Re: Dichromate dilution and speed

Interesting. But what I can not determine from this account is if you
calculated the actual percentage of gum in the final solution (and by
final solution I mean gum + water + pigment + dichromate. In other
words, if you were to start with dry gum and mix it with water, then
add the pigment and dichromate and more water, what would be the
final percent solution of the gum? If on the other hand you start
with something like a mixed gum solution of 14%, when you dilute this
1:1 with water+dichromate you reduce it to 7%, and adding pigments in
aqueous dispersion dilutes the gum solution even more, perhaps to 5%
or even lower in the final solution.

In carbon printing a tissue made with a pigmented gelatin solution
that consists of 5% gelatin would be less sensitive by a factor of
2X-4X than one made from a pigmented gelatin solution that had a 10%
gelatin solution, and one of 20% has more sensitivity over the 10%
solution by a factor of another 2X to 4X.

I don't know how this works for gum but tests which do not account in
any way for the total percentage of gum in the final solution used to
coat the paper do nothing to answer the relevant question. Which is,
when using a pigmented gum solution of a specific percent with a
certain amount of pigment, what is the relationship in terms of speed
and contrast that results from varying the percentage of dichromate
in the solution.

Sam's working method, which is based on a dichromate solution of
approximately 3% or less in the final coating solution, with very
short exposure times and with some of the most saturated colors I
have ever seen in color gum, is compelling proof to me that it is not
necessary to use saturated solutions.

Sandy King

>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 19:12:10 2003