> .... potassium, said to be about half as sensitive as ammonium,
> is also said to give higher contrast (again at the same concentration).
> Another commonly cited generalization is that a 2.5% solution of ammonium
> dichromate has the same speed, contrast and keeping quality as a 3.5%
> solution of the potassium.
I have tested the three dichromates, potassium, ammonium and sodium
against each other. As far as I could tell -- and as whatever source I've
read on the subject has asserted -- the differences in contrast are a
result of the difference in *solubility.*
That is, potassium dichromate is the least soluble in water (15%?),
so it has the least dichromate per unit & the least sensitivity; ammonium
dichromate will make about a 26% (29%?) solution, and is more sensitive;
sodium dichromate I believe goes to about 50% and so is the most sensitive.
Given the same ratio of gum to dichromate in tests of all three, the
potassium would be the least sensitive, so the highlights would get less
of a hit, making the print the most contrasty; sodium would be the most
sensitive, that is, the highlights would expose more completely, hence
have the longest scale, or be the least contrasty.
My tests bore this out. In other words, you could conceivably get the
same contrast using potassium dichromate in a 1-part gum, 2-part
dichromate ratio as using ammonium dichromate 1 to 1. At the same time,
there is more dichromate staining as you go up the sensitivity ladder.
But since a long soak will usually remove that, we don't care.
>
> Sodium dichromate is much more soluble so if as Sandy says Davis is
> using a saturated solution it would be much more concentrated and
> therefore probably much lower in contrast. This might indeed explain
> why Judy is getting unworkably high contrast with an ammonium or
> potassium sensitizer.
>
That's BRILLIANT, Philip, though I suspect other differences may have been
at work as well, such as the fact that Phil's prints were tri-color. Tho I
should add that my "unworkably high contrast" was of course only with the
daylight bulbs. My sensitizer was ammonium). I have some sodium
dichromate on hand and if life cooperates will test it.
Judy