Dark reaction in dichromated colloids

But that's not what I really wanted to talk about. I wanted to talk
about the dark reaction, but I made the mistake of doing my first dark
reaction test on that stupid glutaraldehyde paper, so even though the
gum was without a doubt hardened in the dark, it did that thing with the
granulation (this one also did a really cool thing that makes part of it
look like a fingerprint). So I went ahead and did another test on
unsized paper. I left the first one for 12 hours and was going to leave
the second one for 12 hours as well, but I went out a while ago and
tested it with a damp finger; it was clear that it was already
well-hardened after 5 hours, so I went ahead and developed it. The
results are here:

http://www.pacifier.com/~kthayer/html/Current4.html

I really don't care about the dark reaction, since I print as soon as
the coating is dry, which is about 30 seconds from when I coat the
paper, so it's not an issue that has any relevance to my practice. But
I've been lost for months in the rabbit hole of trying to
understand the chemistry of the gum process, and in this search I keep
running into the dark reaction.

(I should probably be sure to start that everyone understands what the
dark reaction is, because it has been misidentified here before. The
dark reaction is a reaction that starts as soon as the dichromate and
the colloid are mixed together, and proceeds without any exposure to
light, hence its name, the dark reaction. This is not the same thing as
a reaction that continues for a time after exposure.)

 What's interesting is the completely contradictory information about
the dark reaction. Eder may have been the first person to mention the
dark reaction. Galinsky some 50 years later surmised that Eder only saw
a dark reaction because he used dirty gelatin; she tried the same thing
with purified gelatin and found no dark reaction. Grimm et al in
1983 reported that they could only get a dark reaction with PVA by
adding sulfide ions; otherwise there was no measurable formation of
Cr(III) ions and no change in viscosity of the PVA even after a week in
the dark. Kosar is totally "with" the dark reaction and spends several
pages on it. Christina, as I recall, found that she could store her
dried coated papers in Montana for days (weeks?) without any dark
reaction. Sasaki et al express concern about the possibility of the dark
reaction interfering with the usual photo reaction of dichromated
colloids, but eventually decide that it's not a problem.

I think the difference between mine and Chris's Montana experience is
the humidity, but I don't know if humidity plays a part in these other
cases where no dark reaction was observed, and it would be interesting
to find out if Chris sees a dark reaction in her more humid
environments.

Kosar recommends raising the pH to 9 to keep the dark reaction from
happening; he says you can store your papers indefinitely if you keep
them basic. Since at pH 8 or higher, the dichromate exists almost
entirely as CrO4-2, which by a number of investigators has been
found not to be photosensitive, obviously you'd need to change the pH
back to acidic in order to use the papers.

I just thought I'd share some of the conflicting information I've come
across about the dark reaction, along with my own observations. I don't
really think it has much relevance to gum printing practice as most of
us practice it, but it's curious that even very careful scientists are
so at odds even about whether it occurs.

Katharine Thayer
Received on Fri Jun 11 19:01:15 2004