U of S | Mailing List Archive | alt-photo-process-l | Re: separations for gum printing

Re: separations for gum printing

Maris, the thing about default CMYK, even with K set to zero, is
that it alters the color values to fit the requirements and
limitations of commercial printing processes with high speed presses
on slick coated paper, with inks especially formulated for those
papers. How the separations deal with these requirements and
limitations, in general, is to cut back on magenta and yellow in the
separations while leaving the cyan at about the same value. This is
how the separations have to be tailored to make the commercial SWOP
process inks print "right," but it doesn't work for gum very well at

If you go to this page


and scroll down past all the text-heavy explanations of the default
Photoshop CMYK settings, down to the last visual on the page,
there's a demonstration visual I made by printing a neutral grey
patch on gum from (a) RGB-CMY separations (separations made by
inverting RGB directly to CMY -- if anyone is unclear on how that
works I have another page on that here


and (b) CMY(no K) separations. You can see, both from the color
values in the file and from the printed gum patches themselves, that
while the RGB separations print a reasonably neutral grey patch, the
CMY separations with k set to zero print a heavily cyan biased patch.

By the way, I find it curious that people say they can't make heads
or tails of RGB separations, they all look the same, but CMY(K)
separations seem more intuitive. I'd say exactly the opposite, and I
think it's probably more what you're used to. To me, RGB separations
are straightforward but CMYK separations don't make sense. :--)

I enjoy seeing how people tailor separations to their own needs and
desires; I'm not arguing that you shouldn't print CMY(no k)
separations, but just pointing out how they are calibrated in such a
way that they don't produce a neutral color balance when printing in
gum with balanced pigments. So if you're going to use them, you
might want to cut back on the concentration of your cyan pigment to
match. To me, it just makes more sense to print with the straight
CMY separations you get from inverting RGB, that way I know that I'm
working with the color values straight from the original file, not
distorted to meet the needs of some printing process that has nothing
to do with gum printing, but if you like the CMY(K) separations
better, you may want to read through my discussion of the default
settings and see where you could change the settings. It's been
years since I've spent a lot of time with that, and I might not be
remembering this right, but to the best of my recall, I couldn't find
a way to change the settings enough to get the values back to
straight CMY. (And if you could, of course, then they would be
identical to the RGB-CMY separations that you don't like).

On Nov 22, 2008, at 6:32 AM, sam wang wrote:

Hi Loris,

In CMYK, the black (K) is doing a lot of definition in the mid to
lower tone areas. Part of the reasons for using CMYK is to avoid
too much ink pileup in very rapid press runs. That's why in order
to use it, it is important to specify exact requirement for your
particular needs, otherwise leaving it at default may be fine for
the run-of-the-mill offset, but way off for your alt.

To see the differences, you might duplicate an RGB file and convert
the dupe to CMYK. Then duplicate both of these and invert them to
negative. Then compare the channels of each file and the
differences would be very evident. (By the way, your example looks
like you inverted it to negative BEFORE converting to CMYK. Correct?)

Back to Marek's original question - RGB is the "quick and dirty"
way to separate colors. CMYK with K set to 0 may be better for some
but likely it would require some tweaking - it's another case that
there's no sure way for everyone or every situation. (Would love to
see some resulting gums when you have them, Marek.)


On Nov 21, 2008, at 7:03 PM, Loris Medici wrote:

See the comparison here:

(Curved negatives from RGB and CMY K=0 separations.)


21 Kasım 2008, Cuma, 11:09 pm tarihinde, Loris Medici yazmış:

Hi Marek,

I first planned to start to make color gums by CMYK separations -
> because
it seemed natural in context of "printing" + because it was the
used by Keith. (I had not done anything serious with gum other
than few
step tablet tests before participating his workshop...) Anyway, I
the idea and started with RGB separations because it required
only three
negatives, in other words less costly (1. both negative cost + less
layers) (2. yes K=0 also requires three negatives, but I wasn't
considering this at that time, and probably would not in the
future, if I
wasn't reading your message).

I have an image which is currently giving me a hard time; I get
highlights and midtones but muddy / lifeless / flat shadows. I
tried to do
CMY K=0 separations and when I inspect the negatives on screen I
see that
it would print even worse, because there's no useful detail in the
shadows. The RGB separations show good texture in the shadows,
compared to
the CMY separations. How come?


P.S. I'm at my mother's house now, will post the comparison
images later
and ask for your comments.

20 Kasım 2008, Perşembe, 8:52 pm tarihinde, Marek Matusz yazmış:

Since CMYK space and separations are intended for printmaking,
which is
what we do in gum I started making CMY separations with K set to
There is a visual difference on how the pictures look on screen
in these
two color spaces (anyone interested should try to toggle between
Photosop) and also how the Y separation looks compared to B
channnel in
RGB color space. It also seems like the colors are less muddy. I
done a few pictures so far, but I want to see if anybody is doing
different methods of generating separations for tricolor gums.