Why not have a look in the relevant literature; any decent book should cover this. In Anchell/Troop, for example, there’s a whole chapter devoted solely to intensification and reduction. It states, for example:
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Ansel Adams used intensification to expand roll film negatives by at least one paper grade,
or in Zone System parlance, N+1. The intensifier he used was selenium toner diluted with
water 1:2. His technique was to first re-fix the negative in plain hypo then soak the negative
in selenium toner 1:2 for 3 to 5 minutes.
To dilute the selenium, he recommended using a working-strength bath of Hypo Clearing
Agent (HCA). He followed the toning bath with a second, full-strength bath of HCA, without
selenium, and then a thorough wash.
The purpose of re-fixing the negative in plain hypo before selenium intensification is to
ensure that there is no residual hardener. Hardener will cause staining in both negatives and
paper when it comes into contact with selenium. If you are confident that your negatives have
been thoroughly washed, or if you use fixer without hardener, then re-fixing should not be
necessary. But when in doubt, re-fix.
The list of intensifier formulas includes uranium, silver, copper, selenium, chromium, mercuric
chloride, and mercuric iodide. Some of the formulas, such as uranium, create a high
degree of intensification but a low degree of stability. Others, such as chromium, do not intensify
nearly as well, but the image is reputedly quite stable.
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You can also find recipes for a wide variety of intensifiers there. Kodak has also
published at least five recipes for this purpose
. You might find this interesting too.
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Good luck
Nils
