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Making temperature was 42C. Finishing (sensitization) temperature was 60C as instructed by PE in all of his posts concerning sensitization. There should be no ammonia left after neutralizing and washing, if I have understood it correctly.
I managed to get information from the local company that sells the food grade pig gelatin in question.
Their gelatin is imported from Germany. All the batches come from the same manufacturer, they say. They even told me sulphur content according to batch analysis. It's < 50 ppm, and they say there's only little variation between batches.
So, given this information, I made some calculations:
50 ppm(wt) = 0.005 wt-% = 0.05 mg/g
Our emulsion used on average 3 g of gelatin during making at 42C -> 0.15 mg Sulphur
As we compare to hypo (Na2S2O3.5H2O) used in sensitization figures, it has molar weight of
(158.108 + 5*(2*1.007 + 16.00)) g/mol = 248,178 g/mol
S has 32.065 g/mol
-> ratio between hypo and S is 7.73x.
-> 0.15 mg S corresponds to 1.16 mg hypo
We had 0,0765 mols of silver
Final corresponding figure during making is: 15 mg hypo / mol silver
Then, a calculation of final Sulphur level, after addition of gelatin just before wash.
Note: This has not gone through any kind of heat treatment, as it was quickly
chilled, washed and then put to rest, but this is important for later treatments.
Gelatin total 18.5 g -> 0.925 mg Sulphur ~ 7.15 mg hypo
Same Ag content
Final corresponding figure after making is: 93 mg hypo / mol silver
1) Sulphur is in a different form in gelatin than in hypo. Do they react at the same ratio, so that I can convert them based on weight as I did above?
2) Are some Sulphur compounds washed away in emulsion washing?
3) They told me it's below 50 ppm. What we cannot know is if it's really near to 50 ppm or lower, like 25 ppm.
In any case, if it contains already about 100 mg/mol hypo-equivalent, or a little less, it's no surprise that adding 100 mg/mol more did what it did. We're gonna try sensitization without added hypo at all next time.
07-09-2010, 09:24 AM
Making temperature was 42C. Finishing (sensitization) temperature was 60C as instructed by PE in all of his posts concerning sensitization.
Going with a lower temperature might still be worth a try, but I imagine you've already decided that. You seem to have this whole science thing figured out :).
(And, I add my gold at 52C. That's the very top temp I can seem to go without problems, and sometimes I think that might be pushing it. But, again, that's Steigmann's gold, not hypo. For all I know, they're apples and oranges.)
Thanks for tip, maybe we go at 50C at the next try. At least it could help if we have too much sulphur to begin with in gelatin.
07-09-2010, 10:13 AM
I have made this formula and variants from 40C to 60C. I have done all sensitization with sulfur at 60C. The reason is that the formation of Silver Sulfide from the breakdown of Sodium Thiosulfate is quite good at 60 deg C. The original work was by Sheppard in the 20s, IIRC, and involved the use of Allyl Thiourea. It is what is formed from the sulfur compounds in gelatin.
So, sources of Sulfur include Allyl Thiourea, Thiourea, any Thiocyanate and Sodium Thiosulfate (usually Pentahydrate). These all break down with heat to react with Silver Halide to form Silver Sulfides of some sort. Gold reacts with this to increase threshold speed and fog so it adds a layer of complexity. It is used at the ration of 3 Sulfur / 1 gold or even less gold.
Early documents on Sulfur + Gold are found in the BIOS reports and also in publications by Sheppard and by Woody Thomas (Woodlief) of EK.