Reading some really old books- great information

Dichromated Gelatin.
dannybee
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Reading some really old books- great information

Post 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.

Martin
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Re: Reading some really old books- great information

Post by Martin » Tue Feb 21, 2017 1:39 am

Interesting,. What book is it taken from?

As an aside, potassium chromate sensitized gelatin showed nice photo speed around 400nm in my experience. It was roughly on par with potassium dichromate or ammonium dichromate. I always wondered about its efficiency in the 450-460nm range.

As your quote mentioned, chromate sensitized layers have very good keeping properties.

dannybee
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Re: Reading some really old books- great information

Post by dannybee » Tue Feb 21, 2017 12:03 pm

Hi Martin
I was given some old books as a gift, because I wanted to learn the carbon process, but when I started to dive into the books their where many more wealth of ideas and understanding . First book is "how to make old-time photo" 1981 the second is a great book of old paper 1840 -1870 " Photographic Reproduction Processes" 1891 it can be found at forgotten books, a great wealth of old ideas

Martin
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Re: Reading some really old books- great information

Post by Martin » Wed Feb 22, 2017 4:49 am

Thanks, Danny.
http://www.forgottenbooks.com/en is quite interesting.
Just saw they‘ve some Robert Hunt there. I found him quite fascinating lately.

lobaz
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Re: Reading some really old books- great information

Post by lobaz » Wed Feb 22, 2017 1:49 pm

Check also www.archive.org
It seems that forgottenbooks.com shares much of its content.

Martin
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Re: Reading some really old books- great information

Post by Martin » Thu Feb 23, 2017 1:53 am


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