From: Katharine Thayer (kthayer@pacifier.com)
Date: 07/04/03-03:14:46 PM Z
Judy Seigel wrote:
>
> Chris, you weren't testing the 3 dichromates, as you apparently believed.
> You were testing the 3 dichromates at their saturated solutions. Which
> hardly permits the generalizations you offer.
>
> You cannot assume that qualities you find in, for instance, a 30 %
> solution of ammonium dichromate would be the same in a 10% solution of
> ammonium dichromate, and so forth. Your generalizations about what is
> "faster," "sharper," "contrastier" or "stains" more... can only be
> attached to those DILUTIONS, not to those chemicals.
>
> Which essentially was my finding about 5 years ago when i tested all at
> the same dilution (& as I recall mentioned several times on this list).
> The differences between the dichromates USED AT THE SAME PERCENT SOLUTION
> are trivial AND variable. For instance, odds are you'll find as much
> variation with a change of size, paper, gum or pigment as from chemical to
> chemical AT A GIVEN DILUTION.
>
I've already commented generally on this argument in another post, but
coming across this post again I think it would be well to respond to it
directly in context, even though I will no doubt repeat myself:
The crucial difference between the dichromates is that at saturated
strength, ammonium dichromate is faster than potassium dichromate. The
fact that if diluted to the same concentration as saturated potassium
dichromate, ammonium dichromate can be made to function similarly to
potassium dichromate, is beside the point, since potassium dichromate
can never be mixed at a dilution which will make it behave like ammonium
dichromate at ITS saturated concentration; in other words the argument
doesn't work both ways. The argument that differences in speed are only
a function of concentration works only up to the percent solution that
marks a saturated solution of potassium dichromate. Beyond that,
ammonium dichromate will always be faster than potassium dichromate, and
to say that the differences can only be attached to the dilutions, not
to the chemicals, is not a useful statement either in terms of chemistry
or in terms of gum printing practice. The fact that ammonium dichromate
can be mixed stronger than potassium dichromate IS a property of the
chemical; the fact that ammonium dichromate at saturation prints faster
than potassium dichromate at saturation, IS a property of the chemical.
The argument would work only if one never wanted to print faster than
potassium dichromate can print, in other words if one never wanted to
use the qualities of ammonium dichromate that can be achieved in those
concentrations between the maximum concentration possible with potassium
and the maxium concentration possible with ammonium dichromate. People
using some types of lights may not be able to use that extra power, but
most can take advantage of it, and it makes no sense to me to dismiss
that important difference between the chemicals by saying it's only a
function of dilution. It would be a function of dilution only if both
chemicals could be mixed to the same maximum concentration.
Katharine Thayer
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