From: martinm (martinm@SoftHome.net)
Date: 07/17/03-12:36:48 AM Z
----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Wednesday, July 16, 2003 1:32 PM
Subject: Re: GUM TESTING
> Since the colloids by themselves aren't that alkaline, it's unlikely
> this would have anything to do with our context.
Frankly, I do not know much about gum. The only thing I gathered was they
were extremely
difficult to "standardize" - a statement not unlike that made about gelatin
for silver halideemulsions. So there might actually exist various kinds of
gum.
Incidentally, pH 8 being only slightly alkaline, is a value you will already
reach if you have very hard drinking water.
> Although I'm quite sure
> that what you say is true, certainly Kosar says this, it has little
> relevance to gum printing.
That may be true. Having absolutely no experience with gum printing, I did
not mean to
teach lessons here. I am lurking at alt.photo to get hints about my field of
interest
(Lippmann photography, holography). I simply thought it might also work the
other way around - since
in the context of holography you'll find many studies about photosensitive
dichromate materials.
> 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.
I see. So the "colloid" is the paper. Would be good to know the previous
history of that
particular type of paper: what kind of oxidizers, what optical brighteners
are involved,
are there free radicals generated? I guess a strong bleach will reduce Cr3.
E.g. if the
paper had been bleached by hypochlorite, UV exposure would form chlorine.
Moreover, under certain circumstances an optical brigthener like
titandioxide may even form
silver halide-like emulsions. That stuff is used for solar cells etc.
Martin
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