From: Katharine Thayer (kthayer@pacifier.com)
Date: 07/16/03-05:32:13 AM Z
martinm wrote:
>
> I guess it is safe to say that there is agreement about the order of
> sensitivity of the chrome compounds. According to Sasaki, Honda, Kikuchi
> (Studies on Photosensitve Dichromated Materials, U. of Tokyo 1979) the order
> of sensitivity can be described as:
>
> CrO3 > (NH4)2Cr2O7 > (NH4)2CrO4 > K2Cr2O7 > K2CrO4
Yes, certainly, I think it's safe to say that this order of sensitivity
is well established. A big deal has been made here, again and again,
of the rather irrelevant observation that if you dilute ammonium
dichromate way down, you can make it as slow as potassium dichromate,
and the non sequitur conclusion drawn from that observation that since
diluted ammonium dichromate can be as slow as potassium dichromate, you
can't say that ammonium dichromate is faster than potassium dichromate.
But the point I try to make every time that argument is trotted out, and
the point of the ranking above, is that while you can make ammonium as
slow as potassium, you can *never* make potassium dichromate as fast as
ammonium dichromate. So far my argument has had little effect on those
determined to dispute that ammonium dichromate is faster than potassium
dichromate.
>
> When comparing speed of ammonium dichromate / potassium dichromate
> solutions, it might be important considering interaction with the colloid. I
> gathered at pH >8 ammonium dichromate was reduced to chromate (and hence
> far less light sensitive).
Since the colloids by themselves aren't that alkaline, it's unlikely
this would have anything to do with our context. Although I'm quite sure
that what you say is true, certainly Kosar says this, it has little
relevance to gum printing.
I think it's well-established with the different colloids, that unless
ammonium dichromate is watered down to about 75% or less of the
concentration of potassium dichromate, the ammonium will be faster than
the potassium. Maybe I'm misreading your line of reasoning, but it
seems like you're trying to come up with a reason why the dichromates
would behave differently in the different colloid media, and unless I've
missed something, I don't think anyone's really arguing that. But it
shows that it must be hard to follow some of these discussions and
figure out what's real and what's not.
> The bleaching of a Cr3 gum layer by sunlight may also be attributed to
> interaction with the colloid. Though I am not a chemist - I would look into
> that direction.
The colloid isn't an issue, because at least in my tests of this, there
was no colloid involved; this was pure dichromate stain, just
dichromates brushed on bare (unsized) paper and exposed. No colloid
whatever.
Katharine Thayer
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