Gum, newbie questions

From: Yves Gauvreau ^lt;>
Date: 11/17/05-11:59:10 AM Z
Message-id: <035b01c5eba0$9ced0180$0100a8c0@BERTHA>

Hi everyone,

I read a lot of stuff on gum printing, here and elsewhere and yesterday I tried a few things (my first steps) to see how it goes. As I expected the results where so so, well very so so if you see what I mean.

I understand pretty well the principles that a dichromate when expose to light (UV) in the presence of a coloid as the effect of rendering the exposed area more or less insoluble to water and thus any pigments or whatever else is mix with the coloids get's trapped inside those insoluble areas. In other words, there is no magic in this phenomenon.

Now, that's very nice but intuitively I'm quite certain there are many if not gazillions little details that can make a huge difference. For exemple lets consider the following 3 of those little details, the tickness of the emultion, the pigment density or pigment/gum ratio and the transparency or lack of it of the pigments.

The tickness of the emultion which as at least 2 properties that we must consider. First its physical dimension (tickness) implies that the exposing light as some distance to travel until it gets all the way to the paper. If the light doesn't get to the paper it will leave a space between the paper and any exposed emultion above it and at development time, water will get into this space and the insolubles above it will be carried out in the flow thus leaving us with an area with no pigment where there should be some.

In a way the second property as basically the same effect as the above with the following difference, the relative opacity or tranparency, whichever you prefer as what is called an inverse effect. A tick and very transparent emultion may produce similar results to a thin and opaque one.

Basically the pigment density and their relative transparency directly affect the tranparency of the emultion and what I've said above apply for these as well. But something tells me that this thickness and transparency stuff as bonderies beyond which the results will be useless as in my first try.

I think I saw this in one of Katherine text about her approach. Basically one must learn all this through experimentation with the material he/she is using and one must remember that each combination will produce varying results.

All this can take a considerable amount of time to figure out and I wonder if at least intuitively or by experience someone or some of you could guide me and others to the kind of knowledge that could help us kind of troubleshoot our own results. If I go back to what I said above about the emultion flotting away in some areas. I assume this could also be caused by insufficient exposure and possibly by a combination of exposure and "relative thickness" of the emultion. Say I see what most would call staining I think, what could be the cause of this. I'm sure you can understand what I mean but in the event you'd prefer to give your opinion, solution, etc. to specific question I or others may have, please let us know.

Maybe finding a proper working emultion could be arrived at by some systematic testing approach. Any hints on how to do this testing would be excellent as well and less demanding to you.

Thanks in advance to all
Received on Thu Nov 17 12:16:35 2005

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