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