Clarifying sulfur and sulfur + gold treatment
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.
Clarification on the Clarification
I was re-reading this and related threads today at lunchtime and I am having trouble wrapping my head around how to make up the solution necessary to use for sensitizing. No doubt I'm making this much harder than it is by going through this in my head while I am supposed to be doing real work. :whistling: Gotta get this off my mind.
Let's assume for example, my usual batch contains 6g of AgNO3 ending up with about 150ml of emulsion. I want to use sulfur + gold at a rate of 3:1. Furthermore, I have data that tells me that the sulfur is sodium thiosulfate (Na2S2O3·5H2O) used at 3.8x10^-6 mole per mole Ag. I will also use Chloroauric Acid (HAuCl4) as a 1% (w/v) solution for the gold.
- So, at 169.87 g/mol for AgNO3, that is 0.035 mol of Ag.
- at 248.18 g/mol for Na2S2O3·5H2O, 3.8x10^-6 mol is 0.00094 or about 1mg.
- and I would want 1.3x10^-6 mol HAuCl4 per mol Ag
- so at 393.83 g/mol for HAuCl4 (assuming trihydrate) that would be 0.00049 or 0.5mg
(Have I got it right this far?)
OK. If I make up a 1% solution of thiosulfate (1g/100ml H2O) each ml contains 10mg (0.01g thiosulfate. Likewise for the gold chloride solution. If I add the Na2S2O3 and HAuCl4 to a solvent (H20?) in a ratio of 3 to 1 I have my 3:1 ratio. So let's say in 100ml total, I have added 3ml thisulfate solution and 1ml HAuCl4 solution.
If I had a mole of silver at this point it would be simple enough, I suppose. But I have 0.035 mol of Ag. This is where I get stuck! I can use Denise's Steigman's formula because she says how to make and use it.
What sort of a solution do you make? :whistling: I'm lost.
Thank you all for any help.....