Re: GUM TESTING/CLEARING

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From: Katharine Thayer (kthayer@pacifier.com)
Date: 07/16/03-02:19:06 PM Z


kateb@paradise.net.nz wrote:
>

> For me, it was always that I'd underexposed, and it can also be related to the
> thickness of the coating. Unless you're applying a measured coat of emulsion
> with a rod in a defined paper area, you're always going to get a variation -
> this could make a slight and undetectable difference to exposure time, with the
> result that some prints will not expose as well as others. If the coat's too
> thick, it will just wash off because the gum next to the paper surface won't
> have received enough light to harden. Of course if you have a thicker emulsion
> layer there will be more pigment (physically)in the thicker coat than in the
> thinner one - if you see what I mean.
>

Kate's points are all excellent. I might say one thing a bit
differently, although we may well be saying the same thing in different
ways: to me the issue of exposing all the way through to the paper and
the issue of a too-thick coating flaking off are two separate issues.
There is a range of coating thickness within which it's simply a matter
of exposing enough to harden the gum all the way through to the paper,
(and BTW this is true even of very thin layers; even the thinnest of
layers looks a lot deeper to the ions and photons involved than it does
to us). But beyond a certain thickness or pigment concentration, you
could expose all day and that stuff is still going to flake off as soon
as it hits the water.
kt


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