From: Larry Roohr (larry.roohr@comcast.net)
Date: 07/31/03-08:28:36 AM Z
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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