Sir John Herschel : Inventor of Sodium Thiosulfate Photographic Film fixer as well as the Cyanotype Process.For people beginning film processing at home, some of the common questions are “What is the correct fixing time for Agifix rapid fixer?” (or any other fixer), “Do I need to add a hardener in my fixer?” and “How long can I use the fixer working solution before it exhausts?”.

If you have encountered these questions, you are in the same boat as Sir John Herschel, the inventor of Hypo. But we now understand the fixing process much better and have more concrete answers to all these questions compared to Mr. Herschel. So here is our current understanding of things:

Film Fixing time : Different films require different fixing time. A quick way to determine fixing time is this – take the working solution and put a small piece of film in it, shaking occasionally. (a 1 inch piece of film leader, for example). Measure the time it takes for this film to become completely clear/transparent (usually 1 to 3 minutes). This is called clearing time. the fixing time should be thrice the clearing time (usually 3 to 9 minutes).

Fixer reuse : You can keep reusing the working solution till the fixing time doubles from that needed for the freshly mixed fixer.

Hardening fixer : Many fixers come with a hardener, either in a separate packet or some times already added. Film hardener is usually some form of alum that makes the gelatin harder, so it becomes more resistant to accidental scratching during and after processing. This is a relic from the old times where the emulsion used to be quite delicate.  There is usually no need for hardener for modern films. Having said that, a hardener is useful when processing film in extremely hot conditions, where there are chances of excessive swelling of the gelatin making it more susceptible to damage.

Over fixing : It is a common misconception that fixing times are not critical. Actually, one CAN over fix – Acid fixers will start eating up the image forming silver (i.e. bleaching the film/print) if it is left for too long in them. Also, the longer you leave the film or paper in the fixer, the longer you will need to wash it to wash away all the fixer from it. If not washed properly, your film will degrade over years because the residual fixer in the film or paper will bleach away the silver.

The fading that you might have seen on really old black and white photographs is most of the time due to over fixing.

For best archival processing, one should fix only as much as needed and then wash the film/paper for long enough to get rid of the fixer completely, preferably in hypo clearing agent (a 5% solution of sodium sulfite). But it’s always better to err on the side of a little over fixing than underfixing, because you can always wash the film longer. Alkaline fixers are more fullproof against overfixing and also wash away more easily.