From: Richard Sullivan (richsul@earthlink.net)
Date: 07/31/03-08:52:55 AM Z
This could be interesting for gum printers in the southwest looking for
something new:
http://www.alchemy-works.com/incense_mesquite.html
maybe yes and maybe no.
--Dick
At 08:53 AM 7/31/2003 -0500, you wrote:
>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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