Re: GCR and UCR

Etienne Garbaux wrote:
>
> Katharine wrote:
>
> > [needing to replace tricolor neutral density with black density] is
> > only true for printing inks, due to the impurities they contain; it
> > has nothing to do with the pure pigments we use in gum printing.
>
> > I was simply pointing out what happens when you do that; what happens
> > doesn't make any sense for gum printing, was my point.

First of all, before I even go on to read the rest of this, a mild
objection to what seems to me an inexplicable linking of two unrelated
fragments from two completely different posts. The first fragment was
about the issue of whether you can get black by mixing three colors in
gum printing. The second was about how Photoshop shifts density from the
color printers to the black printer when creating CMYK separations. The
two have nothing to do with each other, and I can't imagine why they
were put together here.

>
> I'm not sure it's as simple as impurities in printing inks

To save time, I cut and pasted some things I quoted in an earlier
reiteration of this discussion in July:

[To quote the Photoshop manual: "In theory, equal parts of cyan, magenta
and yellow combine to subtract all light reflected from the paper and
create black. Due to impurities present in all printing inks, however, a
mix of these colors instead yields a muddy brown. To compensate for this
deficiency, printers remove some cyan, magenta and yellow and add
black."]

[To quote a different source about CMYK and printing inks, which
unfortunately I didn't get the reference for and can't find again: "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."

(or, for that
> matter, the purity of gum pigments, where "purity" means faithfulness in
> upholding one third of a tricolor scheme).

No, I didn't mean "purity" in the sense of being a pure hue, I meant
purity in the sense of being uncontaminated. although the two are
related, as the reference above suggests, for example the impurities in
the cyan muddying it with yellow and magenta.

This idea that it is impossible to make black with color seems to be one
of those deeply held dogmas that don't easily yield to reason, fact or
evidence, even though it is demonstrably true both in gum printing and
in painting. That's all I can say, and there's no point in going
around in circles about it. Carry on,
Katharine Thayer
Received on Tue Sep 14 23:16:16 2004