Re: Gum problem(s)

From: Christina Z. Anderson ^lt;>
Date: 11/20/05-08:44:20 AM Z
Message-id: <001b01c5ede1$4b2026f0$566992d8@christinsh8zpi>

> Thirdly, some good or evil fairy in this house has stolen my Kosar. Thank
> goodness, dammit.
> Judy

Not thank goodness--I had to buy that damn book from for $135!
Sell it if you find it and fatten your children's inheritance.

I posted a number of years ago my "Kosar's
Top 10" that got hotly debated for a while on this list. I personally really
enjoyed reading more scientific info on colloid hardening, but, with gum,
for example, there are **so** many variables that get adjusted along the way
that some of the facts are points of interest in theory only.

Those of you who don't have $135, Kosar is posted again, below.

But I haven't yet found any pigment to spontaneously harden gum.

Katharine, in thinking about the humidity thing, when I moved back to MT and
went from 40-70%
indoor humidity down to 25%, I at the same time was switching my curves and
to match my curves to a standardized 6 minute exposure time while working
with PDN. So *theoretically* I cannot make any humidity judgments unless I
were to go back down South and use my new curves down there.

In *practice* my gum exposures have been consistently 5 to 6 minutes, and I
chose the 6 minute time up here in MT because it provides me with a deeper
colored gum layer (meaning thicker).

Furthermore, I also have chosen to use a 15% am di instead of 7% just for
the sole reason that it's easier for me to remember :) I just cut my
dichromate in half instead of trying to remember 1/4 tsp amounts--e.g. in 4
tsp I mix 1 tsp stock gum/pigment, 1 tsp plain gum, 1 tsp water, and 1 tsp
am di. It's a 1:1 mixture except the am di side of the 1:1 is cut in half
with water. With the less thick commerical gum arabics I replace that tsp of
water with gum arabic. That way it is much easier to coat--it doesn't
fisheye or streak.

The point being after that long, convoluted paragraph is, with different
curves, a 6 minute time now, and 15% am di all could theoretically point to
low humidity requiring longer exposure. I could try printing a an old neg
with 7% am di, too, here to compare...

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 added 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 fridge, 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.
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
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.
Received on Sun Nov 20 08:47:34 2005

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