<< The proportion of dichromate does affect contrast from my experience. I
use two dilutions of a 29% amm dichomate mix. One at 2:1 (2 parts
sensitiser to 1 part gum) and one at 1:1. The 2:1 gives less contrast and
I usually employ it around the second or third coating.
>>
This is a good example of what I said before: that a variable is not a simple,
single variable! What we call the "proportion of dichromate" or sometimes the
"amount of dichromate" or the proportion of amount of sensitizer is affecting
at least two variables and these 2 variables are affecting the range of the
final print.
First, let's consider Sam Wang's example. He is using dry dichromate powder.
He mixes the pigment-gum mix first and then add .25g dichromate to his 10 ml
mix, thus the proportion of dichromate is 2.5%. If he wants to examine the
effect of the proportion of dichromate, he can use .5g and double the
proportion. Nothing else changes much, thus he can compare two prints and
study the difference. I believe his experience says that there is no (or not
much) difference in contrast.
But the above example of using 2:1 or 1:1 (sensitizer:gum) is different. While
it is true that the proportion of dichromate is different in both cases, the
viscosity of the mix is also affected! (because it is not just more or less
dichromate, it is also more or less water).
Rounding the dichromate concentration to 30% (for easier discussion), the 2:1
mix has 20% dichromate, and the 1:1 mix has 15% dichromate. Both 20% and 15%
are pretty high concentration and more than enough to harden the gum, so I
suspect that that is that is not the reason why contrast change. The reason is
more because of the dilution of the whole mix.
Now think about it, when you have a diluted mix, if you brush it on normally,
you have a thinner coat on the paper. If you try to imagine and compare the
thinner coat and the thicker coats in your mind (like a mind microscope), you
can actually know that the 2:1 will indeed give you a lower contrast.
Well, these kind of things are so hard to describe in words. Maybe one day I
will try to write a better description.
Dave S