How To Test Your Fixer In The Darkroom by Tim LaytonI get a lot of questions from photographers on how to know if the stop and/or fixing bath has been exhausted.
I don't use indicator stop baths and since I mix my own chemicals, I personally just mix fresh stop bath for every development session.
I like this question because stop and fixing baths definitely should not be used beyond exhaustion because it can lead to spots and stains on negatives or prints, which unfortunately may not appear until a later time.
If you don't fix long enough, or if the fixer is exhausted in combination with improper washing, you can negatively impact the long-term permanence of your negatives and prints.
Free Analog Photography Journal by Tim LaytonAn overworked stop bath ceases to stop development, which is not what we want to happen. This happens because the developer that is being carried over into the stop bath eventually neutralizes the acid in the stop bath.
As a general rule, you can mix a fresh batch of fixer and take a sheet of film, if you are a large format photographer or a portion from a roll of film, and with the lights on, see how long it takes for your film to fully clear. I actually cut up my sheet film into smaller portions for these tests and you could do the same thing with roll film too. I have a special old film box where I keep these films.
For example, if it took 2 1/2 minutes for your film to clear with the fresh fixer, you should document that time and double it for your fixing time. In this case, your film should be fixed for 5 minutes. Next, you can perform the same test with your reused fixer over time. Once your original time for clearing of 2 1/2 minutes has doubled, you need to discard the fixer and mix a fresh batch.
Keep in mind that different films require different cleaning times. So, if you are using different films, make sure you know their clearing times for each film.
You could also use fresh fixer every time you develop your films and prints, but that is not necessary as long as you manage your chemicals properly. It is really important to properly wash your films long enough. I use a hypo clearing agent to help shorten my washing times significantly.
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Hi David, thanks for commenting and the question. My wash times vary by film. For example, I found that by using a hyper clearing agent bath for 4 minutes, I was able to reduce my wash times to less than half of what it was. You can get a hypo testing chemical to verify films are being washed properly. Conventional darkroom practice recommends washing film for 30 minutes or longer, with a flow of water sufficient to change the water in the washing container at least three times. This is not needed when non-hardening fixers are used and I have found the time to be half that when using hypo clearing agent. If you are using a non-hardening fixer you can use the Ilford method to really shorten your wash time and also save a ton of water.
Fill the developing tank with tap water at the same temperature as the fixer (+/-5 °C or 9°F)—maintaining a constant bath temperature during processing is necessary to avoid reticulation of the emulsion; Invert the tank five times and drain it completely; Fill the tank again, invert it ten times, and drain it completely; Fill the tank again, invert it twenty times, and drain it completely. The film is now washed.
Thanks Tim. This holds true for wet-plate too. What about rinse times? :-)
Tim I completely agree with you. I do the same things for decades