Zitat: 

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 washing.

The purpose of re-fixing the negative in plain hypo before selenium intensifi cation is to

ensure that there is no residual hardener. Hardener will cause staining in both negatives and

paper when it comes in 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.