From: Dave Rose (cactuscowboy@bresnan.net)
Date: 07/16/03-09:07:23 PM Z
Greetings from Big Wonderful Wyoming,
I print gum with both ammonium and potassium dichromate. I use both as
saturated solutions. Why bother diluting AD down to the same approximate
stength as PD? That's the same as bolting a 1 bbl carburetor onto your
Mustang's V8 so that its performance is equal to a Pinto. A Mustang no
faster than a Pinto? Yes, it's a ridiculous argument.
AD is faster and will produce lower contrast images. AD will leave more of
a dichromate stain. PD is better suited for printing higher contrast and
'cleaner' images. AD vs. PD arguments are pointless. They each have their
own particular strengths and weaknesses.
Many of my gum prints are made with 3-8 exposures. I often use both AD and
PD - not combined but in seperate and subsequent exposures. AD is great for
bringing out subtle highlight tones while using a weak concentration of
pigment. A heavy dose of pigment with PD will add punch and contrast to the
shadows.
As for prints washing out or disappearing during clearing, I've never had
that problem. My guess is that a fragile, wet, and freshly developed image
might be susceptible to this damage. The only prints I've ever subjected to
a clearing bath have been completely air-dried and 'cured'.
Best regards,
Dave Rose
----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Wednesday, July 16, 2003 5:32 AM
Subject: Re: GUM TESTING
> 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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