Re: Gum problem(s)

From: Katharine Thayer ^lt;>
Date: 11/20/05-11:36:48 AM Z
Message-id: <>

This is a good example of what I was talking about; see my earlier
response to Judy. Now I'm *really* late for breakfast, bye,

On Nov 20, 2005, at 6:44 AM, Christina Z. Anderson wrote:

>> 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 chose
> 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...
> chris
> 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
> 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.
Received on Sun Nov 20 18:58:18 2005

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