The work of Mees and Sheppard at Kodak established that it was allyl thiourea in gelatin that was causing the sensitization. Their work was the first published on this subject. It led to consistant speed in film and paper thereafter.
Before that, most companies had 3 grades of old gelatin. Hard, medium and weak or soft ripening gelatin. (This is blooming in German, not to be confused with the Bloom Index which relates to viscosity. Many amateurs make this crossover in translation and try so hard to get differences in speed and contrast by varying Bloom Index when it is the wrong variable.)
The mixtures of these 3 grades of gelatin made the final emulsion have the contrast and speed that was desired, but it varied due to the diet of the cows.
This is how Mees discovered it. The cows were eating too much mustard seed and this increased the sulfur content of the gelatin and caused the Kodak emulsions to go into fog.
Kodak kept this as a secret untl the publication noted above. Therefore, they converted first to the oxidized deionized gelatins before anyone else.
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