From: Sandy King (sanking@clemson.edu)
Date: 11/11/01-11:12:46 AM Z
Sorry if this message is a repeat. I had a computer crash and can't
tell from my message program if this went out or not.
Judy Seigel wrote:
>
>In gum printing "maximum black" is a function of the ratio of pigment
>added. Period. Trust me on that.
OK, that is fine with me. I don't know enough about gum printing to
even imagine otherwise.
>
>> ...I test with the stock solutions that I use to
>> replenish, and these solutions are kept topped off. I top off the
>> working solution with the stock solution, then add back to the stock
>> as necessary.
>
>Sandy, you'll have to explain that to a mere gum printer -- I can't follow
>it, but it does sound like you're testing with new solutions. Plus, if you
>top off working solution with stock solution, I don't see how you can tell
>diddle, since the concentration would be indeterminate...
My actual working method is based on continual replenishment of
working solutions of a given % from stock solutions of the same
strength. I store the working solutions in the refrigerator to keep
them at the right temperature for sensitizing, while the stock
solutions are stored in a cabinet in the dark. We are really talking
about quite a number of bottles of solution since there would be two
for every strength sensitizer I use. Since I keep all of the stock
and working solutions topped off it is correct to say, as you have
observed, that I am working with new solutions.
However, I also have quite a number of duplicate bottles of different
strength solutions that have just been standing around for years,
some probably 5 years or more, most without ever being opened, others
partially full. Everything was mixed with distilled water, of course.
All of these solutions appear to be of the same color as the day they
were mixed and show no signs of going bad. What I know without doubt
is that I can pull out one of these old bottles of dichromate
solution, mixed perhaps over five years, and test it against a new
solution of the same strength mixed yesterday, and get basically the
same contrast and speed, allowing of course for some reasonable
variation in the speed and contrast of different tissue and for the
effects of temperature and humidity.
I do remember that in the remote past I worked with dichromate
solutions mixed with tap water. Solutions mixed with this water would
in fact change color and develop sludge and other residue on the
sides and bottom of the bottle after a few months time, and
eventually turn brown or green.
>
>I usually print with in effect 10 to 12% solution, but still at least
>twice your strength... Also, as per my reply to Katharine... I don't think
>the processes are so much alike. The gelatin for instance is the crux in
>carbon, may not even be present in gum.
>
All this may be as you say. But my best judgement tells me that our
unlike experiences with dichromate solutions are probably not due to
the differences between gum and gelatin but to our different methods
of mixing and storing the dichromate solutions, or *possibly* some
form of contamination you have not accounted for.
Sandy King
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