Dichromate dilution and speed

I've been experimenting with five new gums while I decide which one will
succeed the Photographers' Formulary powdered gum as my gum of choice.
As part of the familiarization process, I've been running some
comparison tests, following Judy's method (thanks, Judy) of putting some
water into pigment to make it liquid enough to be measured out by
dropper, to ensure very precisely calibrated amounts of pigment in small
amounts of gum so that comparisons between gums won't be muddied by
inadvertent differences in pigment concentrations caused by inexact
measurement of small amounts of paint. I'll report my findings re gum on
my website when I'm finished.

Since I've got this liquid pigment at the ready, it was very simple to
run some comparisons to test the assertion that prints made with diluted
dichromate don't print any slower than those made with saturated
dichromate, all other variables (negative type, pigment concentration,
paper, etc etc) held constant.

My results do not support the idea that there is
little difference in speed between diluted ammonium dichromate and
saturated ammonium dichromate. I found instead that reducing
concentration from saturation to 1/5 of saturation, following
Christina's description, required 5X the exposure time to make a
properly exposed print. (Prints exposed for the same time, or even 2x or
3x as long as saturated dichromate prints, were so grossly underexposed
the gum simply ran off the paper leaving no image at all.) I ran this
test on two different gums and got the same result both times. This is
too little data to prove anything in a greater sense of course, but it
certainly casts doubt on the assertion that was made. I'll be happy to
post test strips and test prints if people want to see them.

I don't doubt Sandy's finding that for carbon printing, there's no
increase in speed above .5% concentration of dichromate, but this
observation is not, in my considered opinion, relevant to gum printing.

I did notice a marked increase in contrast with the diluted dichromate,
but I found the increased contrast horrifying rather than pleasing. I
like to make the basic printing with a pigment concentration that gives
the longest scale the gum is capable of; the step wedges for the
saturated ammonium dichromate showed 8 nicely differentiated steps for
one of the gums and 7 for the other; the test prints had very nice tonal
gradations in the highlights and open shadows that could be deepened, if
desired, with a second short printing with a stronger pigment
concentration to produce a print with full and deep tonal range. This is
how I like to print, (more recently I've printed the basic printing
lighter and omitted the shadow printing altogether, resulting in a
deliberately high-key print).

The prints with the dichromate diluted to 1/5 of saturation had a
sharply truncated range: 3 steps for one gum and 4
steps for the other, resulting in harsh contrasty prints not at all to
my liking. This of course is an extreme dilution and so an extreme
increase in contrast, and less drastic contrast alterations can no doubt
be made using less extreme dilutions, but since I started with the
contrast where I wanted it, why make life difficult for myself?

Contrast is an interesting issue and more complicated than many realize.
You can hold pigment concentration constant and change
dichromate dilution to change contrast, or you can hold dichromate
concentration constant and change the pigment concentration to change
contrast. One method isn't better than the other; it's just two
different ways to achieve the same end. It's not the case that people
using saturated dichromate are limited in contrast range; every contrast
desired can be attained by simply adjusting the pigment concentration up
or down. It's only when you increase dichromate above 1:1 gum/pigment to
dichromate, especially when you get up around 1:2, that I would agree
that added dichromate is senseless and reduces the contrast more than is
useful.

Determining where the point is, that minimum point of dichromate
dilution Sandy referred to, that marks the break between just enough
dichromate and more dichromate than necessary, is a much more difficult
problem than he seems to think. First, the point would have to be
different for every pigment concentration. And since the pigment
concentration necessary to produce a certain shade of "density" is
different for each pigment and each manufacturer's version of that
pigment, there would have to be a break point for each
pigment/manufacturer combination. One for Daniel Smith hansa yellow, one
for Winsor & Newton hansa yellow, at etc, etc, etc. at each "density"
range. Second, the point for each pigment concentration would be
different for every contrast range desired. A person wanting to print
very contrasty images would have a different break point than a person
wanting to print images with a long scale, as I do. And so on and so
forth. It seems to me that to find all those break points would be a
very boring life's work for some unfortunate soul, and that no one would
ever look at the volume(s) of tables that he produced when he was done,
although no doubt every gum printer would have it in his or her library.
These determinations, in my experience, are much better arrived at
through an intuitive process than an analytical one, and the answer will
be different for each gum printer or sometimes different for the same
gum printer as his style evolves, and that's just fine.

I've been printing gum the way I do for so long that it's second nature,
like breathing; it wouldn't make sense for me to have to learn gum
printing all over again. But for someone starting out, I can see that
there could be an advantage to starting with diluted dichromate. One
could start with very little pigment, avoiding altogether the pigment
staining problem that beginners tend to have, and get a fairly high
contrast print, which is what beginners seem to like (I'm going by my
own experience as well as by observation; when I first started printing
gum I was happy with a print with two or three tonal steps, or even only
one dark value against a background of colored paper, perhaps.) And then
if later you wanted to print with a longer scale, you would have to
learn to decrease contrast and increase the tonal range by using a more
saturated concentration of dichromate. I learned the other way, starting
with high pigment concentrations and saturated dichromate and and
getting a fairly contrasty print, and then when I wanted to print a
longer scale, had to learn to lower the pigment concentration to
increase the scale. They are both valid approaches, as I said, just
totally different approaches. To assume that one is invalid because the
other works, is to fail to understand the versatility and complexity of
the gum process.

In principle I agree that it's a good idea environmentally to reduce
dichromate, although given the fact that I use .675
grams of dichromate for a printing session of three or four prints,
depending on size, even at saturated strength it would be a bit of an
overstatement to accuse me of "polluting the environment." But still, I
would make the change if I didn't think my prints would suffer for it.
It's possible, as I said earlier, that one could learn to achieve the
same results by altering dichromate concentration rather than by
altering pigment concentration, but it would require starting over, and
I'm not interested in doing that. I'm also not interested in putting up
with substantially increased printing times. So, call me a polluter if
you like, but since I was told by a staff person at the state agency
regulating toxic waste that "if you were polluting the environment I
would tell you so, but you aren't" I'm comfortable with that and will
continue in my sinning ways.
        
Katharine Thayer
Received on Sun Nov 30 13:33:27 2003