by dannybee » Mon Feb 20, 2017 1:26 pm
I have been reading some very old photography books and thought I would share some of its content
When ammonia or a caustic alkali are added to a solution of bichromate, the color changes gradually from a orange to lemon yellow. This is because the bichromate is converted into a neutral chromate. Neutral chromates can be converted to bichromate by adding acid - even such weak acid as actic acid or citric acid.
One way to prevent the spontaneous hardening of gelatin, such as occurs in a bichromate gelatin, is to add ammonia to the sensitizing solutions. This is added to the point of forming neutral chromate. When exposed to light, this ammonium chromate decomposes to ammonia and ammonium bichomate and gives a sensitivity almost as great as that of bichromate-sensitized gelatin.
Colloid characteristics
When organic colloids are impregnated with a bichromate, they tend to become light-sensitive. This is true of colloids both animal and vegetable in origin - such as gelatin , albumen , casein, gum arabic and shellac,pyrooxyline - and some synthetic materials such as cellulose or polyvinyl esters
There are other organic substances, which can be photochemically reduced, whose reduction products will react with a bichromate solution. These too can be used to sensitize colloids. For example: A gelatin sensitized with a 2:7-sociumanthraquinone siculphonate will keep indifinitely in the dark. After exposure to light, the gelatin may be soaked in a 2% per cent solution of bichromate and the gelatin will be tanned in the same way as if it had been bichromate-sensitized in the first place (G. Koegel 1925)
Sensitivity can be increased threefold by adding , to bichromated colloid layer , a metal salt which precipitates on contact with a chromate but not with a bichromate. In this situation, the metal salt, itself , does not contribute to the tanning action. Rather, it reacts with the neutral chromate which is formed during exposure and increases the tanning. The best results have been obtained with cerium and lithium, which precipitate with chrmates at ph values between 8 and 8.5. T the ph , of course must be maintained between these limits.
I have been reading some very old photography books and thought I would share some of its content
When ammonia or a caustic alkali are added to a solution of bichromate, the color changes gradually from a orange to lemon yellow. This is because the bichromate is converted into a neutral chromate. Neutral chromates can be converted to bichromate by adding acid - even such weak acid as actic acid or citric acid.
One way to prevent the spontaneous hardening of gelatin, such as occurs in a bichromate gelatin, is to add ammonia to the sensitizing solutions. This is added to the point of forming neutral chromate. When exposed to light, this ammonium chromate decomposes to ammonia and ammonium bichomate and gives a sensitivity almost as great as that of bichromate-sensitized gelatin.
Colloid characteristics
When organic colloids are impregnated with a bichromate, they tend to become light-sensitive. This is true of colloids both animal and vegetable in origin - such as gelatin , albumen , casein, gum arabic and shellac,pyrooxyline - and some synthetic materials such as cellulose or polyvinyl esters
There are other organic substances, which can be photochemically reduced, whose reduction products will react with a bichromate solution. These too can be used to sensitize colloids. For example: A gelatin sensitized with a 2:7-sociumanthraquinone siculphonate will keep indifinitely in the dark. After exposure to light, the gelatin may be soaked in a 2% per cent solution of bichromate and the gelatin will be tanned in the same way as if it had been bichromate-sensitized in the first place (G. Koegel 1925)
Sensitivity can be increased threefold by adding , to bichromated colloid layer , a metal salt which precipitates on contact with a chromate but not with a bichromate. In this situation, the metal salt, itself , does not contribute to the tanning action. Rather, it reacts with the neutral chromate which is formed during exposure and increases the tanning. The best results have been obtained with cerium and lithium, which precipitate with chrmates at ph values between 8 and 8.5. T the ph , of course must be maintained between these limits.