Sandy King
>Sulfuric acid dissolves copper sulfide, resulting in copper sulfate and
>hydrogen sulfide. You should have noted the odor, though the amount was so
>small it might be indeterminate. In commercial copper etching, a gum or
>resin is used for the coating.
>
>Just for the record, unlike collodion and other 19th century wetplate
>coatings, the sulfur in gelatin actually acts as a sensitizer for silver
>halides, which is why dry gelatin emulsions, in addition to their
>convenience, were faster than the wetplates, leading to immediate
>popularity. As gelatins are variable in their sulfur content, insuring that
>various gelatins are mixed to create a compound with a standardized sulfur
>content is one of the reasons for today's uniformity in modern emulsions.
>
>Just remember, though, that experimentation is the highest form of
>expression! Techniques must be mastered before the art is possible.
>
>Sil Horwitz, FPSA
>Technical Editor, PSA Journal
>silh@iag.net
>Visit http://www.psa-photo.org/