When pickling silver in a boiling solution of sulfuric acid (10-20%) the surface of the metal turns black since shortly. I think the solution is contaminated but no idea with what,

Is is not a problem (I just prepared a new solution which does not blacken the work) but I am curious what the black deposit is.

 When I heat the silver to dull red (600oC) the black is gone and dues not come back after colling down.The only foreign material I put in the pickle are plastic, stainless steel or titanium tongs.

 Any ideas on this ?

Could be from the stainless steel. I was taught never to put anything with steel in the pickle pop or that is the result you get. Always use plastic or copper tongs.

Perhaps you should only use copper tools in the pickle , I find they are the safest to use and have used them always ,but if in doubt drop  piece of waste silver in before the piece you are pickling. Safest to use a bit of waste metal than wreck  something you have worked on  for hours.How long did you leave the work in the pickle? Sometimes it takes a while to work.

I follow the opinion of Bentiron. Already a small bit of binding wire can sulfuric acid-pickles make useless.

A good insensitive and health harmless pickle confesses from 10 gram alum salt and 90 gram (millilitre) water. Work temperature approx. 50 degrees Celsius. (122 degrees of F). This pickles is insensitive also to light impurities (iron). Alum salt costs only few cent in the drugstore .

Edmund

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