There has been so much discussion on film fixing on this forum that I'm surprised, frankly, that there are so many "methods" that deviate so far from established, standard practice.
I'm not the real expert here on the forum (for that, search on posts by PE, Ian Grant, and Ralph Lambrecht, just to name a few) but, I'll try to give the essence of what I've learned from them and other sources.
For starters, the "fix film for twice the clearing time" rule is valid,
generally, and as a starting point. The clearing time, by the way, is easily observed by doing clip tests as described above (place a drop of fix on the film emulsion side, wait 30 sec., fix as normal while observing and note the time the clear spot is no longer visible). Twice this time is the
minimum time the film should be fixed.
With fresh fixer, this results in times that are often significantly shorter than the manufacturers' recommended times. That's because the manufacturers' times include a generous fudge factor for fix that is near exhaustion. And, because it really doesn't hurt your film to fix longer than minimum, it's better to err on the side of longer to be safe (Note: this is not so with fiber-base prints!). Generally, 3-4x the manufacturer's recommended fixing time will do no damage whatsoever (i.e., 10-30 minutes of fixing time depending on fixer type and film), and, if I recall correctly, was even advocated by Kodak and others for fixing T-Max films to get rid of the pink cast.
You should do a clip test before each batch of film fixed in a given mix of fixer to determine the new (longer) fixing time for the next batch. As the fixer approaches exhaustion, the fixing time will approach the manufacturer's recommendation.
There are a couple of things to consider when calculating the fixing time that modify the "twice the clearing time" rule. First, the clip test does not take into account the exhaustion of the fixer as fixing progresses. This is negligible if you do a clip test and then process a relatively small amount of film in a large amount of fixer. However, if you clip test fresh fixer and then use a small quantity of that fix to process a large amount of film, the fixer will weaken as fixing proceeds, effectively increasing the average clearing time and, therefore, the total fixing time. If you do a clip test after fixing, and find that twice the clearing time then is greater than the time you fixed the film, you have underfixed (return the film to the fixer and fix longer). This is one good reason for a "fudge factor."
Second, modern T-grain films and some newer emulsion formulations contain much more silver iodide than "traditional emulsions." Many recommend three times the clearing time for such films (T-Max and Delta films). Since it can't hurt...
The second part of the "twice the clearing time" rule is that you should discard the fixer when the clearing time for a given film reaches twice that of the same film in fresh fixer. This is a good (and perhaps the best practical) measure of fixer exhaustion. However, it means doing a clip test after fixing batches of film that have a chance of having exhausted the fixer. Following manufacturers' capacity recommendations is also good, but usually more wasteful, since the manufacturers have built in a generous safety factor.
Fixer builds up by-products rather quickly; this significantly shortens its life. The exhaustion of fixer due to this build up depends more on the total volume of the solution than the dilution of the fixer. A rather concentrated fix will reach twice the clearing time for a given film well before the fixing agents themselves are depleted due to this build-up.
Using "twice the clearing time" to determine fixer exhaustion yields negatives with an acceptable level of residual silver (fixer by-products that won't wash out) for "general use," or "commercial use." If processing for optimum permanence, the fixer capacity is less than this and, usually, another fixing method is used (see below.)
If you are interested in getting the most out of your fixer, there are a couple of strategies. The first is to use a two-bath fixing scheme, fixing for twice the fixing time in fresh fix in each bath. To get the most economy from this, you must have a large enough through-put to be able to use the first bath to exhaustion (twice the clearing time) and then replace it with the second bath (which is still relatively fresh) and be able to do this through a few cycles. This also generally yields negatives that have a significantly smaller amount of residual silver than the single-bath method does. (This would be of interest to those processing negatives for optimum permanence.)
In light of the above, I like to fix with a generous "fudge factor" to ensure that proper fixing is being done. I usually fix in two baths, 2x clearing time in each bath for "traditional" films and 3x clearing time in each for T-grain films.
Another strategy is to use a more dilute fixing solution (and fix for appropriately longer times). This results in a greater percentage of the fixing agents present being used before the build-up of by-products kills the fixer. It won't yield "cleaner" negs like the two-bath method unless you discard this more dilute bath well before the "twice the clearing time" limit has been reached.
When I have only a few negatives to process, often use a single bath of rapid fix at "print strength" (e.g., 1+9 for Ilford Rapid Fix) and use the fix to well "below capacity."
There are some who favor a very dilute fix used one-shot. The problem here is determining the minimum amount of fixing agents needed for a given area of film. If you are willing to do the testing, and have the patience to sit through the longer fixing times needed with dilutions of 1+20 or more, this is likely the most economical way to use fixer and yields negatives with a very low level of residual silver.
There are lots of threads on the above methods and lots more details, but, please, can we agree on the basics:
1) how to find a minimum fixing time for a given film (clearing time x 2 or longer).
2) when to toss the fixer (clearing time x 2 or less).
3) fixing longer up to a point does no damage and is good insurance. (So, you can, and I think should, build in a reasonable fudge factor).
And that:
1) fixing film by inspection (i.e., just until it clears) does not fully fix the film.
2) using exhausted fixer, even for a long time, will not give you fully fixed film.
3) doing clip tests before and after each batch of film fixed is a reliable way of determining a) fixing time and b) fixer exhaustion. (So, use it.)
Finally, following the manufacturers' recommendations for times and through-put (i.e., capacity in x number of rolls, etc. per liter of solution) are good guidelines and take all the above into consideration along with a safety factor and will yield fully-fixed negatives with a generally acceptable level of residual silver 99.9% of the time. If you aren't going to do clip tests, then at least follow the directions.
Happy Fixing
Doremus Scudder
www.DoremusScudder.com
