Re: Dichromate dilution and speed

The amount of 14 baume gum liquid gum in the solution is held constant
at 50% throughout the experiments. In other words, each emulsion = 5ml
total gum, water, and pigment, and 2.5 ml of it is gum. Hope that
answers your question.

Sandy King wrote:
>
> 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 19:37:26 2003