Although ammonium dichromate is allegedly flammable and therefore more
hazardous than potassium, and I have not made relative flammability tests
to disprove this, permit me to suggest (as many of us have said on this
list -- and elsewhere -- numerous times) that "this type of information
readily available for free", not to mention in such highly vaunted books
as "OverExposure" and "Health Hazards," is not very meaningful.
That is, the "serving" unit is taken as 8 hours a day, 5-day-a-week
exposure to quantities in the vicinity of carload, ie., industrial-type
use. Some list old-timers may recall my posting the warning on the label
of my jar of Light Impressions gum arabic -- dire, dread and fearsome. Yet
an advisory from the Gum Arabic Institute (or whatever the name, courtesy
of D. Sullivan) points out that over half the VAST amounts, ie.,
boatloads, of gum arabic imported annually into this country are for use
in food.
Meanwhile, my generally incendiary, pierced, smoking, tatooed, daredevil,
devil-may-care bad boy and girl students and I have been using ammonium
dichromate for years and years and seen nary so much as a french fry, let
alone a flame.
There are several reasons for the current hysteria: first, the great human
truism: two wrongs make a right. Because real risks and toxicity were
overlooked for so long (dichromate poisoning was not really diagnosed
until some 20 years into the process, if I have read my old manuals
correctly), we freak over them now. (See also "Asbestos" and "Lead in
paint and glazes."] Second, given the litigiousness of our society,
heaven forbid someone gets a canker sore down the line and sues, if we
haven't posted worst case scenario (or even if we have, needless to say).
I take the trouble to point this out because scare-mongering sets up
a vicious cycle. As usage diminishes, the product gets harder and harder
to get, ultimately may even be discontinued because there's no demand for
it-- thus confirming its hazardous status.
Although wonderful gum prints can be made with any of the 3 dichromates,
my tests show ammonium dichromate the better, more versatile sensitizer,
at least in comparison to potassium. You can dilute it to the strength of
potassium dichromate if you want, but by using it saturated (29% solution)
can get a more sensitive emulsion with less dilution of the gum/paint
mixture.
Judy