From: Christina Z. Anderson (zphoto@montana.net)
Date: 07/31/03-07:53:59 AM Z
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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