Re: Dichromate dilution and speed

All other things being equal, less dichromate in the mix will produce a
slower emulsion, a shorter scale, and higher contrast. Any experienced gum
printer should know that.

I've used countless variations of gum/pigment/water/dichromate ratios. I've
tested my materials extensively. The best approach to gum printing is as
Katherine suggests, "through an intuitive process".

Dave Rose
Big Wonderful Wyoming

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Sunday, November 30, 2003 4:37 AM
Subject: 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 21:17:18 2003