There are a many gelatin types for making silver gelatin emulsions and most current textbooks don't discriminate between them. Here is some terminology to help you out.
1. Food gelatin: These are active gelatins with lots of additives. At my current estimates as little as 30% of the weight is really gelatin in some brands. Using these can lead to foggy emulsions.
2. Active gelatins: These are active gelatins that interact with an emulsion during making and can cause increases in speed and contrast. They are loaded with thoureas and allyl thioureas. For those concerned about the toxicity of thiouread, be forewarned that it is a natural product in gelatin due to the cows and pigs eating mustards. They can lead to foggy emulsons.
3. Inactive gelatins: These have had all sulfur compounds removed. They are preferred.
4. Pig Gelatins: Not commonly used in photo materials nowdays. They have their maximum swell at pH ~9.
5 Bone Gelatins: Made from cow cartialge. Common consituent of current photo products. Minium swell at about pH 4.5.
6. Gelatin Bloom: This is a measure of gelation strenth AFAIK. We never used this at EK. In my personal experience, the higher the bloom, the higher the apparent viscosity and the less pepper grain and aggregation in an emulsion. It also gets harder with higher bloom. Lower bloom gelatin hardens more rapidly though in my experience. This is very subjective and difficult to assess.
7. PA gelatin: Gelatin treated with phthalic acid. Used in coagulation washing of emulsions (PA washing). This gelatin is softer, hardens more slowly and adheres less well unless regular unmodified gelatin is added.
Using low gelatin when making an emulsion results in larger grains and higher speed. Higher gelatin results in lower speed and finer grain. This is a generalization.
Heating gelatin for too long or at too high a temperature decreases viscosity and hardneing capability.
Are bone gelatins the same thing as Ossein?
(maybe a stupid question, but I had to ask!)
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Originally Posted by Jerevan
Yes, they are.
Pig gel is made mostly from pig skin with some bone and cartilage, but bone gel is made mostly from cattle bones, with some cartilage and skin.
Bone gelatin is made by treating the materials with lime and is sometimes called limed gelatin. To be used in photography it must be alkali and calcium free.
The only thing I can add to this is that I saw a great improvement in the smoothness of my coating (salt prints - Richeson brush) when I switched from Knox gelatin (grocery store) to 250-bloom photo gelatin from PF.
This is quite the case Juan.
Originally Posted by juan
The store bought Knox gelatin has a lot of extra stuff in it.
A lower bloom gelatin, about 75, might be very good to try as well.
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I was curious to know what exactly the Bloom value is, and found this useful description--
Originally Posted by Milligan & Higgins website
This may be true for making photographic emulsions based on silver salts. However, carbon is photography, right? If right, I have used both porcine source gelatins, including plain Knox from the grocery store, and ossein source gelatin intended for photographic applications. Both gave very good results for me in making carbon tissue.
Originally Posted by Photo Engineer
I searched and searched for a postable definition, but could not find one suitable. Thanks for this one.
Higher Bloom Index (BI) means greater strength and greater hardness and greater viscosity. They are all subtly related, but there is sometimes no exact correlation. We did not use BI at Kodak. Even so, Eastman Gelatin produces gelatins with listed BI values from 75 to 250. These generally relate to fractions obtained from the purification process of the gelatin. Using either 75 or 250 is best. I use 250.
Poise or centipoise is the metric measure of viscosity. We did not use it at EK. Even so, viscosity is an important measure for photographic coating, so we had an even more accurate measure called RBT (Rolling Ball Time). This measurement was a very accurate metric for judging suitability for coating.
Originally Posted by sanking
Both bone and pig gelatin work just fine in making photographic emulsions and coating them. Kodak has changed to bone gelatins due to the greater availability, purity and the isoelectric point. Processing seems to be better in pig gelatin.
As for edible gelatins, the added ingredients tend to interfere with precipitation and subsequent operations as well as being rather low in gelatin content, so they will work as gelatins per-se, but have drawbacks in that you are not sure what extras are there or how much gelatin you really have.
In such a case, the emulsion content is critical to achieve a given grain size and you can never tell the exact gelatin content of food gel.
Very interesting, PE!
Two questions, though:
1.) Are pig gelatins considered active or inactive gelatins?
2.) What kinds of things are added to food gelatins that cause such an issue with the photographic processes?