From: Katharine Thayer (kthayer@pacifier.com)
Date: 05/09/01-06:22:22 AM Z
Hi Pete, long time no hear.... Hope all is well over on your side.
I guess I was assuming that Dave is an accomplished enough gum printer
to have got beyond pigment stain and the like, and so I was looking for
subtler reasons why his colors might turn out dull when he wants them
bright. But you're certainly right that this could be a problem for
some.
But I'm curious, what is your light source? And what do you use for your
sensitizer? The reason why I'm asking this is because I've noticed that
the people who use certain kinds of light sources tend to report this
brown/green dichromate "stain" you're talking about, especially with
the faster ammonium dichromate, and for that reason some of them prefer
to use potassium dichromate to avoid it.
The only time I've ever seen this "dichromate stain" is once when I
experimented using the sun for printing and overestimated the exposure
time necessary. And in fact, (I have a feeling we've been around this
question before on this list) this is not what I mean when I use the
term "dichromate stain" and I'd prefer to find a different term for it.
The thing I call dichromate stain is the yellow-orange color of the
original dichromate and it comes from the dichromate not being
completely washed out of the paper. The one you're talking about, I
would prefer to call by a different name, because to me it's something
different, an actual chemical or physical change in the dichromate
compound. Some chemist on the list said once, as I recall, that it is
the result of overexposure, and the dichromate compound actually changes
to chromium oxide, and that's why it's difficult if not impossible to
clear the way you'd clear a normal dichromate stain, because it isn't
the same material chemically.
I've been writing a manual containing the gospel of gum printing
according to yours truly, and so I've been thinking a lot about some of
these issues. And it says something about the difficulty of coming up
with assumptions and standards and rules and procedures that we can all
agree on, that each of us who have some expertise in gum and are called
upon to teach it and talk about it, eventually end up writing our own
manuals.
Katharine Thayer
Peter Fredrik wrote:
>
> Dear Dave,
>
> Dave Rose wrote:
>
> > Hello Katherine,
> >
> > Could you recommend suitable pigments to achieve "brilliant and saturated"
> > results?
> >
> > So far I've experimented with: Cyanotype, Phtalo Blue, Cadmium Yellow,
> > Quinicridone Red, and Alizarin Crimson. Printing test 4x5 negatives (photo
> > of a gray card and color chart), I've been able to get good color balance,
> > but the color is anything but brilliant. I'd love to get Cibachrome
> > intensity in a 3-color gum print.
>
> I operate a similar system to gum /bichro using whole egg as an emulsion and
> have for the past few years managed to achieve highly saturated coloration in
> my prints . In formulating this process the following
> pigments have been most helpful in obtaining strong saturated colour
>
> .Cyan = Phalocyanine Blue (ASTM PB15) has a lot of names Windsor, Monstral blue
> etc. and comes as previously stated in a green or red shade. I use the red
> shade most of the time. This is an easy colour to use, but it does have a high
> tinting strength , and therefore liable to stain.
>
> Magenta = Acra Violet (Quinicridone Magenta ASTM PR 122) will need more coats
> to reach the same colour value as the phalocyanine blue
>
> Yellow = Hansa Yellow Medium (Arylide Yellow ASTM PY 73) a nice transparent
> pigment
>
> I have found however that the problem of dull colour does not lie with the
> pigment, but considerations
> which are primarily to do with
>
> 1) pigment stain
>
> 2) dichromate stain
>
> Most pigments don't stain but the water-colour paper does ! in fact it is made
> for that express purpose Water-colour paper is made to work for water-colour
> painting not gum / PVA/egg printmaking .The function of the water-colour paint
> is to stain the paper. It has to go into the surface and get attached to the
> paper fibres and not come out. With sensitive sizing and precise balancing of
> the gum and the pigment mix a skilled gum printer can travel along a razor edge
> of technique whereby the pigment emulsion can go into the paper and come out
> leaving no pigment behind to adulterate the image. It is hard work a true
> labour of love , and the gum printers that achieve this control in multicolour
> work have my sincere admiration.
>
> I have dealt with this topic in a previos thread. So I will just repeart my
> findings --:
>
> >From an e;mail I sent to the list in regard to dichromate stain
>
> >
> > So do I
> > .
> > Okay let me try to explain .
> >
> > I have been for years concerned with trying to obtain maximum colour
> > saturation
> > in photo-alt at the same time as achieving creative flexibility.
> >
> > When I first started the brown stain proved a problem degrading the cyan to
> > a
> > muddy grey knocking the edge of the magenta and affecting the yellow only to
> > a
> > slight extent .
> >
> > This problem was quickly solved by using any of the following clearing
> > procedure's
> > 5% sol bath of either
> > sodium sulphite , sod/pot metabisulphite, pot alum, liquid acid hardener, or
> > a
> > long soak wash in running water 12 hrs /24 hrs, all of these baths gave much
> > cleaner colour the most noticeable was the effect on the cyan which went from
> > a
> > muddy grey to a fully saturated blue, so far so good.
> >
> > However I noticed as time went there seemed to a slight green cast appearing
> > which
> > was a will of the wisp effect hard to predict .At first I thought it was
> > badly
> > mixed pigment . I was using acrylic colour so I did a series of tests without
> > any
> > colloid present and mixed them physically with a paint brush ,the result no
> > green
> > caste. So it had to be the colloid /bichro mix .Working on this surmise I did
> > a
> > further series of tests with out pigment the results of which were most
> > interesting .With a full exposure the colloid went brown using different
> > papers
> > the shade of brown was different, I also used three different colloid to
> > cross
> > check on a control sheet of fabriano artistico
> > 1) W&New gum , 2) egg,, 3) gloy gave the following results --: 1) strong
> > brown 2) green brown 3) medium brown
> >
> > These tests were then subjected to the clearing baths and as expected they
> > each
> > turned a soft green grey .The colour of turquoise that has been let down
> > with a
> > slight grey tone. So how could I get rid of this residual stain a number of
> > other
> > chemicals were tried with no effect.
> >
> > Then I remembered work I had done on bromoil year's before many bromoil
> > bleaches
> > contain a small amount of sulphuric acid, which had the effect of turning the
> >
> > gelatine colourless before inking, and as similar chemistry is employed I
> > decided
> > to give it a try.
> >
> > BINGO it worked the brown stain disappeared and no green stain reappeared,
> > however
> > the stain did not in fact go away on heavy exposure a light neutral grey tone
> > was
> > created, but this did not pose any problems as it just made the colours in
> > the
> > shadow regions slightly darker and in effect undetectable, in normal
> > practise.
> >
> > So there we are,
> >
> > All my best,
> >
> > Pete.
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