From: Peter Fredrik (pete@fotem.demon.co.uk)
Date: 05/12/01-12:56:41 AM Z
My dear Katharine
Katharine Thayer wrote:
> Hi Pete, long time no hear.... Hope all is well over on your side.
Fine just getting a bit older and not wiser
>
>
> 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.
Sometimes the simple problems are the worst
>
>
> 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
I use a bank of iron gallium point source lights which are very hot, very actinic
and very fast my normal exposure is in the region of 60secs I am sure you are right
in your assumptions. Incidentally I have found that the Chromic oxidation stain
only starts to appear when the exposure is twice the minimum required to make the
coating insoluble this seems to hold out for egg PVA. and gum.
>
> to use potassium dichromate to avoid it.
I agree but for me time is of the essence
>
>
> 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.
What we have here is a problem of semantics. I don't think it is important what
things are called as long as the majority understand the terminology. The
dichromate processes are in fact governed by the effects of oxidation this is the
mechanism that produces the tanning effect. The greater the exposure the greater
the oxide present. I suspect that the oxide is always present after exposure, even
though in small quantities it is invisible and I take your point strictly speaking
as such it is not a stain ,but by common parlance it is known as a brown stain .
I think we could describe the yellow dichromate stain in your terms as yellow
dichromate retention stain without any difficulty. And as this effect does not
often happen with other substrates, such as plastic, wood ,ceramics, etc.. . My
effect could be stated as Chromic oxidation stain as this effects all dichromated
emulsions.
What do you think ?
>
>
> 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.
I too am embroiled in a similar project perhaps we could collude from time to time
in this mutual endeavour mine is to be called " The Book of Temperaprint ".
My best wishes
Pete
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