For using rapid fixers on film. Do a clearing test with the film(s) you use with freshly-mixed fixer to establish a baseline.
To determine fixing time for a specific batch in used fixer either:
~ Do a clearing test on the film immediately before fixing and fix the film for a minimum of twice the clearing time. Note that the clearing time will increase as the fixer is used or degrades due to age. When the clearing time in the used fixer reaches twice that in fresh fixer, the fixer is no longer good and should be replaced.
... or:
~ Fix all your films for a minimum of 4x the clearing time in fresh fixer. This is the time needed for the last films through the fixer just as it reaches exhaustion. The earlier films get more fixing time, but this makes zero difference with film. This eliminates the need to do the clearing test before each batch. You do, however, have to keep an eye on throughput and test the fixer for exhaustion as it approaches the end of its capacity. So, do clearing tests to determine when to discard the fixer and mix fresh before your throughput reaches the manufacturers' published capacity, or earlier if the fixer has been stored for a longer period.
This, IM-HO is best practice for small darkrooms without access to lab monitoring equipment.
For RC prints, using throughput as a guide is more practical, since a clearing test is more complicated. The trick is to test for residual silver (inadequate fixing) when the manufacturers' throughput capacity is being approached. More below.
For fiber-base papers, two-bath fixation is recommended to extend the capacity of the fixer. One- bath fixation works, but has a rather small throughput capacity if the goal is to fix prints to "optimum permanence" standards. According to Ilford, the capacity for one-bath fixation of fiber-base prints cannot exceed 10 8x10-inch prints per liter if processing for optimum permanence. (Note that for "commercial" or "general-purpose" levels of permanence, the capacity is around 40 8x10-inch prints per liter. Most users of fiber-base paper these days are making art prints and desire the best standard of permanence they can get, though.) The reason for the reduced capacity is that the levels of dissolved silver in a fixing bath for fiber-base prints needs to be lower than that for film or RC paper. If not, the paper base bonds to fixer byproducts and retains these, which then cannot be washed out.
With two-bath fixing, one can process up to 40 8x10-inch prints per liter of bath one, but that is then followed by bath two, which stays relatively fresh and finishes up the rest of the fixation not reached in bath one. After the capacity for bath one has been reached, the first bath is discarded, bath two is promoted to become bath one, and a new bath two is mixed. This can be repeated through 5-7 cycles before both baths need to be mixed fresh.
Testing for adequate fixation is important to determine throughput capacity. One can just use the manufacturers' recommendations, but testing is not difficult. I tested my workflow to find my own general capacity and now just test a print or two toward the end of that throughput.
Tests for residual silver in clear areas of the print (which show if fixation is adequate or not) include the ST-1 test and the selenium toner test. The ST-1 test is good, but requires a separate and fresh test solution of sodium sulfide. I find the selenium toner test more practical, since I always have the toner on hand and it never goes bad. In both cases, one simply puts a drop of the test solution onto an unexposed portion of a processed print (or an unexposed test strip processed just for testing purposes). The drop is rinsed off after three minutes; any stain other than a very faint yellowing indicates inadequate fixation. Search here for specifics on the tests.
Best,
Doremus
I find the selenium toner test to be more practical and just as