From: Christina Z. Anderson (zphoto@montana.net)
Date: 07/01/03-06:39:50 AM Z
<Sandy said> What I find is that at
> a given percentage solution ammonium dichromate gives more
> sensitivity than potassium dichromate. If you reduce the percentage
> solution of either you will get a more contrasty image and less
> sensitivity. If you increase the percentage solution you will less
> contrast and more sensitivity. If you do this in gum I would bet a
> lot of money that you would observe the same relationship.
> With carbon you can get the same results with either ammonium
> dichromate or potassium dichromate, so long as you adjust the amount
> to account for the greater sensitivity of ammonium dichromate. I find
> that a 3% solution of ammonium dichromate will give almost the same
> results in terms of speed point and number of printable steps as a
> 4.5% solution of potassium dichromate.
> Sandy King
>
Sandy,
Now this makes sense. Bottom line--the so called "lower contrast" of
am di would be directly related to its faster speed, which you could adjust
by exposure and amount added in your sensitizing mix (or pigment and
exposure time of course)--not that it is necessarily inherent in am di
itself...
Maybe the reason am di gets a bad rap in gum sometimes is people don't
realize how fast it is, and overexpose, and thus their image gets
overprinted highlights.
I've come across a motherlode of stuff on gum from the 1900's and it
seems at that time that they were using additions such as sodium hydroxide
to the gum to neutralize the acid, ammonia drops to the sensitizing
solution, manganese sulfate to the same, methyl alcohol, and lemon juice
(infrequently on the last) to vary their formulae. They said to do such and
such but do not give the reason why, I guess assuming that we all know why
or that we don't care, or maybe it was a secret or hocus pocus. I kept
coming across this manganese sulfate and being puzzled until finally one
author explained that manganese sulfate in combo with am di reacts most
quickly to evolve gas under the action of light (?) and was used in the
ozotype process and is particularly beneficial in the gum bichromate process
therefore. Any clues as to the use of this or what it is?
Also, nowadays is there a source of fish glue or is it called something
else, and if it is "fish" glue is it stinky?
Chris
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