From: Christina Z. Anderson (zphoto@montana.net)
Date: 07/31/03-09:07:57 AM Z
Hi Larry!
Yes, that's it. I don't think you need a constant temp water bath by
any means--it just means that it may be bests for repeatable results to use
a specific concentration measured out by grams instead of the "dump and
pour" method I do. I put the pound of dichromate in a jar and cover it with
distilled water, assuming it'll have a saturated solution at the top of the
jar and sludge at the bottom, and that the saturated solution at the top of
the jar will be (approx) 10% for pot di and 30% for am di. Now I realize
that isn't a good method at all unless my temperature of my darkroom is
fairly constant within 5 degrees or so. In MT it is--I am in a basement
where it remains cool and dry. Here I am in a floor level darkroom which is
about 80 degrees in the summer and wet.
The only thing I will now change in practice is to measure my
dichromate solutions with a scale before I mix. But that's OK--I was going
to do that anyway because I want to figure out the minimal amount I can use
and still get acceptable times of exposure.
Chris
----- Original Message -----
From: "Larry Roohr" <larry.roohr@comcast.net>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Thursday, July 31, 2003 9:28 AM
Subject: Re: Kosar's Top 10 Gum Facts
> Christina,
>
> Thanks for doing this, lotsa meat there. Am I reading #2 correctly? Does
> this mean the saturated solution content per volume varies with temp?
yikes.
> Maybe a constant temp water bath is in order.
>
> Larry
>
> ----- Original Message -----
> From: "Christina Z. Anderson" <zphoto@montana.net>
> To: <alt-photo-process-l@sask.usask.ca>
> Sent: Thursday, July 31, 2003 7:53 AM
> Subject: Kosar's Top 10 Gum Facts
>
>
> > Good morning all!
> > I finished taking notes out of Kosar's 60p chapter on Dichromated
> > Colloids (AND a momentous day--finished all my notes from all
> sources--what
> > a relief-68pp 11pt type is DONE). Since I know not everyone has Kosar
> (out
> > of print, costs $135 or more to get used) I thought I'd share the items
I
> > found of interest in that chapter.
> >
> > MY KOSAR'S TOP 10:
> >
> > 1. Certain pigments may be found to react with dichromate causing
> > spontaneous insolubilization without any exposure. (this is probably the
> > source of why some pigments "don't work")
> >
> > 2. Ammonium dichromate at 15 degrees Celsius (59 F) is 30.8% soluble.
At
> > 30 degrees Celsius (86 F) it is 89% soluble! Thus, the method of not
> > measuring out dichromates and always keeping them in saturated solution
is
> > probably not a good idea if temp varies greatly in your workplace.
> >
> > 3. Two reasons for am di's faster speed is its high solubility without
> > precipitating and its lower pH than either potassium or sodium
> dichromates.
> > It is pH 4.5. 2.5% ammonium dichromate is the same speed, contrast, and
> > keeping quality as 3.5% potassium dichromate. In a comparison chart
using
> > albumin, gum, and process glue, these are the comparative speeds of the
> > three dichromates: ammonium is 100/100/100 potassium is 20/46/65 and
> sodium
> > is 28/100/100. Note the different speeds for the different colloids,
> except
> > for ammonium dichromate.
> >
> > 4. Viscosity varies not only from batch to batch, but with age of gum,
> > which makes the sensitizing properties inconsistent.
> >
> > 5. PH, temperature, and moisture all affect printing speed.
> >
> > 6. Adding an alkali to the gum/dichromate mix: this changes it from
> orange
> > to lemon yellow; if so much is added it is converted into a
monochromate,
> > and the light sensitivity drops to 25%. The higher the pH of the layer,
> the
> > longer the required exposure. Chromates, thus, are slower than
> dichromates.
> > With ammonia, you may start out with a high pH in solution, but
due
> to
> > the volatility of ammonia, it evaporates during drying and the pH of the
> > coated layer returns to a lower pH. If a solid alkali is used (sodium
> > hydroxide or carbonate) the alkalinity of the dried layer remains the
> same.
> > The useful life of a sensitizing *solution* is greatly increased
with
> > addition of ammonia. If pH is 8 or higher, deterioration of solutions is
> > practically nonexistent (note: not coated paper).
> >
> > 7. Humidity: The presence of a certain amount of moisture in a coated
and
> > dried layer is necessary for the hardening reaction. When dry, the
> moisture
> > remaining varies with relative humidity. Completely dehydrated or
fully
> > swollen coatings do not show any light sensitivity at all, but in
between
> > the sensitivity is high when the humidity is high. Sensitivity doubles
> with
> > increase of 30% humidity.
> >
> > 8. Paper will keep, coated, for even 70 days in the ice box, or 3 days
at
> > room temp. If paper is dried at room temp high enough to dehydrate
> coating,
> > dark reaction does not occur and consequently shelf life is very good.
> > (Katharine, with the relative humidity in Montana being so low, this is
> why
> > I could use sensitized paper for so long without dark reaction ruining
it
> > there. Here in MN where it is dripping, this is not the case).
> >
> > 9. Raise in temp increases rate of chemical reactions, and for each 10
> > degree centigrade raise there is a 3x dark reaction rate, if rH is
> constant.
> >
> > 10. In there was the answer to my manganese sulfate question.
Apparently
> > "back in the day" they added various things to the sensitizing solution
to
> > speed it up, and this was one (that didn't work). Cupric chloride added
> to
> > dichromated glue increased its sensitivity 2-4x, with just 1/10 of a per
> > cent. The action that happened was to either promote the reduction of
the
> > di ion to chromic ion, which then hardens the colloid, or to partially
tan
> > the colloids themselves. All these methods have also been found to
> > accelerate the dark reaction. Thus it is not good to store these papers
> at
> > all. Manganese sulfate was first suggested, but this did not improve
the
> > sensitivity, but it was one additive that did not increase the dark
> > reaction. Copper sulfate and cobalt chloride were not as good. Other
> > sensitizer increasers were copper, cobalt, nickel, and rare earth salts.
> >
> > As they used to say on Saturday Night Live Coffee Talk, "Tawk
amungst
> > yorselfs..."
> > Chris
> >
> >
> >
>
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