I ran film washing tests some years ago (or rather had graduate
students run them :-) and the results surprised me. It turns out that
you need to do very little washing to get down to a trace amount of
thiosulfate. If you use a washing aid, you literally don't have to do
more than rinse the film off.
The methodology for the test was suggested by one of the George Eaton
books on photo preservation published by Kodak. (I believe - if there is
interest, I could look up the exact details.) Basically, after the test
washing period, you apply HT-2 solution to a clear area of the film
emulsion, wait the requiste 2 minutes, and then use a densitometer to
compare a double layer of the stained film with unstained film base
plus fog. There is a table that you can use to relate the difference in
density to the amount of residual thiosulfate in the film.
It turned out that it was difficult to get any stain, literally
requiring testing right out of the fix. If there was as much as a 1
minute wash, or 2 minutes in a washing aid combined with a few seconds
rinse, the stain wasn't measurable.
So, perhaps the answer is to not put so much effort into washing the
film. It is likely that the negative is at greater risk from
contamination during the printing process than from any potential
residual fix.
Carson Graves
carson@ileaf.com