Re: GCR and UCR

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.

I'm not sure it's as simple as impurities in printing inks (or, for that
matter, the purity of gum pigments, where "purity" means faithfulness in
upholding one third of a tricolor scheme). If pigments merely absorbed
some wavelengths, let the rest through to reflect off the white base, and
filtered the light again on the way to the viewer's eye, there might be
something to this. But they don't. Pigments *reflect* light on their own,
so the final visual effect depends on both the color layers' *transmission*
and *reflectance*. Whatever the reason for not printing yellow last in
lithography, this is the reason it is not usually done in tricolor pigment
printing -- yellow has the highest *reflectivity* of the pigments, and
consequently limits visual print density where it is applied. [I realize
I'm mixing two threads of this discussion -- order of printing the layers
and reasons (or not) for substituting tricolor neutral with black -- but
they are interrelated.] Try this thought experiment: Put down a nice black
pigment layer on paper. Now do anything you please with yellow pigment on
top of the black, and the end result will always be less dense
[reflectively] than the original black layer. This will be true even if
the black layer is deep, deep blue (i.e., deep cyan plus deep magenta).
The yellow will do some amount of color correction to remove the blue cast,
but it will also *lower* the net reflected density, not raise it like
adding density should in theory.

Dyes do not suffer as badly from this, but even so I always found that I
could make better dye transfer prints with a black layer. (In that case,
the K layer was not a total substitute for the neutral portion of the
colors, but rather a very light touch to kill the slight reflectance of the
color dyes.) [Note that using transparent dyes in a diffusion process
doesn't really give you layers the way gum (or even C-prints) are layered
-- the gelatine ends up with a relatively homogeneous mixture of dyes, as
if you mixed C, M, and Y food coloring into liquid gelatin and then let it
set.]

Now, having said all that, I'm certainly NOT suggesting that it is
impossible to get good results with no K layer or by printing yellow last.
As Kirpal Singh so often pointed out, there are many, many paths to
enlightenment -- although some are generally easier than others. One thing
we can say about printing yellow last is that D-max will not be as high as
it could have been with the same materials printed in a different order.
But if maximizing D-max is not necessary for an image, this won't be a
problem.

Best regards,

etienne
Received on Tue Sep 14 20:36:35 2004