Re: Colors of Dichromate Stain (was:Re: Sodium Bisulfite

From: Katharine Thayer ^lt;[email protected]>
Date: 10/19/04-07:14:52 AM Z
Message-id: <41751347.26A8@pacifier.com>

Alberto Novo wrote:
>

> As for the speed of the process, I think that a clearing process should rely
> on reasonably short times of action. What is "reasonably short" is again a
> matter of taste.

I think perhaps my point has been missed. Bisulfite is faster enough
than sulfite that it wouldn't make any sense to choose sulfite for
clearing; I think that's been well established, and that was what I
reported when I tested them in comparison with each other. There is
simply no disagreement between us on this point.

But time of action aside, and the choice of bisulfite for clearing taken
for granted, I find it interesting that given enough time, sulfite
appears to have the same effect as bisulfite, contrary to the assertion
that sulfite simply can't work because of its alkaline nature, and that
both of them appear to have a different effect from clear water or salt
water, both of which have been recommended here as being as effective as
sulfite or bisulfite for clearing. That was the distinction I was making
between water or salt water, which simply fade the stain to a lighter
brown, and sulfite or bisulfite, which actually react with the stain to
produce a blue-grey or green-grey product; I don't believe that water or
salt water can be said to "clear" the stain in the same way.

As for the colors, I'll have more to say about that later when I have
samples to show. But my distinction between the green and the blue and
brown referred to film scraped off the support, in which the color
becomes more evident because the gum is more concentrated. In the film,
as in our tests, the colors are much less visible and would be much more
difficult to distinguish. But one point that should probably be made is
that I did the sulfite/bisulfite tests both on mylar and on paper, with
identical results, so I wouldn't put too much into the hypothesis that
the paper makes a big difference. And after all, gum works the same
whatever support people put it on: glass, aluminum, mylar, paper,
whatever.

>
> I think that this matter is interesting in particular if referred to prints
> made with subtle, transparent colours, which may be shifted towards unwanted
> hues by the gum stain.
>

Sure, but I think most gum printers are well aware of this and do clear
if they are unlucky enough to be plagued with dichromate stain in their
practice. What interests me at the moment is the blue-grey tinge that
can result from treating the stain, which reinforces my conviction that
printing in such a way to avoid dichromate stain altogether is the best
way to go.

And yes, I think the chemistry of the dichromate stain is interesting in
the same way that the chemistry of everything that relates to the gum
process is interesting. But I think all we've managed to establish here
is that it is a more complicated problem than it appears at first
glance.

Katharine
Received on Tue Oct 19 14:10:46 2004

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