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

Re: separations for gum printing

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 ->
it seemed natural in context of "printing" + because it was the
used by Keith. (I had not done anything serious with gum other than
step tablet tests before participating his workshop...) Anyway, I
the idea and started with RGB separations because it required only
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
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
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.