Re: Water and gum coating

Hi Ryuji,
      The myth is in all gum books, passed along for generations--e.g. coat
paper in room light as it is not sensitive until dry, but dry it in the
dark, blah blah blah. I mean, I coat *and* dry my paper in room light, so it
doesn't make any difference to me one way or another, but all you have to do
is stick a wet coated paper under UV light for a brief moment to see that it
immediately exposes.
     I first read to the contrary in Kosar, and then came across the same in
Nadeau who pointed it out and also traced the history of the myth. Gassan
and a couple others also said it was not true, but infrequently is this
mentioned. I know Sandy has said that it is light sensitive but quite a bit
slower, with gelatin. I did not find it radically different in speed with
gum, but it would be so hard to test as you say (at what point is the layer
really wet to just damp). If you wanted to be sure the layer remained wet
it'd have to be a thick one and then that would affect speed, too, etc. etc.
Chris

> I don't know the context where such myth was generated. But
> dichromated gelatin in sol form (lots of water) is sensitive to UV
> irradiation. If I see any reason for such an argument, it might be
> the lower concentration of dichromate (same amount of dichromate, more
> water before drying), longer average distance between gum molecules,
> or both. But these factors can vary in early part or late part of the
> drying process.
>
> When you read scientific papers on dichromated colloid processes, it
> should be noted that "speed" of the material, or some other term, is
> defined in different ways in different studies, and some of them may
> use definitions that are very far from whta's used in practice. Some
> of them talk about speed of the material although the material does
> not make visible image at all. But there are reasons why they were
> interested in light sensitivity of the material though the
> manipulations used did not allow to make images. I wouldn't blame them
> for their results being not directly applicable to the problem at
> hand.
>
> --
> Ryuji Suzuki
> "You have to realize that junk is not the problem in and of itself.
> Junk is the symptom, not the problem."
> (Bob Dylan 1971; source: No Direction Home by Robert Shelton)
Received on Wed May 19 07:41:59 2004