Re: Gum problem(s)

From: Christina Z. Anderson ^lt;>
Date: 11/18/05-04:20:44 PM Z
Message-id: <004201c5ec8e$593abce0$5d6992d8@christinsh8zpi>

I hope I just didn't send a blank email to the list but i think it

Anyway, Yves, the difference is twofold: i am at 5000 feet; UV rich here.
With Montana full sun at 5000 feet, if I exposed between 10 and 2 during the
day during spring to fall, anything over several minutes was overexposed.
That has to be adjusted to lower altitudes and also seasons, winter
producing less UV.

Direct sun vs. shade is another variable.

Then there is your dichromate--30% am di vs. 10% pot di.

Then there is your negative--bullet proof analog Kodak direct dupe film vs.

When I was making those exposures in the sun, I was using bullet proof
analog and 30% am di in the sun, btw.

One more thing: my gum coating is about one half to a teaspoon per 11x17
print--thus 2-5ml. 11x17 is about 5 times as big as 5x7...and a thicker
layer requires more time.

Second, I don't think I'd take the humidity thing too far. I have gone from
South Carolina to Montana, from 50-70% humidity to now 25% humidity, and my
gum exposures are the same. The length of time I can successfully keep a
coated and unexposed gum print before it succumbs to dark reaction is
probably changed, though.

As far as registration, though, the lower humidity is a pain in the popo...

> 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
> this.
> 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.
> Thanks
> Yves
> ----- 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
> an
>> > intuitive feel of the various parameters involved and how they affect
> the
>> > 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
> a
>> > 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
> the
>> 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
> choose
>> 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
> you've
>> 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
> 4,
>> coupled with a normal development time of between 1/2 hour and 1 hour
> (less
>> if sprayed) and with spray development the layer is quite stable. It
> works
>> for me, under UVBL with diginegs, but your light source is way different
> and
>> 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
> the
>> >> observation that when you first soak your paper in the water face up,
> all
>> >> 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
> theory.
>> > At
>> > this time I don't see how it's even remotely possible but I could be
> dead
>> > wrong on this.
>> You're not--I'm just explaining why the theory might have been suggested
> in
>> the first place because the observation not based on science seems like
>> that.
>> Size that paper.
>> Chris
Received on Fri Nov 18 16:21:22 2005

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