Re: Gum problem(s)

From: Yves Gauvreau ^lt;[email protected]>
Date: 11/18/05-02:18:52 PM Z
Message-id: <04ac01c5ec7d$4b62f210$0100a8c0@BERTHA>

Hi Christina,

I was going to ask you why you, exposing under the sun need 0.5-3 minutes
and Jo, exposing under the sun needs 20-30 minutes. I assure you I saw
figure like this somewhere and I believe them to be true.

Katharine, as given me reason to believe that relative humidity is a major
player in the gum dichromate matrix as it is for the gelatine dichro matrix.
A small change in relative humidity may require a significant change in
exposure in the mid to lower range of RH. She is suppose to get me facts on

I've got 30 %RH here and this kind of confirms that I need 20 minutes or
more to get correct exposure and she normaly needs only 2-3 minutes which
would mean that the relative humidity is much higher at her place then mine.


----- Original Message -----
From: "Christina Z. Anderson" <>
To: <>
Sent: Friday, November 18, 2005 1:17 PM
Subject: Re: Gum problem(s)

> > I kind of decided at this time to use only a 21 step tablet to acquire
> > intuitive feel of the various parameters involved and how they affect
> > results. Also, I'm not expecting the top most 10-11 steps to print any
> > thing
> > else then "pure" white. As for the 20 minutes well I would argue that it
> > depends a lot on your light source, the composition of its light and the
> > minimum and maximum optical density of the negative you're using be it
> > digital or whatever else. In my situation I could argue that I could use
> > bit more time because I can see a tiny difference from step 1 to step 2
> > and
> > I assume overexposure would start to happen when these lower steps begin
> > to
> > look all the same, yes/no???
> Of course exposure depends on your light source--if I were to still use
> sun, if I remember the exposures were 30 seconds to a couple minutes. Now
> under UVBL it is 6.
> What I have decided with test strips and gum is, since you're hardening a
> deeper and deeper layer of pigmented gum, sometimes you just have to
> a time that gives a stable coating and go with it. In other words, if you
> expose a print at 20 minutes and get a nice deep layer of color in the
> shadows and still have highlights that clear, then 20 minutes is perfect.
> If you expose at 6 minutes and your image washes away, then you know
> underexposed. I can expose at a range of, let's say, 10 to 4 minutes, and
> yet I've chosen 6 because I get a deeper layer of color with 6 than with
> coupled with a normal development time of between 1/2 hour and 1 hour
> if sprayed) and with spray development the layer is quite stable. It
> for me, under UVBL with diginegs, but your light source is way different
> I wouldn't begin to suppose I could extrapolate a time for your light
> source.
> >> BTW, the idea that the dichromate may, in fact, expose the layer at the
> >> bottom (I wouldn't have a clue if it did or not) probably stems from
> >> observation that when you first soak your paper in the water face up,
> >> the dichromate leaches out immediately. This only takes about a minute
> >> to
> >> happen, and then you can lift the print out of the water and watch the
> >> dichromate stream off. The layer is still perfectly intact. I have no
> > idea
> >> chemically why this is so--the chemists of the list can answer that
> > one--but
> >> I have observed sometimes in coating a paper with magenta that in my
> >> brush
> >> strokes the yellow of the dichromate brushes at the paper base and the
> >> pigment seems to be on top, so perhaps the dichromate is of a different
> >> weight or whatever the term is chemists use...mole...blah blah..
> >>
> >
> > As I've writen elsewhere I have a big, very big problem with this
> > At
> > this time I don't see how it's even remotely possible but I could be
> > wrong on this.
> You're not--I'm just explaining why the theory might have been suggested
> the first place because the observation not based on science seems like
> that.
> Size that paper.
> Chris
Received on Fri Nov 18 14:17:09 2005

This archive was generated by hypermail 2.1.8 : 12/01/05-02:04:50 PM Z CST