Re: Gum problem(s)

----- Original Message -----
From: "Yves Gauvreau" <gauvreau-yves@sympatico.ca>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Friday, November 18, 2005 1:18 PM
Subject: Re: Gum problem(s)

> 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" <zphoto@montana.net>
> To: <alt-photo-process-l@sask.usask.ca>
> 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 16:07:03 2005