Gum Bichromate: Pigments as Color Filters?

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Hi All,
I've been spending some time trying to assess the various theories and observations that have been bruited about re gum, pigment hue, exposure times and curves. Much of what's been said doesn't square with my experience, and much of what's been said is contradictory. For example, several months ago someone asserted in a thread I can't find now, that "red" is a difficult color to print because it blocks UV and so the UV has a hard time exposing all the way to the bottom of the gum layer, so red will print with higher contrast than other colors. I'm paraphrasing from memory here, but I think that was about the jist of it. The implication, if this theory were true, would be that "red" would require longer exposure than other colors. I personally have not observed this.

Then someone in a different thread observed that "red" prints faster than other pigments, although I suspect in that case it was PR 209 at an undersaturated concentration, so saturation or pigment concentration may have been the crucial factor in the speed of the mix rather than it being "red." Elsewhere, I've read that an orange red requires much longer exposures than a bluer red, not something I've ever noticed in my experience printing lots of different pigments, and in fact I compared PV19 and PR209 this week and found their exposures (established by step tablet of course) to be exactly the same. (There's another issue in comparing these two pigments, in that their pigment strengths are very different, but that's a different issue). Another statement I've read is that blue prints faster than other pigments, also something I've not observed.

When I was printing a lot of tricolor years ago, using PR 175 (a dark orange red), pthalo or ultramarine, and PY110, all at fairly saturated concentrations on unsized Arches Aquarelle, in a year-round damp climate, my times for all three colors were almost always about the same, about 3 minutes as I recall. It never occurred to me that I'd be required to defend that observation; I just used exposures that worked. But when someone came along who stated categorically that "blue" "red" and "yellow" each required quite different exposures, I shrugged my shoulders and said well, not always. I thought maybe it was that the red and yellow I was using were deep-valued as red and yellow go, and so maybe similar to the value of cyan, and that might explain why my exposures were so similar for different colors. And in fact when I ran some tests later on different pigment combinations for tricolor, I did find that a very light-valued yellow, such as PY151, did require a different exposure than the darker-valued PY 110, for example.

I'm in a different climatic environment now (although still not a temperature-or-humidity-controlled space) and using different pigments and a different paper, so my current times can't be compared exactly with my earlier times except to say that in a different climate, with different pigments and different paper, the times for "blue" "red" and "yellow" are still quite similar. Running tricolor tests with PV19, Prussian blue and PY97, on Arches bright white sized with gelatin and glyoxal, my times recently have been Prussian, 3:00 minutes; PV 19, 2:45; and PY 97, 2:15 minutes. Not much difference between them, but blue is the longest, not the shortest, and the exposure for red isn't way longer or way shorter than the other times as various assertions and observations might suggest. And, as I've said in another thread, my curves generated by ChartThrob were essentially the same curve for each of the three pigments as mixed for tricolor. In other words, these recent observations are consistent with what I've observed more informally in years of printing gum: I haven't seen much evidence to support the currently popular notion that pigments behave in predictable ways in gum printing as a function of their hue (wavelength). What I see is that pigment mixes behave differently based on their strength, saturation and concentration and other characteristics of pigments, but that the hue of a pigment doesn't predict reliably how it will behave.

Katharine
 
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mkochsch

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I'm just going to throw a couple of question on this.
The saturation of a colour on the final print can be controlled either by the amount of pigment in the mix or exposure. True or false?
Different pigments have different weights. Could this be a factor.?
Would/does the pH change in different gum mixes because of the different pigment chemistries? Other chemical changes?
Which pigments do you use more of? Do you need to add more of one pigment to make it saturated to your liking?
~m
 
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(1) False, at least as I'm defining saturation. When I refer to the "color saturation" of a mix, I'm talking about the color as mixed, not the color as printed. The color as printed, if exposed properly (and I assume most people know how to expose their prints properly) will give (where film is clear) the darkest tone the color saturation of that particular mix allows. Each color mix has its own DMax, which coincides with the deepest color that mix can print, and the proper exposure will give you that DMax, whatever it is. Further exposure will not darken the tone, or increase the color depth, beyond that point, only block the tones up more steps at the same value.

(2, 3) I don't know if weight or pH are factors in determining exposure; I don't know if I care. :--)

(4) Every pigment requires a different amount to reach an equivalent color saturation in the mix, and different brands of the same pigment require different amounts, as I've said so many times I'm starting to feel like a broken record, and as Demachy was saying 100 years ago. Just a few examples: PR 209 takes much more pigment to reach a particular color saturation than PV19; PBk11 takes much more pigment than lamp black; ultramarine takes much more than pthalo, and so forth. They're all different.

I'm not interested in delving into all the factors that might influence exposure, although if you are I'd be glad to see your results. My point here is that I don't see any evidence that a pigment's hue is a major determinant on exposure. In other words I don't see any support for the assertion that "blues" or "reds" as a group behave in any predictable way that would separate them from other color groups.
kt
 
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When I refer to the "color saturation" of a mix, I'm talking about the color as mixed, not the color as printed.

That's why I attached samples of pigment mixes to a post in another thread, as a way of visually demonstrating the color saturation to which I mix a particular set of pigments when using them for tricolor printing.

PBk11 takes much more pigment than lamp black

Just to give a sense of what I mean here (since I don't measure pigments, only judge them by eye, and can't give exact grams of pigment): to get a lamp black mix that will print very dark requires a couple of squeezes of the paint tube into 15 ml gum; to get a mix of PBk11 that will print the same dark (say reflection density of 1.8) requires emptying an entire tube of paint into 15 ml gum. This makes a mix that is so stiff that it can hardly be brushed, but it does work if handled properly.
 
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I'm just going to throw a couple of question on this.
The saturation of a colour on the final print can be controlled either by the amount of pigment in the mix or exposure. True or false?

As I said above, the depth of the darkest color a mix can print is, in my experience, almost entirely a function of the color saturation of the mix (not the amount of pigment in any absolute sense, since the amount of pigment required to achieve a certain color saturation varies widely, as has already been noted), not by exposure. The only way to control the DMax of the print by exposure is to reduce maximum color saturation by underexposure, and I don't know why anyone would underexpose on purpose. Overexposing has no effect on color saturation, as I've said.

Here's a rough visual I came up with to make this clearer (I hope). I coated two pieces of paper with separate mixes of PV19 at different saturations. Then I exposed each of the papers, moving a sheet of black paper across the coated paper at 30 second intervals, producing the DMax (or maximum color saturation, if you will) of the mix at each exposure, unmediated by glass or by film As you can see, only the underexposed sections show a difference in color; once the paper is exposed at reasonably close to the optimal exposure, then the DMax stays the same with increasing exposure. (The light mix shows some dichromate stain yellowing the color as overexposure increases, but I'm quite sure if that were cleared, the color would be the same as the less overexposed sections to the left. Hope this is helpful,
 
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Sorry, now I'm "blogging," but I just thought of another way of showing the same thing:

Here's a set of step tablets printed on PR209. The strips (and the background area around them) were exposed for 1, 2, 3, and 4:24 minutes. Note that the background tone is essentially the same hue and value (color saturation) for the background area that was exposed for 1 minute as the background area that was exposed for 4:24 minutes (background areas covered by glass during exposure but not of course by film). In other words, a one minute exposure already exceeded the threshold for producing DMax for the mix in the border areas. In the strips themselves, a little more exposure was required to produce DMax under film, but I hope you get the idea: underexposure that's pronounced enough to fail to print DMax is also pronounced enough that you'd never print that way (no one would choose 1 minute as the exposure time for this mix, for example). I prefer judging exposure by the number of steps printed and retained, rather than by DMax, which to my mind is a necessary but not sufficient criterion for exposure.

P.S. I'm not sure what any of this has to do with the original issue?
 
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E Thomson

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I may be able to clarify some issues here. The experiences you cite make sense to me based on my knowledge of pigment manufacture and type. I've had a career in fine art materials, which is a world that is contiguous with but largely foreign to the photography world; assumptions from one world don't always work in the other.

The assumptions you find to be mistaken were guided by the model of the color filter, I surmise. The notion that red and red-orange would take longer to sensitize all the way down through the layer assumes that the pigment is behaving like a red filter. But you find otherwise and that would make sense if the pigment is indeed a pigment, not a dye.

Historically, the list of permanent, artist-grade pigments has been dominated by crushed or pulverized inorganic minerals or metals; iron-oxides, cadmiums, cobalts, lead sulfates, copper oxides and the like. Many are synthetic only in the sense that they are precipitates from simple chemical reactions. These sorts of materials can be extremely permanent since they are non-reactive, chemically or photochemically. The standard for permanency on an artist's palette is ridiculously high; the 'A' list consists of colors which simply don't change, ever, when subjected to laboratory-simulated conditions of indoor viewing.

Pigments are granules; they have substantial mass. They have been pulverized and graded by size but they are still granules. They act by reflectance, absorbing certain wavelengths and reflecting others back to the eye.

Dyes on the other hand are completely different. They have no granularity, they are colored solutions, acting like gels to transmit one wavelength of light through, to bounce off the white substrate and back to the eye.

Materials that are pigment-like will have relatively little variation one to the next, with respect to how much UV light they will pass. The pigment particle itself is opaque to UV, whether we see it as blue, red or green. And a similar amount of UV will penetrate a matrix of pigments, whether they are blue, red or green, provided they are dispersed similarly and of similar grain size.

Not so with a dye of any sort. A red dye will do its job of passing only a select wavelength through, excluding UV in this case. Alt-process photographers are familiar with this from the recent use of inkjet negatives or inter-positives printed in red ink on dye-based systems.

So, the behavior you notice, Katherine, would be what I would expect from traditional artists-grade pigments; however there will be some exceptions. Late twentieth century pigment chemistry has produced a range of synthetic-organic ('coal-tar'-sort) dye-based pigments which behave very differently than traditional pigments. They are highly transparent and high-staining and generally act somewhat differently than other pigments. They are produced by staining an inert, colorless pigment particle like barium sulfate with the dye and in this respect they are like a handful of traditional pigments called "lakes"; alizarin crimson is the best known. Pthalocyanine blue is an example of this sort of new pigment.

So, to make a blanket statement is impossible, and even traditional pigments have different degrees of transparency because of subtle refractive diffrences when suspended in their medium. But you seem to have chosen a list of pigments which are similarly opaque, one to the next and block UV uniformly.

One could predict the behavior of pigments with some reliability if one knew their precise nature. Artist's pigments are largely single compounds. Pure cobalt stannate. Pure iron oxide. Pure cadmium sulfide. And they are sold by their chemical name or by a name which is constant and can be referenced, such as Raw Sienna, which is always (in any reputable maker's line) an iron oxide, which as a general class will have known opacity. A reliable resource for pigment qualities is Ralph Mayer's The Artist's Handbook.

As an aside; I am curious how you come to be referring to colors by their industry names, like PR 209. What are your sources of supply?
 
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Hi Reck,
Thanks for your thoughtful reply. It's very helpful, and supports my observations; I appreciate the additional information very much. I haven't found, in practice, pthalo blue to be that different in exposure time from other pigments, but I haven't done a systematic study of that either.

My own personal favorite source for information about pigments used in watercolor paints is Bruce MacEvoy's website,

http://handprint.com/HP/WCL/waterfs.html

It's well-researched and usually accurate, and keeps up to date with currently available watercolor paints.

As to the use of color index international pigment numbers, this is the only unambiguous way of designating pigments, and since the characteristics of the pigment are such an important variable in gum printing, IME, I prefer to refer to pigments precisely rather than using the marketing name given to the paint, which is often misleading as to the actual pigment. Bruce MacEvoy also designates pigments this way.

Welcome to hybrid photo!
Katharine
 
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E Thomson

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Hi Katherine,

I'm glad some of that helped support your experience. Just to clarify: pigments derived from dyes, like pthalo blue and the profusion of newer synthetic-organics, are still pigments and behave more like pigments than dyes (or filters). They have been "laked", i.e. used to dye a colorless particle such as barium sulfate or calcium carbonate (or in the case of pthalo, I believe, further treated so as to precipitate out as a granule). So the dye becomes a granule with some opacity. Nevertheless, it's generally true that dye-based pigments behave with a lot of transparency, sometimes radically so. So some aspect of the dye behavior often carries over by some mechanism.

I am particularly interested in these highly transparent pigments. They are "something new" in the sense that traditional artist's palettes have never had any such thing with any permanency; alizarin crimson has been allowed on the palette only with reservation as it is so fugitive. They have a remarkable characteristic of having one distinct hue as a mass-tone and a very different hue when thinned out or tinted out with white. For instance an earthy green will tint out to a bright yellow undertone. And so the possibility exists to work with one pigment and achieve a color effect as if two or more are involved; a monochrome image with a range in hue. For me, this could be interesting both as an ink in photogravure and as an accumulated layering effect in temperaprint (similar to the gum process). One of the first artist's paint manufacturers to incorporate a large list of these was Old Holland.

Lastly, I've been watching inkjet makers and their pigmented inks. I note that even pigmented inks are being used for UV-filtering effects. I also note that the nozzles of the current machines are perishingly small and require a pigment particle much smaller than anything achieved for artist's use. So we can assume these 'pigments' are very different than traditional ones, perhaps a sophisticated variation on the synthetic-organic "lakes".

Bruce MacEvoy's site is extraordinary, no other word for it. And his information on pigments looks entirely reliable. What a resource; permanency ratings and Wittgenstein all in one place. Thanks very much for that and for the Koch-Schulte site.
 
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So, to make a blanket statement is impossible, and even traditional pigments have different degrees of transparency because of subtle refractive diffrences when suspended in their medium. But you seem to have chosen a list of pigments which are similarly opaque, one to the next and block UV uniformly.

Just a point of clarification: when I report observations that fail to support a theory, that's all I'm doing, reporting observations that fail to support a theory. I'm not generalizing from my observations or even suggesting a new hypothesis on the basis of my observations. I'm not saying, or even suggesting, that all pigments behave the same, just because my exposures were similar for different pigments in the examples I gave. My position, as stated in a post earlier in the thread, is that pigments behave differently for all kinds of different reasons, but that my observations over decades of gum printing do not support the theory, speculation, or whatever you want to call it, that pigments of different hue ranges require different exposures (eg blue pigments print faster and red pigments print slower) as a result of the more or less efficient UV filtering of pigments based on their hue (wavelength).

I found your comments about inkjet inks fascinating; perhaps that provides a partial clue to why inkjet inks do seem to (although as someone pointed out in another thread, not always in a lawful and predictable way) block UV more or less depending on their color, but pigments in the gum emulsion don't.

I'm enjoying this conversation,
Katharine
 
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E Thomson

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Last time I looked, the technology of inkjet pigments was heavily under wraps and apparently not much has changed. I find almost nothing very revealing. One might have to troll through endless patent applications to get a good idea what they're doing. But this site,

http://www.roumazeilles.net/news/en...-pigment-or-dye-based-a-technical-comparison/

had a couple points of interest. A brief quote:

"Pigment technology is difficult to master (taking into account the small size of the particles to create in large quantities). You have to manufacture nano-sized particles that will be coated in a thin resin layer.

The resin will be used to protect the pigment against external mechanical and chemical attacks, but it also helps in transporting easily the particle inside the ink (and through the nozzles) and fixing it onto the paper.
It is easily seen that these contradictory constraints make this technology more difficult to master."

Two points. 1) The notion of "encapsulation" which Epson talks about in its literature. Each pigment particle is coated with resin (polymer). So pigment manufacture has come a long way from the muller and slab. 2) "Nano" size. Inkjet pigments are nanoscale, meaning 1-100nm, which is of course at the cutting edge of manufacturing technique. Interesting to think that the wavelength of visible light is 400-700nm. And that the variable size of different molecules spans the nanometre mark.

So these are no ordinary pigments. I happen to know that CaCO2 (calcium carbonate) is extensively made at nanoscale and it is one of the traditional "lake" pigments, the neutral carrier for a dye. Perhaps it's used in inkjet technology. But clearly, the definitions of dyes, pigments and particles start to break down at this level.
 
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