I have recently been re-reading a rather rare and excellent textbook on photography, "Fundamentals of Photographic Theory" by James and Higgins, and ran across something interesting.
Earlier, I had stated here that there were basically 3 types of gelatin known in the early years of emulsion making, Soft, Medium and Hard "blooming" gelatin related not to Bloom Index, but rather to their ability to sensitize emulsions via the sulfur finishing method described in early patents using allyl thiourea. I had also stated that modern gelatins were oxidized and contained no sulfur sensitizing compounds. This is our modern inert gelatin.
Well, it seems that the inert gelatin we use today was also known as an apparent natural product at the turn of the century (19th to 20th) and known up to the mid 40s of the last century. James and Higgins note that early emulsion makers used any gelatin of these 4 types (not 3 as I had originally stated) in any manner that suited them such as in the make, in a post make addition, and in a final "ripening" step. So, such early formulas are totally ambiguous and J&H warn in their book to beware of such ambiguity.
I have reviewed Wall and Baker both, and find no mention there of any classification of gelatin although they would be virtually forced to know this if they were to make a useful emulsion. This very fact makes me feel that there was less than full disclosure in these books.
My contact with the German Agfa formulas from the 40s indicate that the Agfa engineers knew of 4 types of gelatin, at least... schwerreifend, mittelreifen, kraftigreifend and weichreifend, although English texts only refer to 3, except for the reference above.
Therefore, I again warn potential emulsion makers to beware that the gelatin designations in most all English texts do not convey the full import of gelatin type and usage. Any gelatin can be used at any time.
Modern emulsions take the following general divisions in the make:
Precipitation - inactive gelatin
Ripening - heating with no silver halide solvent or sulfur compound
Digestison - heating with a silver halide solvent but no sulfur compound
Finish or chemical sensitization - heating with a sulfur compound or a sulfur + gold compound.
Any of the above 4 can be combined in any fashion, but the emulsion maker with experience can see this just by reading the formula.
Old formulas use the first 3 terms interchangably and we cannot clearly distinguish what is going on unless the gelatin type is clearly stated as is done in the German Agfa formulas for example. Old English texts do not clearly disclose this for some reason.
I suspect that all of the early authors were being coy about their formulas even in publications. The general readership would not know the difference, and the engineers at the major companies would just be amused.
This is intended to alert those who rely on old texts for information. You might make an emulsion, but the speed, contrast or the amount of silver needed to reach a given result might be far off from what is possible with the 'right' formula.
I would also like to add that many of these formulas include "finals" which are things like pseudo surfactants (Everclear) and hardeners (chrome alum), but in fact, many additives were reserved for overcoats which were necessary to the final function of the photo material. Most of us do not use overcoats, and adding these chemicals to the emulsion layer can often be harmful, and as a result, even the best coating sometimes does not perform up to what is reported as a 'result' in the early textbooks where many coatings had overcoats.
These are some general precautions regarding earlier texts as explained by J&H and also by my re-reading of many formulas. I hope this helps some of you.
Can you please tell me what section of the book that references this? I want to find this in my copy of the book and read more about it. I am curious if there is any mention of this in the Mees and James book.
Can you please tell me what section of the book that references this? I want to find this in my copy of the book and read more about it. I am curious if there is any mention of this in the Mees and James book.
Thanks,
Bob M.
Hi Bob,
I know you're as fanatic a book collector as I am. If you have The Theory of Photographic Process by Mees, 1942 (1945), look on page 98. Mees talks about four 'classes' of gelatin there.
I can't find any similar reference in the James and Higgins that Ron mentions, but I don't have all the editions (yet! as Kirk says). I love the old books and I don't think I'd be making emulsions today without their guidance.
Keep on collecting!
d
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Oh, page 25 is near the middle of the chapter on gelatins, and they describe the 4 types of gelatin and mention that one is inactive in that text forward to the end of the chapter. It is pretty much as Denise states in the Mees book she references above. In both cases, the reference is weak. They make no strong mention of it, but merely note it in passing even though the impact is huge today. I suppose they did it due to confidentiality or the fact that the 4 types were common knowledge.
1948 ed. - Chapter 2, "The Photographic Emulsion", page 25, mid page has the heading "After-Ripening".
"Moreover, batches of gelatin which were very similar in their usual physical and chemical properties sometimes would differ enormously in their photographic properties, as measured by the sensitivity of emulsions prepared from them."
Is that what you are refering to? Where does it mention "4 types" of gelatin. 1960 ed. covers "silver halide" and "Preparation of the Emulsion" on page 25.
He mentions that there were several used and mentions the inert gelatin just as Mees does. Since I have been able to actually find 4 varieties in searches, I feel that they may have been aware of 5 different types rather than 4, but he does not mention them specifically by name, just that there were some that changed sensitivity and others that did not. The German literature is more revealing than the English literature on this subject.
James is very circumspect in his assertions, as this new method of inert gelatins was just being published in the literature.
New gelatin and emulsion information 
Linear Mode
