[Alt-photo] Test for residual hypo in salt prints

Ryuji Suzuki rs at silvergrain.org
Wed Aug 9 23:39:02 UTC 2017

For practical purposes and troubleshooting, I recommend the simple 
silver nitrate test and don't bother with methylene blue test.

Silver nitrate test is easy and sensitive enough for practical purposes. 
If you see anything more than pale yellow, revise the washing process. 
Static soak is not efficient, however long. Slow flow of fresh water is 
wasteful. Reasonably vigorous agitation with a few changes of water is 
probably most effective.

Residual thiosulfate does not stay in the same chemical form over time 
in the print, but it slowly degrades to products including harmful 
active sulfur compounds. Methylene blue test doesn't pick up the latter, 
so it is only useful for laboratory testing of freshly prepared test 
specimen. The sensitivity and quantitative measurability is better, but 
these advantages are of little practical impact.

Ryuji Suzuki
"Don't play what's there, play what's not there" (Miles Davis)

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