Friday, June 27, 2008

Color Processing

Color negative developing (C-41 process) and color print developing (process RA-4) are very similar. The principal difference is in the color developer formula in the first step; and the combining of the bleach and fixer steps in process RA-4 with a bleach-fix mixture (blix), which dissolves both the silver halides and the elemental silver leaving only a dye image.

Mixing the bleach and fixing agent in process RA-4 is optional; they are often mixed to minimize the number of processing steps.

In color reversal processing the film is developed in an MQ (metol/hydroquinone) developer similar to a black-and-white developer, followed by a rinse or a stop bath. The film is fogged in the reversal step, and is then developed in a color developer.

Next, the film is then bleached to remove the black-and-white negative image (metallic silver is removed by the bleach), while keeping the color positive image (dye is not affected by the bleach).

The film is optionally washed to minimize the carry-over of the bleach to the next bath, fixer. Film is then fixed, washed and dried conventionally. In some old processes, the film emulsion was hardened during the process, typically before the bleach. Such a hardening bath often used aldehydes, such as formaldehyde and glutaraldehyde.

In modern processing, these hardening steps are unnecessary because the film emulsion is sufficiently hardened to withstand the processing chemicals.

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