From: Judy Seigel (jseigel@panix.com)
Date: 02/08/02-04:19:15 AM Z
The Tests
Science, the newspaper tells us, has now got a computer program that puts
data from work in progress into code, so researchers do not know how
things are coming out. The human desire to have results go one way or
another apparently affected ability to read tests accurately.
In a similar vein, I have read that studies by doctors who like exercise
"proved" that heart patients benefit from exercise, and studies by doctors
who don't like exercise "proved" the contrary. Which is to say, my tests
make it dramatically clear beyond the possibility of doubt that the
stronger the dichromate in an emulsion mix, the stronger the pigment
stain.
They should also prove that you cannot tell the pigment-stain story
without the dichromate story, that differences in dichromate cause
differences in pigment stain. You will also see that they are so simple
and obvious, it's hard to understand why something along these lines never
replaced the gum-pigment-ratio event that will not die.
The Dichromates:
Mix dichromate with distilled water. Potassium dichromate is soluble up to
about 10% (10 grams in 100 cc). Ammonium dichromate is soluble up to 30%
(30 grams in 100 cc). Sodium dichromate is soluble to *around* 100% (my
books refuse to make up their minds). The range of concentration possible
with the potassium dichromate is too small to read results easily, so use
either sodium or ammonium.
The volumes given are far more than needed for test strips, but are easier
to measure. If you have droppers or pipettes or syringes for measuring
small amounts, choose your own volumes. All tests should be with the same
dichromate (ammonium or sodium), so that differences can *only* be due to
test variables. The figures given are for ammonium, and assume a 30%
solution. In a cool room, some dichromate may precipitate out. That's OK,
there's range to spare.
The Paper
Since we're TRYING to show stain, ideally the paper should be shrunk & not
sized. A surface size on paper that's never been wet can forestall stain
on many first coats; the water washes it away & raises the nap, after
which it stains more (unless given a coat of size).
Procedure:
Make up 50 cc of a 30% solution of ammonium dichromate (15 grams in 50 cc
water), and 50 cc of a 10% solution of ammonium dichromate (5 grams in 50
cc). Wait a day before doing the tests, so the crystals will be fully in
solution.
Test #1: Choose a tube watercolor that likes to stain. Mix enough of it
into 10 cc of gum arabic to make a strong color. Combine half of this
gum-pigment mix with 5 cc of the 30% dichromate and coat two strips of
paper. Combine the remaining half of the gum-pigment mix with 5 cc of the
10% dichromate solution and coat two more strips. Reserving one strip of
each concentration for the next test, expose one of each concentration
under a 21-step for a bit longer than your usual gum exposure. Develop
face down in water for half an hour, let dry and compare. Although the
gum-pigment ratio remains the same, the stain will change with the
dichromate.
Test #2: The 10% strip may have been too underexposed to be read clearly.
Expose the remaining two strips for twice the time (or as much longer as
you figure it will take for the weaker sensitizer to get enough tones to
show stain). Develop and compare as in #1.
The 30% strip should have more pigment stain than the 10% strip. Yet the
gum-pigment ratio has remained the same.
Test #3: Coat two strips as above, except with half as much pigment in the
gum. Expose for the compromise time arrived at in test #2. The total
amount of stain will be less than in test #1 (which has more pigment), but
the difference in stain between the two strips should be comparable.
Try any other papers on hand and compare.
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