Recently I bought an "uncleaned"
Tetradrachm of Demetrius II. After removing some tarnish there was revealed major encrustations of Horn silver on the
face and some on the
reverse as well. I could find no
good instruction for removing it. Battery (sulfuric) acid will take it off but devastate the coin as well.
Chemically, horn silver, or chlorargyrite, is mineralized silver chloride (AgCl). A
fine emulsion of AgCl is used in photo film. Exposure to light releases metallic silver. When the emulsion is dissolved away leaving behind black silver deposits on the film this creates the "negative". The chemical used to dissolve the emulsion is called "hypo" or fixer. I ordered some of this chemical (it is quite safe) from a photo supply
shop. It
comes as a concentrate which I used straight from the
bottle.
My first photo shows the coin as I photographed it for my
gallery. Clearly the horn silver deposits are a problem. The next photo is a close-up of the main problem
area after some pre-cleaning with ammonia -which proved unsatisfactory for long-term use because of its volatility.
When the "hypo" arrived I
applied it with a Q-tip and rubbed vigourously. After about an hour's
work the deposits were
reduced significantly. This is tedious
work. It smetimes seems that no progress is happening and then suddenly it happens. I tried warming the hypo before applying it and this helped a
bit. After about 3-4 hours total it seemed that little further progress was being made. There may
still be some
work to be done but now Demetrius looks a lot better.
