Dichroic fog

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mska59IT
Posts: 5
Joined: Thu Jun 06, 2019 6:17 am

Dichroic fog

Post by mska59IT » Sat Aug 03, 2019 4:58 am

Hello, some experts say that not using the acid stop bath in the film development increases the risk of having dichroic fog on the negatives. The book does not talk about this risk and how to reduce it, could you give some information on the subject? Thank you

Bill Troop
Posts: 8
Joined: Wed Mar 09, 2016 2:06 pm

Re: Dichroic fog

Post by Bill Troop » Wed Aug 07, 2019 3:03 am

This kind of dichroic fog occurs when developing in the presence of a strong silver solvent, for example, a fixer. Because of the way our typical use of developers and fixers has changed over the years, this is no longer a problem in most scenarios. In order to achieve dichroic fog because of not using an acid stop bath, film must be transferred from a strong developer into an exhausted fixer that is no longer sufficiently acid to stop development. Because of the strength of the developer and the weakness of the fixer, development will continue for some time in the fixer. If it continues long enough, dichroic fog may result. Several things work against this happening today: 1: we tend to use dilute developers which do not have the strength to continue significant development in the fixer solution whether it is acid or alkaline; 2: when we don't use a stop bath, we intentionally use a water rinse, and we know we must rinse for long enough, at least 30 seconds, and with continuous agitation and flow of water, to remove the developer from the film; 3: we tend to use rapid fixers which clear quickly enough not to allow the formation of dichroic fog; 4: we tend not to overuse fixers as was a more common practice in the past. Today, dichroic fog is most likely to result during the development stage, when we use an extremely solvent super-finegrain developer. This is one of many reasons why super-finegrain developers are no longer popular. There is more discussion of these issues in the upcoming edition of Film Development Cookbook which should come out late 2019 or early 2020.

mska59IT
Posts: 5
Joined: Thu Jun 06, 2019 6:17 am

Re: Dichroic fog

Post by mska59IT » Wed Aug 07, 2019 11:46 am

Many thanks for the in-depth explanation, I look forward to reading more about in the new edition of Film Development Cookbook

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