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Hardening gelatine, pigments for gum

Hello Judy,

let's try sorting this out.  I claimed, that glyoxal is probably not as
effective an hardening agent for gelatine as chrome alum or formaldehyde.
By "not as effective as" I principally mean that glyoxal-hardened gelatine
is less resistant to high temperatures and/or physical abrasion as the
alternatives.  My source is "hearsay": a professional chemist told me.  He
and a fortiori I may well be wrong.  But not being a chemist myself, I
wouldn't know how to set up and interpret reliably an experiment.
Apparently, you did some testing yourself and found glyoxal at least as
good as, perhaps even better than ChrAl or FdH.  What I do not dispute is
that for common gum printing the three candidates for hardening are
probably all good enough.  But our little discussion was sparked off by a
case where gelatine is being subjected to some extra pressure (i.e. soda).

> >As for "clearing gum" in alum. Consensus of conservators seems to be that
>> >alum bath degrades the color -- especially ye gods 30 minutes. (No I can't
>> >document, but have read  5,627 times.)
>>
>> Depends on the color pigments.  Alum does absolutely no harm to the more
>> stable pigments (eg earth pigments) that IMHO one _should_ prefer for gum
>> prints anyway.
>
>Again, do you have any sources for the statement about alum?

Yes, 4 sources:  common sense (;-), qualified hearsay (a befriended
chemist), a monograph on the chemistry of pigments (in German), the work of
Heinrich K"uhn.  Let's take only the first and the last source.  Clearly,
colour pigments are of _very_ different chemical composition.  Chemically
there is nothing that all of them have in common.  It is therefore
extremelely unlikely, that alum has a degrading effect on all of them.
Hence, the sweeping "consensus of conservators" is extremely unlikely to be
true.  (In fact, I doubt whether there is such a consensus, as opposed to
an unwarranted generalisation handed down in the amateur literature.)
Heinrich K"uhn used mostly earth pigments for his gum prints and cleared
with alum (as apparently most of his contemporaries). I don't know what his
prints looked like 100 years ago but they still look good enough to me.  He
was a dedicated amateur chemist and had a very discerning eye.  If he
didn't complain of a degrading of colours during the alum bath, I shan't
either.

>As for what one should "prefer" for gum printing -- an interesting premise
>which it would be most interesting to hear explained, tho from here it
>seems difficult to do a tricolor print with earth colors.

But Judy, you must know perfectly well that some pigments are less suitable
for gum printing than others.  Technically, because the more suitable
pigments allow you to build up good densities with only a few printings and
no staining problems.  In short: with such pigments you get nicely
contrasty prints with clear highlights ... if it is that what you want.
Here we get to the aesthetic aspect.  Of course, everyone is at liberty to
use whatever pigments he or she likes.  To my taste, however, tricolor
printing is an aberration -- at least if you are aiming at "natural"
colours.   There are better ways of achieving this.  But, to repeat, this
is a matter of taste.  We don't need to agree on this.  But can't we agree
on this:  there is a large array of pigments, eminently suitable for gum
printing which are, for all we know, not ill-affected by alum?  And, for
all I know, even with the colours you prefer for tricolor printing, chances
are not too bad that they emerge unharmed from an alum bath.

Andre

PS
BTW I prefer _not_ using alum for clearing: simply because it takes too
long and extends the final wash.  I prefer using metabisulfite.