>Also, all of the chemicals so far discussed:
>alum, sodium bisulfite, and sodium metabisulfite are hardeners.
to this Luis.Nadeau@itchy.mi.net (Luis Nadeau) replied:
>
> As far as carbon printing (i.e., pigmented gelatin) goes, potassium alum is
> a weak hardener. Chrome alum, acidified, is a much more powerful hardener.
> The bi and metabisulfites, by themselves, are softeners. There is more on
> this in my carbon books.
>
Also, at least one other person replied with a similar statement.
This sent me back to my source (I couldn't dream it by myself), which I
finally located under a huge pile this weekend. It was from a Patrick
Dignan newsletter in the 70's describing a formula for clearing gelatin
silver emulsions (of thiosulfates, not dichromates). A lot of the
formulas (including this one) were compiled in a funky pamphlet called
"150 Black and White (sic) Popular Photographic Formulas" (copyright,
1977).
The formula contains a small amount of sodium bisulfite, and about that
Dignan writes: "The Sodium Bisulfite is in the formula only to lower
the pH to 7 - 7.5 to prevent softening the emulsion of film. If papers
are to be used, leave out the Bisulfite to deliberately get more
softening..."
Now, my question is:
1) Is the above statement correct? (I've confirmed from other sources
that sodium bisulfite does indeed lower the pH of a solution, but what
about the "preventing softening"?)
2) If the statement is correct, then how is my inference that sodium
bisulfite is a hardener incorrect.
I'm assuming that I have drawn the wrong conclusion, so what I am
asking is for an explanation of how I've gone wrong. Also, I am
assuming that whatever goes for sodium bisulfite goes for sodium
metabisulfite.
Thanks for any enlightenment.
Carson Graves
carson@ileaf.com