Iodide in emulsions
A while back I had a furious argument offline with an APUG member regarding adding iodide to an emulsion. He insisted that iodide would cause an effect called 'renucleation' which led to fog and I said that it could increase contrast and speed. It got rather vitriolic, so I wanted to clarify things.
I posted an AgCl emulsion recently based on an Agfa/Kodak AgCl emulson with Iodide addition, and here is an explanation.
Iodide, up to about 3% can be added to an emulsion to increse either speed or contrast or both.
Here is what happens.
Iodide adds as a surface effect, and so it goes after the finer grains which are slower and more numerous. Since the speed of slower grains increases, the darker areas gain in 'darkness' which causes speed and contrast to go up. Very simple and known for over 50 years. Add iodide, increase contrast!
However, if you add too much, there is renucleation and then the emulsion goes into fog. Therefore, the careful addition of iodide is a method of increasing contrast and/or speed of any given emulsion. It is an art.
There are restrictions and other variables on this. I am not prepared to discuss them, but I do refer you back to the basic AgCl emulsion posted earlier with the iodide addition.
An interesting point in this is that one individual insisted that no iodide was added to the Agfa AgCl emulsions. Ian Grant and I would laugh at this, as KJ in Germain is the symbol for KI in English or Potassium Iodide (Jodide) in both cases. A language barrier that caused an argument in a particular case.
Just looking quickly at some emulsion formulae it is perhaps worth clarifying a few points which confirm what Ron is saying.
In some commercial Bromide paper emulsions the ratio of Potassium Bromide to Potassium Iodide can typically be around 60 KBr :1 KI (by weight), with faster emulsions the amount of Iodide is often higher 40:1 is fairly typical.
Potassium Iodide is also used in Azo type emulsions as Ron has indicated, and the formulae have been published, in this instance the ratio of Sodium Chloride to Potassiun Iodide is around 130-220 NaCl: 1 KI by weight
Of course not all emulsion formulae contain Potassium iodide and many used to contain Cadmium Chloride, and occasionally Lead Nitrate, but one thing is for certain the Agfa Lupex - Azo type formulae does contains KI.
Some of the other formulae will be posted here on APUG as soon as the Article section is finally revamped.
Last edited by Ian Grant; 10-13-2007 at 06:36 AM. Click to view previous post history.
Reason: Add NaCl:KI ratio
Thanks Ian. The bottom line, for those interested is that Azo paper is an AgCl/I emulsion, not purely AgCl as some have claimed.
Also, some added notes are in order. Please look again at the Azo type formula I posted and see that the iodide is added AFTER precipitation. This type of control does not normally work during or before precipitation!
Also note that Cadmium salts work poorly if at all in bromide only emulsions. And, note that Cadmium salts are used at much higher levels than most other salts for contrast control. They work to increase contrast by another method. Cadmium salts sharpen or 'scoop out' the toe of the emulsion thereby raising contrast.