,740 are referred to.