Re: Hardening


FotoDave@aol.com
Fri, 29 Jan 1999 13:03:25 -0500 (EST)


In a message dated 1/29/99 2:50:13 AM Pacific Standard Time,
cor@ruly46.medfac.leidenuniv.nl writes:

> So this Alum can also harden a gelatin
> emulsion I presume, but can it also prevent digestion by micro-organisms?

Yes, it can too. It will harden the gelatin the same way, except that alum
works much slower (than formaldehyde or glyoxal) and it is weaker. Well,
perhaps slower is the better description as given enough time, it can harden
the gelatin to a point that a piece of paper become crispy!

Due to its slowness, if you want to use alum as hardener, it is best to
prepare the paper in advance (for example, one week ahead) although I have
used it immediately also, but I haven't done a lot, so I don't have
accumulated data to tell.

While alum is slow and continual in hardening gelatin, I believe it can harden
at least a little (probably quite a bit in fact) very quickly too, otherwise
the whole concept of using alum in hardening fixer won't work at all.

> I think that introducing salts in paper for gum etc. is not a good idea..

This has been discussed before and mentioned again recently. Since we wash and
wash and wash our prints, the alum will get washed out too, so it is ok
(unless your process doesn't require a wash after exposure).

Dave



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