Chemical sensitization (finishing) emulsions
There has been a lot of activity on APUG about chemical sensitization of emulsions (finishing) both on threads and in my private mail so I thought I would start this thread on the subject.
When an emulsion is made, it has little sensitivity and often low contrast. Old textbooks don't describe this fact (or fault) because the formulas use active gelatins which carry out the sensitization step as the emulsion is digested or even as it is precipitated.
Modern gelatins have had this active ingredient removed. It is allyl thiourea. Today, it is common to use sodium thiosulfate pentahydrate. It is put onto the emulsion at the rate of about 10 - 100 mg / mole of silver and since it is a surface phenomenon, it is based on the surface area of the emulsion. But, I hasten to add that there are a lot of caveats here. Halide type and other factors enter into this.
Now, here is what goes on.
After the emulsion is made and digested, you may either add the hypo, or wash and add the hypo. In real practice, after washing, the emulsion is adjusted for both pH and conductivity (pAg in technical terms, which is a measure like pH but for silver ion). Before washing this is not done.
So, we have added hypo to the emulsion, typically at 60 deg C, and now we hold it for anywhere from 30' to 90' taking samples and coating them. Usually, this is done every 10'. The emulsion gains in speed and contrast until it begins to fog. Just before the fog point is the optimum finish. This treatment can gain you up to 1 - 3 contrast grades and several stops in speed.
The finish is slower in unwashed emulsions.
You can use a mixture of hypo and gold salts in a fixed ratio to obtain even more speed but at lower contrast. This is the sulfur gold finish which is used in most films today.
This process cannot be done on wet plates or coated materials, only on dilute melted emulsion prior to coating.
Other things that can be done... Adjust pH. Lower pH within reason, using citric acid will increase contrast. Addition of thiocyanate will accelerate the finish, but also the rate of fog formation. Adding benzotriazole will retard finish and fog.
These are some of the 'knobs' available to the emulsion maker.
You must be aware though that 'real' emulsion finishes are hours long procedures with many many steps, conducted at controlled rates of heating, cooling, pH and pAg. There are dozens of addenda that can be added to control rate of finish and curve shape.
The best we can do at home is either plain sulfur or sulfur + gold at the optimum time for our emulsion and 60 deg C. This will do and can achieve a good high-speed emulsion of up to ISO 100.
I'm being conservative and am going to aim for about ISO 50 though or maybe 25.
Hope this helps.