Since the visible image in a gum print is made of pigment, the serious gum printer will spend some time becoming familiar with the pigments he/she uses.
Choice of pigment has more to do with personal aesthetic choices than with how well the pigment "works" for gum printing, as it is my belief that any pigment will work if it is properly understood and used appropriately; that is, there are no pigments I have found that are inherently incompatible with the gum printing process. For example, it is repeated in gum literature often enough to be a truism that chromium pigments are incompatible with the gum process; however I have printed with chromium oxide green (PG 17) and viridian (PG 18) which are both chromium compounds, with no discernable difficulty.
My approach to pigments, and my recommendation to beginning printers, is to choose a few pigments and learn them well, rather than trying a lot of different pigments. This works for me in painting as well as in gum printing; I can mix every color I want from a few basic colors, and so I work with a fairly small palette. I also buy single-pigment paints rather than mixed paints, preferring to make my own mixtures from pigments I know well rather than rely on manufacturers' mixes.
A note about nomenclature: I always try to refer to pigments by the pigment number as well as the pigment name, as the number is the only unambiguous way of designating pigments. When I use the name of a pigment, understand that it's a pigment I'm naming, not a paint. If a pigment is well known enough that its name is unambiguous, such as pthalo or ultramarine, I may refer to the pigment name without always giving the number. Whenever I refer to a paint rather than a pigment, I will always use quotation marks for the color name of the paint. An example to illustrate this distinction: Gamboge (NY24) is an outdated, fugitive pigment that is used by only one manufacturer, Winsor & Newton, in their paint "Gamboge Genuine." All other paints given names including the word "gamboge" (of which there are many) are made with pigments other than gamboge.
Either watercolor paints or dry pigment can be used in gum printing. Dry pigment is perhaps the better way to go for a serious worker, as you know what pigment you've got to start with; you are in complete control of the pigment and of what is added to it along the way. However, I prefer the convenience of not messing with powdered pigments, so I've always used paint and can offer no guidance to those wishing to work with powdered pigment.
If you choose watercolor paint, be sure to use professional or artist grade paint, not student grade, and as long as you're doing it, why not stick with the best quality lines? An excellent reference source on pigments,
gives reviews of all the major watercolor paint manufacturers, along with a wealth of other information about watercolor paints and pigments; the beginning gum printer would do well to read these reviews of manufacturers' lines before picking a paint brand.
The brands of watercolor paint I use are M. Graham and Daniel Smith. I didn't decide on these brands after testing all the brands available and deciding that these are "best" for gum printing, these were simply the brands most conveniently available to me when I started printing gum and I haven't found any reason to be dissatisfied with them. But if I had started with other high quality brands of paint, I believe I would be equally satisfied. In other words, I simply don't believe that there is any magic associated with particular brands of paint. But for what it's worth, it turns out that both of the brands I use are rated highly by handprint.com.
Since paint color names often have little relationship to the pigments that are in them, a gum printer who wants to understand the pigments he uses will need to rely on sources such as the handprint.com site given above, or the books "Hilary Page's Guide to Watercolor Paints." or "The Wilcox Guide to the Best Watercolor Paints." Handprint.com is hands down the best source for pigment information, in my opinion, but having one of the two books is handy because they contain indexes by manufacturer's paint listings; by following that index to the page, you will find the pigment and its number, which you can then use in handprint.com (which is organized by pigment number) to get the relative pigment information. Some manufacturers (M. Graham for example) give the pigment names in their paint listings on their websites, others don't.) I use the books to supplement the information on handprint.com, but when there's a discrepancy, especially on important information such as the lightfastness of a particular pigment, it's handprint.com that I trust.
Most manufacturers put the pigment number on the paint tube, so if you have paints that you want to check out, just look on the tube for the pigment number. I wouldn't buy paint from any manufacturer that didn't identify the pigment.
Watercolor paints are categorized for watercolor painting on a number of different dimensions, of which not all are applicable to gum printing:
Whether a color is classified as "staining" or "lifting" ("lifting" refers to whether a color can be rewet and removed from the paper once it's dry; staining of course refers to whether a color leaves an indelible stain in the paper) isn't an issue in gum printing, because with the amount of gum arabic that we add to the paint, staining, at least staining as an inherent quality of a particular pigment, isn't an issue. All pigments in gum printing "lift." at printing strength, as an important feature of the process is that when the dried and exposed print is put in water, the pigment lifts off of areas where the gum is soluble. Again, the reason this isn't an issue in gum printing is because of the extra gum that we add to the paint.
Below I've taken two "staining" pigments, PR 175 and pthalo blue, probably the two most staining pigments that I have, to illustrate this point.. On the left are two swatches of each, mixed to printing strength with gum arabic. On the right are two swatches of each, taken straight from the tube. I attempted to wipe away the bottom half of each of these swatches; the left swatch of each pair was wiped immediately after brushing the paint onto the paper; the right swatch of each pair was allowed to dry, then brushed and scrubbed with water and wiped dry. So this demonstration addresses both the issues of staining and lifting.
The pigment mixed with gum to printing strength wiped away cleanly in both conditions (the bits of color remaining are not from stain, but from sloppy wiping) but the paint that wasn't mixed with additional gum stained badly, as the swatches on the right show clearly.. And to make it clear that it's the amount of gum, not the amount of pigment that's the factor in the staining, notice the halo of stain at the bottom of the far right swatch of each color. The water I used to rewet the dry paint, in each case, picked up some of the pigment and carried it outside the main swatch, creating a light tint of color, which immediately stained the paper and couldn't be scrubbed away. The jpegs don't show the stain very clearly, since it's fairly light, but believe me, it's there.
The staining is not a function of the inherent strength of the pigment, (more about strength and weakness of pigments below) as pthalo is a very powerful pigment and PR 175 is much weaker (it takes very little of the pthalo and quite a lot of the PR 175 in say 20 ml of gum, to make a mix that prints well). So the staining isn't from the strength of the pigments, it's from the staining property of the pigment, but as I've just demonstrated, this staining property of certain pigments is a non-issue for gum printing.
You may also find pigments classified into categories depending on how they are made: natural vs synthetic and organic vs inorganic. These distinctions are purely academic and have no importance to gum printing in practice.
Some pigments tend to settle out when used wet-in-wet or mixed with certain other pigments, creating a granular effect which can be very attractive in a watercolor painting. This characteristic is largely irrelevant to gum printing, but not entirely. A gum printer who knows her pigments can take advantage of the settling quality of such pigments to get some interesting effects, especially by choosing judicious mixtures of pigments.
The first considerations in choosing pigments, if you're interested in making prints that will last for generations, are permanence and particularly lightfastness. It's best to get lightfastness ratings from an independent source rather than from the manufacturer, although some manufacturers are better than others about giving accurate lightfastness ratings. I trust the ratings given by Daniel Smith and M. Graham; I do not trust the ratings given by Winsor & Newton. You also need to be aware that Winsor & Newton offers many fugitive pigments that most paint manufacturers no longer carry. The purpose in doing this is to make old-style pigments available to those who want to continue to use them, but it's important to understand that these pigments aren't lightfast and that more permanent colors are available in the same color ranges.
Most responsible rating systems for lightfastness have four or five points on the scale. I am a stickler for permanence and use only pigments or paints rated absolutely lightfast by a source I trust. Others will have different cutoff points for how much leeway they are willing to give a pigment. A person who uses a pigment rated IV on a four-point scale or V on a five-point scale should do so in full knowledge that they are using a fugitive pigment. (And please note that I don't attach any moral significance to using fugitive pigments; if that's what you want to do, be my guest. My goal is simply to encourage informed choices.)
Winsor & Newton, probably not alone among manufacturers, obscures the fugitive nature of some of the pigments it offers by giving lightfastness ratings on a three point scale, labeled "Extremely lightfast" "Durable" and "Moderately Durable." The lowest rating a fugitive pigment can receive in their scheme, in other words, is "moderately durable." I'll just suggest that you would serve yourself and the longevity of your prints better by going to independent sources for lightfastness ratings of Winsor & Newton's paints than by relying on their rating system.
And if you really want to be sure, the very best way is to run your own fading tests on the paints that you use, but even as particular as I am, I've been content to take the word of sources I trust on the matter of lightfastness.
A couple of fugitive pigments to watch out for (note this is not an exhaustive list by any means):
Alizarin Crimson, PR 83. This pigment is still offered by many manufacturers because artists are attached to it and don't want to give it up, but it is notoriously fugitive. The quinacridone reds and crimsons are quite lightfast and transparent and make good substitutes; there is no reason to use this fugitive pigment. (I use a lightfast transparent scarlet pigment PR175, of which more later). And if there's any doubt that Winsor & Newton is quite aware that Alizarin Crimson (PR83) is a fugitive pigment, even though they rate it "moderately durable," consider the fact that in addition to "Alizarin Crimson" made with this pigment, they also offer a paint called "Permanent Alizarin Crimson" made of a mix of two quinacridone pigments.
Indigo, PB 66. There was an earlier pigment called Indigo, which has never had an official pigment number or name since it's never been used in modern paint or dye manufacture, although it can still be obtained as a powdered pigment as "genuine indigo" which is also fugitive. PB66, a synthetic indigo, has been the pigment officially designated Indigo since sometime in the 19th century. This synthetic indigo, like its predecessor, (which it was advertised to be more permanent than) fades like crazy (think blue jeans) and shouldn't be used in any work you want to endure. Only a couple of manufacturers, Grumbacher and Schminke, still offer PB66. All other paints labeled "Indigo" are convenience mixtures of more permanent blues (pthalo, ultramarine, indanthrone) with lamp black.
Gamboge, NY24. Winsor & Newton is the only manufacturer that still uses this fugitive pigment, in its "Genuine Gamboge." All other paints named "Gamboge" use more lightfast pigments.
Chrome Yellow, PY34 "A most unreliable color that can become very dark on exposure to light or to the atmosphere." (Wilcox) has recently been discontinued in all lines, including Winsor & Newton, probably because of its toxic lead content rather than its impermanence.
Various madders, lakes, etc. Most if not all of the pigments that have the words "madder" or "lake" in their names are less than permanent, but one shouldn't go by the paint name, as some manufacturers actually package more permanent pigments with the names of fugitive pigments, a practice whose logic escapes me.
Watercolor books talk about a characteristic called "tinting strength," by which they mean whether a pigment overpowers other colors in mixtures. For example, if you mix even a very small amount of pthalo blue with another color, the mixture will tend toward blue because the pthalo is such a powerful pigment.
This quality is important to gum printers because pigment strength determines how much pigment is needed in a given amount of gum to achieve a given tonal depth, so understanding pigment strength is the single most important key to getting the right pigment concentration for printing a particular pigment. (There are also smaller variations within the same pigment, because different manufacturers use different amounts of pigment per tube, but here I'm talking only about the inherent strength of a given pigment relative to other pigments).
Lamp black is the strongest pigment I know; a very little of it goes a very long way. The weakest pigment I've seen is called Natural Green Earth or Terre Verte (PG 23); it's marketed by Daniel Smith as "Bohemian Green Earth." I had to put (literally) half a tube of it into 15 ml of gum to get this much tone:
In other words, there are some pigments that are so weak that they will never deliver much tonal depth. Many people would say that such weak pigments are of no use for gum printing; I would say that it depends on the goal of the printer whether a particular pigment is useful or not. The point is to understand the characteristics of pigments and use the ones that are most appropriate to your goals.