Older Gelatins and Sulfur
We all should know by now that in the days gone by, the sulfur content of gelatins used for photographic purposes varied greatly, depending on how much mustard the cows liked with their grass. This was figured out in the mid-1920s. I assume gelatins were still using sulfur-containing gelatins up into the 1940s or 50s, depending on the manufactuer.
Todays gelatins have been stripped of these sulfur compounds to give us a more consistent performing gelatin. Since the sulfur is missing in these modern gelatins, we need to add it back in to increase emulsion speed - this is done in the final steps of emulsion making.
If one wanted (and I'm not saying that would really would, but) to better emulate the emulsions of yore, I suspect we could simply some sulfur to the gelatin at the start of the emulsion making process.
I assume thiosulfate would be suitable or would we want to use an organic sulfur compound which may react slower and survive the initial steps of emulsification? I can see not wanting to have the sulfur react immediately with the silver and get locked up inside the crystal, and not be available to increase speed as it would if it was on the surface of the halide crystal.
At what concentrations would we shoot for?
And perhaps more importantly, I realize that there will be more variation in the result with the sulfur added at the begining of the process instead of at the end, as is done with modern film emulsions. But, given that caveat, what other issues should one consider if one was to try this?
The sulfur actually came from sulfur containing amino acids such as methionine and its analogs. This broke down with heat into allyl thiourea. Since this was only used for a few years back in the 40s during the conversion from active to inert gelatins, I have no experience with it.
The order of use commercially was:
Active gelatins from the early days until about 1940 when the work of Sheppard of Kodak was published.
Allyl thiourea and thiourea sensitizations from 30s - 50s
Thiocyanantes from 40s - 50s
Thiosulfate from 50s to present.
This is a rough timeline of sulfur sensitization in films and papers. It varied from company to company as well. While Kodak used the thoureas, Agfa was using the thiocyanates and etc. confusing the timelines.
In addition, in the early days, there were roughly 9 gelatins available, soft, medium and hard (related to allyl thiourea content and giving different speeds and contrasts) and low, medium and high BI (Bloom Index) which related to thickness, viscosity, hardening or whatever. In the early days they had mixes of these but paid less attention to BI than to the allyl thiourea content. They merely measured viscosity and adjusted with water or more gelatin before coating, but this then caused finish to vary (due to sulfur level) and gave rise to variations in products.
Today, we have inactive gelatins in 3 grades (roughly) which are soft (75 BI), medium (175 BI) and hard (250 BI). They are all available from either the Gelita division of Kind and Knox, or from Eastman Gelatin (see the Kodak web page for products). There is little call for soft and medium and it has been vanishing from the market. It is very hard to get. The hard is the most common.
Soft, medium and hard refer to the fractions that come out of the melted purification liquor, soft being the first cut, medium second cut, and hard is third or remaining cut.
Now, as to how to add the sulfur. Kodak and Agfa researched this very completely in the 40s and found that the ripening, digestion and sensitization steps gave better results if separated by time. Ripening is growth of crystals without silver halide solvents, Digestion is growth with solvents, and the sensitization should follow them. This is due to the fact that sensitization with sulfur is changed by both silver halide solvents and by excess halide, so you should have adjusted all of these by dumping in the solvent, halide or whatever, or washing it out. The method chosen is dependant on emulsion type. If you look in your notes, you will see a definition page. It includes some of this to help you out plus other information.
As a final note, the addition of a sulfur compound to an SRAD (Single Run Ammonia Digest) can run you into fog quickly. In fact, Baker states this in his book saying that the fog growth rate is rapid and can ruin an emulsion if one is not careful. He was using active gelatin. We do not, and wash first, so the ammonia + sulfur combination is less prone to causing uncontrolled fog growth. Of course, there are ways around this, but then things rapidly get complex, and even Baker and Wall avoided this. Baker by warning you and Wall by never even mentioning it.
Hope this helps.