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Re: Dichromate and the plate
Liam wrote:
> > You'll all be pleased to hear that my plate did clear after soaking in water
> > for a little while. Print was fine too, though I got a greenish colour
> > rather than the grey I was hoping for. Dirty brush?
Liam, the basic manuals for gum printing often list colors that are NOT
compatible with dichromate, such as, if I recall correctly, chrome green.
I didn't pay too much attention because it was apparent even then that the
books were, let's say, loose, and pigments with the same name could be
different & vice versa... but I suspect there's some truth in there
somewhere, with maybe a white among them. All the whites I think of (zinc,
titanium, and flake, or lead) are metals, which may be a factor. On the
other hand I have in past used zinc white without trouble, tho
post-millennium it's NOT covering very well...
Sarah writes:
> Liam, did you soak the print for a while too? The greenish color could be
> the result of yellowish dichromate that didn't wash out (or light almost-
> yellow tanning from it) combining with the ivory black to give a sort of
> army green. Yellow and black produce such a green.
That sounds right, tho also maybe something in the white combined with the
dichromate. I have a mixing dish that has such a greenish stain in it --
I'd mixed some gesso into an emulsion for something or other -- and
nothing I do, including 100% clorox, has cleared that stain (tho
admittedly I didn't try clearing bath).
> Returning to the thread of whether or not tanning occurs in gum printing, I
> have found that it depends on the density range of negative. My pinhole
> negatives almost always have tanning in the open shadow areas because I have
> to expose so long to print into the dense highlights, but there should be no
Sarah, I'm confused by this use of the term "tanning." Saying that
dichromate tans the paper or whatever, I've meant *hardens,* as in
hardening leather, or film with what they call a "tanning developer."
Then -- at least in my conception, if not in reality -- there's dichromate
*staining,* which is probably related in some way, but not the same. In
other words, I believe there is always some tanning, but not always
*staining* --- and the one (as I understand it) not simply an extreme
version of the other.
> tanning when printing from half-tone negatives, for instance, because
> minimal exposure can be given — just enough to harden the gum in the open
> areas without heading into the secondary effect of tanning. I am interested
> that so many people on the list were able to get rid of tanning with
> prolonged soaks in water. I will see if that works with my prints and report
> eventually.
But, speaking of "halftone" negs, I've been getting the opposite:
consistent staining with a particular neg in 3-color sep printed on my
laser... very puzzling because 21-step on same paper came clear. FINALLY I
printed 21-step & neg on one piece of paper,the first cleared, the second
stained -- & at last the light dawned: That neg, tho it looked dense to
the eye, had when you looked at it closely, plenty of spaces between the
dots ------ aaaarrrrghhh !! (Now I'm going to try some sun-tan lotion on
it.)
And this just in from Liam:
Quote ========================
That was my third gum print so far, and I thought it a good idea to put #1
and #2 into bisulphite, just in case. These were made with ivory black
and Indian red and had had much less washing; they had apparently cleared
OK, and showed no further change in the clearing bath. I don't get it at
all, but I'll clear in future.
===================================== End Quote
Aside from checking the back, as Katharine says -- and our still
unanswered questions as to whether the problem is merely aesthetic or also
archival -- I've found that putting a print next to a piece of the
unprinted, fresh paper also will reveal a yellow cast not evident from
the print alone. For what that's worth.
We ran a thread on clearing dichromate-hardened gum as paper size several
years ago: sulfuric acid (maybe 2% as I recall) was declared the best
clearing agent... It was one great big nuisance, I found by trying it.
Far more trouble than gelatin and hardening, whatever you use to harden,
and you kept having to remix the sulfuric as it got verrrrry slow verrrry
fast. The "finished" paper was indeed a pale green, tho that could be
ameliorated by even more acid bath. But not worth it -- at least in my
use, paper so sized didn't print as well as gelatin sized.
> I am finding the List along with PF to be a sort of graduate school in
> alternative processes!
Thanks for the plug Sarah -- and I take the liberty of adding, in case
there's someone else out there who thinks Post-Factory is where we make
posts, or as a few (apparently deadly serious) folks assumed, I take
photographs of post-industrial sites....
"Post-Factory" means what we do after the factories have gone on to other
bottom lines (which is to say, abandoned not just Dassonville but all of
us), among other things..
Judy
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| World Journal of Post-Factory Photography > "HOW-TO and WHY"
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