Although many gum printers believe that a simple conversion in Photoshop to CMYK gives you a straightforward conversion from RGB to CMY plus added black for extra density, this is simply not the case. Many teachers and printed sources (James, for example) have compounded this misconception by recommending this seemingly simple route to color separations. It's true that if there were a way to get a CMYK file with all the settings turned off, then the separations generated from a CMYK file should be exactly the same as the separations generated from the RGB file. However, the whole purpose of CMYK is to change the color values to accommodate the requirements and limitations of printing inks, papers and presses or other printing devices, and the default CMYK space is no exception.
Note: My comments on this page refer only to this default CMYK space in Photoshop, and not to any other CMYK spaces or profiles that gum printers may have developed or adapted for their own use. There are people who are very sophisticated with Photoshop and who have developed their own CMYK spaces to make separations for gum, and more power to them. This page is not directed toward those more sophisticated CMYK users but to those who just want quick and easy separations for printing gum, to help them think about whether the default CMYK space really makes sense for gum printers. To me, it doesn't, but this should not be interpreted to mean that I'm saying that it shouldn't make sense to anyone. My purpose here is to tell what I do and why it makes sense to me, not to set out rules for all gum printers.
First of all, what is CMYK? CMYK is a family of color spaces that have been developed and are continually being developed to deal with specific problems in printing color images with mechanical devices using specific inks and specific hardware. Every time a new consumer digital printer or ink is developed, a new profile, which is another name for a CMYK space, is developed to optimize its performance.
Which leads to the question: what is the default CMYK space in Photoshop? The default CMYK space in Photoshop was designed (quoting the Photoshop manual) "to produce quality separations for web offset printing presses using SWOP inks on coated papers. " Let's take the default CMYK settings one at a time:
(1 ) Ink Colors: SWOP(Coated). The SWOP (Specifications for Web-Offset Publications) profile specifies inks that are formulated to work well with high-speed web-offset printing presses, printing 20,000-60,000 impressions per hour; heat-set drying of inks after printing; on clay-coated pulp-based paper; 133 lpi halftone screens; and elliptical halftone dots. How relevant is this to gum printing?
The idea that three primary colors can't add to black seems to have drifted over from commercial printing, where, according to my Photoshop manual, "due to impurities present in all printing inks a mix of these colors yields a muddy brown. To compensate for this deficiency in the color separations, the default separations remove some cyan, magenta, and yellow and add black ink." The default CMYK space, in other words, which is set up to adapt to the limitations of SWOP printing inks, alters the color values in the file and adds black to compensate for the fact that these inks can't do what pure pigments can do. By looking at the grey ramp under "black generation" with black set at "none" you can see exactly how much the color values are altered just for considerations involving the limitations of these printing inks alone.
"The color gamut of process (CMYK) inks has always been a problem. Poor purity of the cyan and magenta pigments is usually to blame for the shortcomings in the CMYK color gamut. Cyan is typically contaminated with yellow and magenta, for example, graying the color and giving it a dull muddy appearance. The amount of contamination ranges between 18-26% depending on brand and density. Magenta is contaminated with yellow, if the pigment is rhodamine, and blue if the pigment is rubine. It is common practice to mix them together to obtain a true process magenta. While the hue may improve, the resulting mix will be dull and muddy. ...Color printing inks also use extender pigments -- kaolin type clay."
The pigments used in printing inks for offset printing, in other words, are inferior in quality to the pigments we use in gum printing; for one thing they are notoriously fugitive in the presence of light, as anyone knows who has hung a poster in or near a window. The most commonly used pigment in magenta ink is lithol rubine (PR 57) a pigment not used in watercolor paints. The most commonly used process yellow is diarylide yellow (PY 12) again not used in watercolor paints. These pigments are cheap, inferior quality pigments, not lightfast or pure. The most commonly used pigment for process cyan nowadays, also as far as I can determine, is pthalo, but as explained in the reference above, both the cyan and the magenta that are commonly used contain significant amounts of contaminants. I suppose economic factors preclude the use of purer pigments.
(Coated) The default CMYK space is designed to print with process inks on clay-coated paper.. Since we don't as a rule print gum on such paper, it doesn't make sense to me to use separations that are designed to optimize the printing of process inks on such paper.
(3) Dot Gain: Standard, 20%. When ink is printed on paper, the dots of ink spread and will smoosh into each other if the separations aren't made in such a way as to minimize this problem. But we don't have "dot gain" in gum printing. The pigment is continuous, not in dots, and is held in dried gum during exposure and in hardened gum after exposure; the only time you might see a spread of pigment would be in development if the print were grossly underexposed, but that's a printing problem, totally unrelated to separations.
(4) Separation Type: GCR. There are two kinds of separations: GCR and UCR. To quote from the Photoshop manual again, "With UCR, black ink is used to replace portions of cyan, magenta and yellow ink in neutral areas only (areas with equal amounts of cyan, magenta and yellow). This results in less ink...because it uses less ink, UCR is generally used for newsprint and uncoated stock. With GCR, black ink is used to replace portions of cyan, magenta and yellow ink in colored areas as well as in neutral areas." Since the default CMYK space is designed to print process inks on coated stock, and GCR is designed for coated stock, it makes sense that GCR is the default separation option. But anyone using the default CMYK separations should be aware that this separation type significantly alters the color information in the file, and it alters the color information to adjust to a commercial printing operation that isn't much like the process of printing gum.
(5) Black Generation: Medium. If you pull down the CMYK settings dialogue box, you can see the grey ramp that shows how the color values are altered under this option. It's interesting to note that even if you change Black Generation to 0, the magenta and yellow curves are still altered, as with printing inks, you need more cyan than magenta and yellow to create a neutral grey; and the SWOP ink setting guarantees that the values for magenta and yellow will be less than the value for cyan in a neutral area. In other words, you can't get to a true CMY just by specifying None for the Black Generation setting.
(6, 7) Black Ink Limit: 100%, Total Ink Limit 300%. The ink limit is determined by the maximum density of ink that a particular press can support on a particular paper. The ink limits determine the cutoff points for the CMYK curves.. Thinking about this setting a bit may help you see how it is that when black is added, color must be removed to keep within the ink limits dictated by the press for that particular ink and paper. In gum printing we simply aren't limited in this same way by how much color can be laid down. There are limitations, due to sizing considerations and pigment staining and so forth, but why would we assume that these limitations operate in the same way as the limitations of wet ink laid on glossy paper by a high-speed printing press? It doesn't make any sense to me to use the color values designed to optimize the performance of commercial presses, commercial printing inks and commercial printing papers for our process which is very different.
(8) UCA Amount: 0%. If there were a number here, it would mean you were adding color back into neutral areas that color was taken out of in the service of inks, papers, ink limits, etc in the default space. The purpose of adding color back in is to "produce rich, dark shadows in areas that might appear flat if printed with only black ink." But as the default stands, with 0% UCA, it means that black only is used in those areas, and the colors won't print there, resulting in black only in certain areas.
Let's see what happens to the light green above when converted to this default CMYK space:
On the left, I've put the true CMY values as determined above by the inversion of the RGB file. Next is the CMYK values you get from converting to the default CMYK. The default CMYK space typically has about the same cyan value as the true CMY would call for, but the magenta and yellow are typically considerably altered. Whether this makes sense for gum printing is something that each gum printer must answer for himself.
Perhaps the simplest way to show the difference between the two kinds of separations is by generating both types of separations from a neutral grey color patch, printing these separations on gum, and comparing the results.
On the left is the original grey patch, a neutral 41% grey. The RGB separations were all the same: 59% density for C, 59% density for M, 59% density for Y. The center patch shows the result when these RGB separations were printed on gum. The swirly cloudy patterns in the printed grey are there because I used typing paper for the negatives, and the paper fibers printed on the gum. The gum print is a bit lighter than the original grey, and it has a bit of a pink cast, but it's at least recognizable as grey.
When the grey patch was converted to default CMYK, with no black generation, the color values were changed from the true CMY to optimize performance for SWOP inks. In other words, the CMYK instructions for this grey said to the printer: To get this particular neutral grey using SWOP process printing inks on a high-speed web-offset press you need to use more cyan than magenta and yellow, otherwise you won't get a neutral grey.
The rightmost patch shows what happens when I printed the CMY(no black) separations on gum. I printed the CMY separations from the same kind of negative on the same piece of Lana paper with the same gum coatings, the same exposures and the same development time, to hold everything constant for a pure comparison of the two kinds of separation. The densities of the CMYK separations were C=58%, M=68%, Y=69% K=0 in other words the straight inverse of the positive CMYK values. The point here is that even if you hold the black generation at "none" you still don't get the true CMY values (59, 59, 59) just by telling the CMYK default profile not to add any black. And, not to belabor the point I hope, if the magenta and yellow printers are denser than the cyan printer, then what should be a neutral color is going to print with a strong cyan bias.
This is why I prefer the straight RGB-CMY conversion, because it retains the exact color values that were in the original image, rather than altering them to serve the requirements and limitations of a commercial printing process that has nothing to do with gum printing. For a tutorial in how RGB changes to CMY when inverted, see this page.
Copyright Katharine Thayer, all rights reserved