Re: GUM TESTING

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From: martinm (martinm@SoftHome.net)
Date: 07/15/03-08:31:23 PM Z


> Actually I doubt whether any of that would enlighten our discussion; at
> least a quick perusal of the titles shows nothing that appears to
> relate to this discussion or even to gum printing in general. If you've
> read this literature and know of specific information from it that you
> think would be useful in our context, please bring it to our attention.

You are right - there is no direct connection with gum printing.
Nevertheless, despite considerable differences in processing - the
underlying principles remain the same: you have a dichromated hydrophilic
colloid that is exposed
to light. Upon the action of light, Cr6 is converted into Cr3 which in turn
crosslinks the exposed areas. In gum printing you are exploiting the relief
structure whereas in holography development of DCG layers is usually aimed
to get a volume structure (throughout the layer): the initial water bath
must not dissolve the gelatin but rather swell it. The wet emulsion is
dehydrated in a series of increasingly concentrated isopropyl alcohol
solutions. Finally, the plate is inserted into 100% isopropyl and dried in
hot air.

In the context of DCG extensive research has been carried out with regards
to many aspects of light-sensitive chrome systems. Incidentally, sometimes
"modern" activities converge with old ones. In some respect DCG might
parallel some almost forgotten photographic techniques (like
e.g.orthochromatic sensitized dichromated gelatin or cellulose). By the way,
not everything here is based on gelatin. Gum arabic, casein, polyvinyl
alcohol etc.have been used equally.
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

Ammonium dichromate is said to be considerably faster than potassium
dichromate. The authors point out that this order of sensitivity also
corresponds with dark reaction.

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).
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. Maybe there are gums containing sulfur component that reduce
dichromates...

Thanks for pointing to Kosar's book. It has been on my list of must-have
books for quite a while...

Martin


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