Photographer's Formulary Residual Hypo Test is a simple chemical test that can be used to determine if a print or filmstrip has been adequately washed.
The test is based upon the formation of a yellow stain of silver sulfide - silver thiosulfate on the white of a test print (or the clear area of a negative film strip) when the test solution is applied.
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Ensuring proper washing after fixing is critical for anyone desiring archival prints. I just ordered a new container of this, because I'll soon be getting a vertical print washer and will need to test washing efficiency on all parts of all prints in the print basket. Even though the directions say that archival measuring is beyond the sensitivity of this product, PF is known to be conservative. I don;t know any serious B&W photographer who doesn't test for residual hypo, and this is the simplest way I have found and, in my view and that of many, reliably excellent.
Ordering was very efficient, delivery ok - around 2 weeks to Australia.