Re: Dichromate dilution and speed

Sandy,

I don't know why Sam's gum prints with so much speed. And, in fact, I
don't really care. For me, speed of the dichromate solution is irrelevent
to my process since my exposures are rarely more than a few minutes
anyway. I think Sam's solutions work so well for him because his
negatives are quite low in contrast and match the high contrast gum
solution that results from very low dichromate concentrations.

For those of us doing combined gum-platinum printing, very low dichromate
concentrations are not very useful, except perhaps to boost shadow density
in later coats. For giggles, I tried a 3% dichromate mix with one of my
negs and got very harsh, contasty image. My pt/pd negs work very well
with saturated dichromate levels in the gum-platinum process. I commonly
use 30% ammonium dichromate (relative to the amount of gum) on my first
gum coat, then cut back to 20% or 10% on subsequent coats when I want to
richen and separate midtones and shadows while leaving the highlights
alone.

Also, I'm curious what Sam means by a 1 part gum to 2 parts water. Is this
by weight? I mix 300 gm of powdered gum to 1 liter of water and end up
with slightly more than 1 liter of solution. If Sam is mixing 500 gm
powder to 1 liter of water, then I presume his gum is more viscous than
mine.

And why are we talking about Sam like he's not here? Sam - any thoughts?

Kerik

> So the question remains, why do Sam's gum, which use such a low
> percentage of dichromate (about 3%) in the coating, print with so much
> speed, while others need saturated solutions of 10X the amount of
> dichromate. I could be wrong but I think there is really some dramatic
> difference in technic rather than an accumulation of little differences.
>
> Sandy
Received on Mon Dec 1 16:09:42 2003