You're welcome. Here's a bit more
Originally Posted by konakoa
The Formulary residual silver test kit is the same formula as the Kodak ST-1. It is fairly reliable but not quantifiable, i.e., you don't end up with a value, just a subjective evaluation of staining. I've always erred on the side of caution and took any extra staining at all from an obviously well fixed and washed print to mean inadequate fixing. Read the page I linked to at Unblinking Eye for more info.
As for your suggested test procedure... Sorry, that will only give you a fixing time for the first print. As the fixer is used, the time needed for adequate fixing increases. When doing clip tests for film, you should test used fixer for clearing time (in essence what you are suggesting to do with paper) before each batch. This is impractical for paper. Just use the 10-minute guideline and the throughput as a starting point and test to find your practical capacity.
If you wish to process for optimum permanence, then I really suggest you use two-bath fixing, as I detailed above, and then test for both residual silver and hypo. (The residual hypo test only tests your washing sequence; you can have well washed but underfixed prints, and vice-versa. Really, testing washing is a separate thing, so I'll not address that completely from here on out, but you should do the test to find the efficiency of your wash sequence).
I would test as follows to establish a base line for your particular situation. Test the first print through the fixing and washing sequence. You should leave adequate borders on the print or simply fix and wash a blank sheet of paper. This sheet will certainly have been fixed well if you give 5 minutes in each bath (and, if you use a wash aid and wash for minumum 60 minutes in a good washer, should be washed well). This will serve as a control.
Then, after the recommended fixer throughput has been reached (approx. 35-40 8x10/liter), start marking prints for testing again. Mark you prints so you can keep track and test every few extra prints, say every third one, till you start getting test results that show residual silver.
Now you have an idea of the total number of prints you can fix in a given amount of solution. Keep in mind that this number is far from absolute. If the next batch has less developed silver in it (i.e., more undeveloped halides = lighter areas in the print) then the capacity will be much less. So, use your data as a guide and build in a generous safety factor. And, if there is any question if your fixer is performing well, test again.
And, after you have found a print that shows residual silver, refix it and the last several prints before it in the sequence in fresh fixer and rewash.
Now, if you wish to use single-bath fixing, you will find that your capacity is greatly reduced. Ilford says that for optimum permanence, only 10 8x10 prints per liter can be fixed (as opposed to 40 for two-bath fixing). Your residual silver tests will confirm this. Unless you really have a reason not to set up the extra fixing tray, use two-bath fixing for fiber-base prints!