>coat of gelatin, into which was mixed several ccs of formaline. The plates
>looked great at first, with shiny copper and a very thin, clear coat of
>gelatin. The next morning, however, they had all turned black. My first
>thought was that some kind of reaction had taken place between the formalin
>and the copper, but upon recoating with gelatin with no formalin, the same
>blackening of the plates took place.
Gelatin contains sulfur compounds, and copper (like silver) is very
reactive with sulfur. The blackening is undoubtedly copper sulfide, which
is black/gray.
>I found that a little Sulfuric Acid strips the gelatin and returns the
>plates back to a nice shine so I at least back to home plate.
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/