From: Larry Roohr (larry.roohr@comcast.net)
Date: 08/02/03-08:39:16 AM Z
Thanks Christina.
This seems to mean that Stuarts thick mix of 10ml gum + 2gm AmmDi (dry) +
2.5ml water could be gotten pretty close to with liquid dichromate solution
by keeping your saturated sludge mix at 86Fdeg for an 89% saturated solution
and just using 2.5ml of it, with no nervous-making dry powder mixing, no?
Larry
----- Original Message -----
From: "Christina Z. Anderson" <zphoto@montana.net>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Thursday, July 31, 2003 9:07 AM
Subject: Re: Kosar's Top 10 Gum Facts
> 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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