Old gelatins contained varying amounts of allyl thiourea, and this was finally discovered and refined out of gelatin making the first so-called inactive gelatins. The allyl thiourea could be added then in a carefully measured amount, but this meant that all old formulas (about pre 1950) were obsolete.

These old formulas can be detected if there is no addition of a sulfur containing compound with heat treatment.

Well, further experimentation showed that sodium thiocyanate and sodium thiosulfate did the same thing. It turns out that it was the heat + sulfur compound wich formed Silver Sulfide sensitivity specks on the surface of the crystal. (it is more complex than this but this is good enough here)

Then, just before WWII, Agfa discovered that gold added to the sulfur would give even higher speed, but at the cost of lower contrast. This was learned from Agfa formulas after the war. It is the only item that Kodak (and the world) learned from Agfa formulas that was unknown to Kodak.

This is the history, and now for the details.

Sulfur is added at 3 mg/mole - 100 mg/mole of silver if the average crystal size is in the range of about 1 micron to 0.1 microns just to give an example. Gold is added at 1/3 of this rate, on average. So, as grain size goes up, the Sulfur or Sulfur + Gold goes DOWN!!!!!

The emulsion is heated to 60 deg C and then the Sulfur or Sulfur + Gold is added and held for the correct time, usually about 30 minutes to 90 minutes determined by emulsion type. The emulsion is then chilled and stored for use.

This treatment will add up to 3 - 5 stops to the emulsion speed, and 1 - 2 contrast grades depending on emulsion type. It works least well with pure chloride emulsions and best with bromo-iodide emulsions with others falling in between and requiring other addenda. See Jim Browning's matrix formula in another thread for an example.

You can derive the exact amount of sensitizer to use directly from photomicrographs, otherwise you must determine it by experimentation using time and amount of sensitizer as your variables.

Remember that hypo is the best, but decomposes in solution and therefore, your hypo solution should be made up fresh every month and should be kept refrigerated.

I hope this helps.

PE

Remember that hypo is the best, but decomposes in solution and therefore, your hypo solution should be made up fresh every month and should be kept refrigerated.

I hope this helps.

PE


Hi Ron- I wish I had known you when I was working on the matrix film development. When we had Fotokemika make the film from my formulation, the first trials came out totally fogged (I mean BLACK!). I travelled over to Croatia, and sat with the emulsion chemist, and tried to think of what went wrong. We decided that there was too much hypo being added to sensitize. The amount was about 10 times the normal amount. We proceeded with the regular amount, and everything worked fine. After further scratching my head, I figured out that my hypo solution had slowly gone bad over the 3 year period I was making trial emulsions, and that caused my additions to slowly increase. Stupidly, I hadn't realized that the hypo solution was slowly going bad! It was a major snafu, causing much pain and expense. Experience is so valuable! Other than that problem, the emulsion scaled very well from 4L mixes to 100 L mixes.

Regards - Jim

Thanks for this, PE.

Just one thing... most og the 'old' formula contain a ripening stage anyway - do we add our sulphur compound at this point - or must it be a seperate step added onto the end of the process?

Steve

Steve;

This step was commonly done after the wash step. So, old emulsions can be left to stand to that point in most cases.

PE