Kodak HT-2 residual hypo test
Vestal comments that "No single recommendation can possibly apply to everyone's needs" so you have to test your own print washes...
To make 1/2 liter of Kodak HT-2
water ------ 375 ml
acetic acid (28%) ---62.5 ml
silver nitrate ---- 3.75 g
water to make 500 ml
Put a small amount of the above solution (50 to 100 ml) in a small brown glass bottle with a dropper cap, and keep the rest in a sealed brown glass bottle. It should stay cool & dark, refrigerated if possible. (Just return it to room temp before using.) The solution is poisonous so mark it plainly & keep children away from it... It also causes stains which turn black & are almost impossible to remove...
To use: First develop & fix an unexposed sheet of the paper you are printing on & put it through the washing aid & into the wash along with your prints. (You don't test real prints with anything that stains them permanently: the unexposed sheet is a disposable stand-in.)
Process this sheet as you do yr prints: same development, stop & fix, so your test sheet will be in the same chemical condition as yr prints.
When you think the prints might be washed, say after 1/2 hr of wash -- cut a small sample (eg 1 square inch) from the sheet & put sheet back into the wash.
Blot all surface water drops off emulsion side with a paper towel, then put 1 drop of HT-2 on it. Leave alone for exactly 2 minutes, then rinse off the HT-2 drop with water, blot with paper towel & INSPECT AT ONCE. (It will soon darken.) Do this in subdued light.
A definite brown or yellow stain means wash is far from finished. Still too much hypo. Pale beige stain means you're getting somewhere, but not arrived. No stain at all or only faintest visible stain, means wash is fair & may be good. But it's a good idea to wash another 1/2 hour for luck.
This doesn't guarantee 'archival' or eternal prints. Silver nitrate simply
doesn't react to very small quantities of hypo, et al, which may in time cause stains & fading, it just means you've made a very good try....."
However, Vestal concludes, we don't yet "have an archival standard for b&w photographic prints or a practical critical test that can tell with authority whan a wash is complete. At present, the only truly reliable test is to look at the print after several hundred years. If it's still like new, the wash was good."
End of story.