Re: warning Re: gum mixes

[Katharine Thayer <kthayer@pacifier.com>]

On Jan 4, 2008, at 8:26 PM, sam wang wrote:

> I prefer to use about 1/8 of dichromate in dry form for gum as  
> compared to using saturated solutions. Didn't we go over this many  
> times in the past? I can't understand why one would use more  
> dichromate than necessary to do the job, except because the folks  
> who wrote the literature way back when said to do so.
The idea that there is an amount of dichromate that is superfluous,   
is simply not consistent with my observations.    I did a set of  
tests five years ago or more, available somewhere on my website, that  
compared five concentrations of ammonium dichromate and found a  
linear increase in speed with each increase in dichromate all the way  
to saturated ammonium dichromate, with concomitant decrease   in  
contrast (more steps).   In other words, all the way to saturated  
ammonium dichromate, an increase in dichromate concentration resulted  
in faster printing and more steps printed, holding everything else  
constant (except exposure, of course; the exposure must be adjusted  
for the change in dichromate concentration for a meaningful  
comparison).   It's entirely possible that people might prefer  
printing a shorter tonal range and not mind a longer exposure, in  
which case they are free to consider more dichromate "superflous,"   
but this is purely a subjective call.  I prefer the shorter exposures  
and longer tonal scale of the fully saturated ammonium dichromate.

I had assumed that saturated ammonium dichromate mixed 1:1  
represented the useful upper limit on speed and steps, but recently,  
by accident, I found that one can improve speed and tonal scale  
further by doubling the saturated ammonium dichromate,  in other  
words using 2 units saturated ammonium to 1 unit pigment/gum.   
Printing this way but exposing more than the extra dichromate  
required, I found that I could get a long tonal scale in one coat  
similar to what some of the folks are getting with bleach  
development, although it required forced development.  I haven't  
explored that avenue further, but I think that mix might respond well  
to bleach development.

As for the amount of dichromate going down the drain, the amount we  
use is so small as to be negligible, unless you're making several  
wall-sized prints every day or something like that.  I use .675 grams  
of dichromate for a short printing session of 3-4 prints; this is  
hardly worth worrying about.

Katharine