Water and gum coating (was: tanning theory)

From: Katharine Thayer ^lt;[email protected]>
Date: 05/17/04-08:25:22 AM Z
Message-id: <40A8CB39.202C@pacifier.com>

> "Grimm et al also showed one more step from there. If the dichromated
> PVA does not contain certain amount of moisture, this PVA-Cr(III)
> complex undergoes further photosensitive reaction, which loses water
> and chromium drops out of the PVA complex and renders the PVA soluble
> again. This, they say agrees with the experience from phosphor coating of
> color CRT's, but also seems to agree with some of the careful gum printers
> in this list talking about moisture level in the coated layer during
> exposure."

I didn't see this post that's quoted in Martin's note, so I just have
this quoted paragraph, but I have a bit of trouble with the leap from
the cited research to what careful gum printers on this list have said
about moisture.

It's true that when it's been argued, on the basis of reports out of
literature, that moisture or humidity is a problem for gum printing,
I've countered that since I print in high humidity all the time without
a problem, I can't believe that humidity per se is a problem variable.
I've argued that whenever it's been reported that humidity is a problem,
the humidity is almost invariably associated with heat, and perhaps it's
the heat that's the real problem.

But I recently came across a chart that shows that for dichromated gum
arabic, speed increases almost vertically with increasing humidity.
Since seeing that chart, it has occurred to me that when people have
humidity only part of the year, they might not be aware that there is a
tremendous difference in speed between a damper coating vs a dryer
coating, and don't adjust their exposure enough to account for the
difference. For example, according to this chart I found (BTW, this
chart is almost identical to a similar chart for gelatin in Kosar) a
gum coating that's at 45% humidity will require *15 times* the exposure
necessary to achieve the same level of insolubility in a coating that's
at 80% humidity.

It would be interesting to know whether people have seen these huge
differences in practice. I can't check it out because my humidity is
high year round. But this makes me wonder about something else. I may
have the timing wrong, but wasn't it about the time you moved from
Montana to the South, Chris, that you started working with diluted
dichromate and were surprised to find that you could print with very
short exposure times, not that different from what you were experiencing
before with saturated dichromate? If so, it may be that the increased
speed of the damper coating offset the decreased speed of the dilute
dichromate, and that's why you weren't seeing a big difference in the
speed. Just a thought.

At any rate, the discussions heretofore have centered on whether high
humidity is bad in and of itself for gum printing, and this careful gum
printer is quite sure that it's not. But to use the observation that
high humidity doesn't hurt gum printing to support the idea that a
hardened colloid might dissolve if it becomes too dry during exposure,
is stretching something way beyond where it has any valid application,
and is typical of the kinds of ad hoc reasoning that have made me cross
and frustrated in this discussion before. I assume this is from Ryuji,
and I'll say again, if these are the best arguments you can come up
with, your team isn't doing so well.

I doubt very much if this is a phenomenon which could be reproduced in
the normal gum printing process, and if it did happen, it would be
impossible to interpret. If the chart is right, it seems clear that if
the coating is very dry, it would take a very long exposure to harden
it. If a very dry gum coat were to dissolve in the development water,
there's no way of telling whether it was hardened and then dissolved, or
whether it never hardened in the first place. But given this chart, I
would put my money on simple underexposure as an explanation for the
failure to "insolubilize" rather than on a theoretical assumption that
the gum must have hardened and then redissolved.
Received on Mon May 17 15:21:36 2004

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