Method of preparing photochromic glass

Claim 2

A process for producing a photochromic glass containing silver halide and which has a brown colouration in its darkened state, which process comprises (a) forming a batch comprising tin or a tin compound in an amount which will produce a glass which will exhibit a brown colouration in its darkened state, together with those glass forming components which, in the absence of tin or the said tin compound and when subjected to steps (b) to (f) below, will produce a photochromic glass having a grey colouration in its darkened state, (b) melting the batch to form a molten glass, (c) refining the molten glass, (d) conditioning the molten glass to bring it into a state suitable for forming into articles, (e) forming the molten glass into articles, and (f) subjecting the formed articles to a heat-treatment which will develop the photochromic properties of the glass, steps (b) to (f) being performed under conditions which, in the absence of tin or the said tin compound, will produce a glass having a grey colouration in its darkened state.

Claim 3

A process according to claim 1, wherein the process is performed in continuous manner.

Claim 4

A process according to claim 1, wherein the components used in step (a) are components which, in the absence of tin or the said tin compound, will produce a fast response photochromic alumino-phosphate glass having silver halide crystals dispersed throughout the glass and having a grey colouration in its darkened state, the glass comprising as non-photochromic components in weight percentages: ______________________________________ SiO.sub.2 8.5 to 25% Al.sub.2 O.sub.3 13 to 36.5% P.sub.2 O.sub.5 7.5 to 33.5% B.sub.2 O.sub.3 7 to 28% R.sub.2 O 7 to 20.5% ______________________________________ where R 2 O represents one or more of Na 2 O, K 2 O and Li 2 O, the maximum content of Li 2 O being 5%; the amount of SiO 2 being not less than 16% when the B 2 O 3 content is less than 8% and, as photochromic components, expressed as weight percentages over and above the 100% total of all the non-photochromic components of the glass: ______________________________________ silver, expressed as Ag.sub.2 O not less than 0.05% Cl + Br 0.20 to 2.0%. ______________________________________

Claim 5

A process according to claim 1, 2 or 3, wherein the tin compound is stannous oxide or stannic oxide.

Claim 6

A process according to claim 1, 2 or 3, wherein the tin compound is in the form of a finely divided solid.

Claim 7

A process according to claim 1, 2 or 3, wherein the amount of tin or tin compound added is such as to produce a finished glass which in its darkened state has a brown colouration which is described in accordance with the Hunter L, a,b Colour Scale system by the rectangular region on the a,b-plane of the Hunter system with corners on the (a,b) co-ordinates: (5,1), (1,5), (12,16) and (16,12).

Claim 8

A process according to claim 1, wherein the amount of tin or tin compound used is an amount which will produce 500 to 5000 ppm, expressed as SnO 2 , in the finished glass.

Claim 9

A process according to claim 7, wherein the amount of tin or tin compound used is an amount which will produce 500 to 3500 ppm, expressed as SnO 2 , in the finished glass.

Claim 10

A process according to claim 1, wherein the batch of components in step (a) includes one or more reducing agents.

Claim 11

A process according to claim 9, wherein the reducing agent is starch or potassium hydrogen tartrate.

Claim 12

A process according to claim 1, wherein the batch of glass-forming components in step (a) includes a colouring agent which produces a tint in the glass when the glass is in its faded condition.

Claim 13

A process according to claim 11, wherein the tint is amber.

Claim 14

A photochromic glass having silver halide crystals and tin dispersed throughout the glass, the glass having in its darkened state a brown colouration which is described in accordance with the Hunter L,a,b Colour Scale System by the rectangular region having corners defined by the (a,b) co-ordinates: (5,1), (1,5), (12,16) and (16,12), the 1/2 OD FT of the glass being 120 seconds or less, the glass being an alumino-phosphate glass having silver halide crystals dispersed throughout the glass and comprising, as non-photochromic components in weight percentages: ______________________________________ SiO.sub.2 8.5 to 25% Al.sub.2 O.sub.3 13 to 36.5% P.sub.2 O.sub.5 7.5 to 33.5% B.sub.2 O.sub.3 7 to 28% R.sub.2 O 7 to 20.5% ______________________________________ where R 2 O represents one or more of Na 2 O, K 2 O and Li 2 O, the maximum content of Li 2 O being 5%; the amount of SiO 2 being not less than 16% when the B 2 O 3 content is less than 8%; and, as photochromic components, expressed as weight percentages over and above the 100% total of all the nonphotochromic components of the glass: ______________________________________ silver expressed as Ag.sub.2 O not less than 0.05% Cl + Br 0.20 to 2%, ______________________________________ the finished glass composition further including tin.

Claim 15

A photochromic glass having silver halide crystals and tin dispersed throughout the glass, the glass having in its darkened state a brown colouration which is described in accordance with the Hunter L,a,b Colour Scale System by the rectangular region having corners defined by the (a,b) co-ordinates: (5,1), (1,5), (12,16) and (16,12), the 1/2 OD FT of the glass being 120 seconds or less, the glass being an alumino-phosphate glass having silver halide crystals dispersed throughout the glass and comprising, as non-photochromic components in weight percentages: ______________________________________ SiO.sub.2 8.5 to 25% Al.sub.2 O.sub.3 13 to 36.5% P.sub.2 O.sub.5 7.5 to 33.5% B.sub.2 O.sub.3 7 to 28% R.sub.2 O 7 to 20.5% ______________________________________ where R 2 O represents one or more of Na 2 O, K 2 O and Li 2 O, the maximum content of Li 2 O being 5%; the amount of SiO 2 being not less than 16% when the B 2 O 3 content is less than 8%; and, as photochromic components, expressed as weight percentages over and above the 100% total of all the nonphotochromic components of the glass: ______________________________________ silver expressed as Ag.sub.2 O not less than 0.05% Cl + Br 0.20 to 2%, ______________________________________ the finished glass composition further including tin.

Claim 16

A photochromic glass according to claim 13, wherein the amount of tin in the finished glass, expressed as SnO 2 , is 500 to 5000 ppm.

Claim 17

A photochromic glass according to claim 14, wherein the amount of tin in the finished glass, expressed as SnO 2 , is 500 to 3500 ppm.