Photo grade gelatin has not vanished. It is available from the Photographers Formulary and others and is made by 2 companies in the US and 1 in France.
Photo grade gelatin is oxidized deionized gelatin made extremely pure for the photo industry. Kodak uses gelatin from all 3 of these companies. It comes in several BI (Bloom Index) numbers from 75 to about 300. It also comes as a phthalated gelatin used in making emulsions. The phthalated gelatins are made in several grades too, in order to facilitate the washing step.
Old gelatins that were used for photography were impure, and contained high levels of sulfur containing compounds. That meant that old gelatins had to be graded as to their activity in photo emulsion making. All formulas published in older texts used this old gelatin which did 2 things at one time. It allowed the making and sulfur sensitization steps to be combined into one step. It also made the emulsion less predictable due to the variations in sulfur compounds.
Modern gelatins require a separate sulfur sensitization step to replace the combined steps noted above. The effect of the sulfur is very predictable and controllable.
This is a repeat post of earlier information in a thread on gelatin. There is much more information spread out here including the method of sulfur sensitization.
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