Re: Dichromate dilution and speed

On 30 Nov 2003, at 20:06, Sandy King wrote:
>
> 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.
>
I must be missing something in this whole thread. There is no
reason you cannot maintain the same percentage of gum in a
mixture whether you mix your dichromate in water or not. If mixed
in water, then all you have to do is keep the total volume of water
added to the gum/di mix the same, regardless of how much
dissolved dichromate you use. For example if you ordinarily use 2
ml gum + 2 ml am di in a saturated solution, you can cut the
dichromate concentration in half by using 1 ml am di soln + 1 ml
water mixed in the usual 2 ml gum. Starting with all dry ingredients
might make it easier to calculate _absolute_ percentages, but
doesn't make it any easier to maintain _consistent_ percentages.

Katheryn's test are methodologically sound and demonstrate her
point, at least for her particular combination of
neg/gum/paper/light/time-of-day/position-of-mouth/mood-of-the-
gods etc.

Which leads me to ask the question: what is the percentage figure
for a saturated solution of ammonium dichromate?

>
>
>
>
>
>
> >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
>
>

Regards,
Gary Nored
http://home.centurytel.net/Gary_Nored/
Received on Mon Dec 1 17:53:05 2003