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

Hi All!
     After a week of being offline, how refreshing to come back to all kinds
of exciting stuff! I got 186 spams and 194 alt list messages, so the good
outweighs the bad. I'm now in MN for the summer, trading one sort of
humidity (SC) for another (MN).

<Katharine Thayer asked> 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.

     Not exactly...I started gum printing in SC with saturated right away,
and didn't switch to the lower dilution until the end of last year, actually
right around the time the humidity got closer to the 30% mark instead of the
60% mark (indoors, considering I have a heat/air conditioning unit there).
     However, I was already printing in high humidity in MN over the whole
summer, with the saturated am di.
     The best present I ever bought myself was a Brookstone temp/humidity
digital readout, so now I always know what my RH is.
     Since switching to the weaker dilution, I usually standardize exposure
at 3-5 minutes with my gum layers, and there have been no surprises, whether
30% or 60% humidity. I think the deal is that gum is not as noticeably
affected *in practice* because you can always add 5 minutes of soak time to
compensate if a layer is not budging as quickly.
     I think the ability to use the weaker dilution has more to do with the
digital neg than the humidity--matching the dilution of dichromate with the
curve of the neg.
     My biggest problem with humidity is making sure the paper is always to
the same dryness for registration purposes.
     I talked about this issue last spring or summer, so I am glad it is
coming up again--the myth that a wet gum layer is not sensitive until it is
dry. That is a crock, and that's what I said last spring/summer after
reading Kosar and several others and then experimenting with side by side
wet/dry. I haven't been scientific enough to take a digineg and sacrifice
it on top of a wet layer to see what the speed difference may be, though,
but it sure wasn't slow.
     The other intriguing thing, though, from Demachy's account, that
somewhere might fit into some of the stuff you are investigating, is an
infrequent occurence he had where a gum layer was fine and stable, and for
some reason all of a sudden it solubilized. I can't locate the book at the
moment--still packed--but it is at the end of the book if I remember
correctly where he explains the previous saturation of paper with
dichromate. That there may be a reason that hardened colloid can become
unhardened and why this sometimes happens is very interesting. It has not
happened to me, tho.
Chris
.>
> 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 20:33:03 2004