Re: Water and gum coating (was: tanning theory)

DCG related literature mentions the following "trick" to increase
crosslinking: subsequent to the light exposure the layer is kept under a
warm high humidity environment for a couple of hours.

Martin

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Monday, May 17, 2004 4:25 PM
Subject: Water and gum coating (was: tanning theory)

> >
> > "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.
> Katharine
Received on Tue May 18 11:38:47 2004