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.
Very helpful information -- thanks for the thread! Gotta keep this short because I'm supposed to be working, but how does one know when the 'pre-fog' point has been reached? And to ask an even dumber question, what's a good way to test for fog -- visual inspection, magnifying glass, microscope?
Oh yeah, one more 'easy' question -- if adding hypo is done per mole of silver (I assume you mean silver halide), how can I tell how much silver nitrate has been converted?
If you take a sample every 10' over 30' - 90', you will see good dmin and then fog in the series. The one before the fog starts is the level you want. If you take samples every 5' or more often, your accuracy will improve.
We used to use small porcelain sample dishes or ultra pure blotters for this test. A drop of emulsion in the dish or dipping the blotter and then developing would give us the fog level in the dark.
As for sulfur to silver ratio it is milligrams / mole of silver itself, and you assume that 100% of the silver nitrate is made into silver halide and is therefore present in the emulsion. The number of moles of silver therefore remains constant throughout, but the volume of the emulsion and the % gelatin will change due to the washing step if you use one.
PE, I guess I'm just not getting it. I understand what you mean by fog and I understand using a little on blotting paper and developing to look for it, and I could understand if you said stop as soon as the slightest hint of fog starts showing up, but how do you know fog is going to start showing up on the [i]next[i] sample?
Well, the only way I can suggest is that the first finish experiment is a 'throw away' experiment done with a very small quantity of emulsion.
Also, the fogged emulsion can still be chilled and used to give you an example of what the speed is, albeit a foggy speed. Then too, remember that quite a bit of reflection fog does not show up when coating on film, so when you first see it on a blotter (reflection) then it will not be visible on film (usually depending on level).
So, two things to remember here. First, the initial attempt is a throw away but you can use it for generic tests. Second, if you catch it in time, the finish will not work on paper, but will work on film most of the time.
PE
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Chemical sensitization (finishing) emulsions 
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