Commercial printing vs gum printing

Hi All,
I've been too weary to read much of the intervening but am still
thinking about the stuff I went through so hurriedly the other day and
have a couple more comments:

wrleigh@att.net wrote:
>
>I'm just saying that if you want Black or Neutral Gray, you can't get >>there by just CMY, you need to use Black or Neutral Gray.

This just shows the fallacy of using a commercial printing model where
it's not relevant.

The reason why you can't (or at least can't without a lot of monkeying
around) get a neutral grey or black in commercial printing without
adding black or neutral grey, has to do with impurities in printing
inks, as I quoted from the Photoshop manual the other day, not with any
discrepancy between color theory and reality as it relates to color
photographic methods.

To quote another source, which unfortunately I didn't get the URL 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."

As near as I can determine, the most commonly used pigment for process
magenta is lithol rubine (PR 57) a pigment not used in watercolor
painting. The most commonly used process yellow is diarylide yellow (PY
12) again not used in watercolor paints. The most commonly used pigment
for process cyan nowdays, also as far as I can determine, is pthalo, but
as explained above, both the cyan and the magenta contain significant
amounts of contaminants. I daresay economic factors preclude the use of
purer pigments, and all the "color science" that Bill refers to in
commercial printing is an effort to push inferior materials to do what
they really can't do very well. And I also daresay that adding black is
a much cheaper way to deal with the muddy colors than buying purer
pigments.

We are not under these commercial constraints; we can buy very pure
pigments because we use small amounts, and furthermore the issues of
dot gain and trapping and out of gamut colors and all those things that
occupy printers are simply not issues that concern us.

I'm getting to my point, which is that it may be more useful to think
of gum printing as a branch of painting than as a branch of commercial
printing, if we have to attach it to something other than itself.

In painting, it is very easy to make a neutral grey or a neutral dark
that's quite black with three, or more commonly with two, colors.
Handprint lists a whole page of complements that mix to form a neutral
grey (chroma <2 as measured by spectrophotomoeter), and as a painter, I
never use black pigment or grey pigment but always make my neutrals by
mixing complements.

For tricolor gum printing, of course, we use not complements but
primaries, but the three primaries used with intelligence can always
produce neutral greys and a dark enough neutral black to look black to
most reasonable people. Where a muddy brown results from the tricolor
mix, it's due IMO to the choice of pigments or possibly in some cases to
an imbalance in the pigment saturation among the three colors, not to
any great discrepancy between color theory and reality, a discrepancy
that simply doesn't exist in gum printing to the extent that it exists
in commercial printing.

Katharine Thayer

>
>
>
> --
> Bill Leigh
> wrleigh@att.net
>
> >
> >
> > On Wed, 7 Jul 2004 wrleigh@att.net wrote:
> > > ... Printer
> > > manuafacturers spend millions to get the proper combination of colors to
> > > produce the proper results, and I find it extremely difficult to believe
> > > that someone working in their basement (or wherever) combining
> > > approximate amounts of paints with unknown amounts of specific pigments
> > > can achieve what the manuafacturers are unable to do with large expense.
> > > Colo printing is quite analagous to gm printing, in that the printer
> > > lays down 3 or 4 successive layers of colored ink or toner, expecting
> > > the combination will!
> > > yield the desired effect.
> > >
> > > Please post 2 instances of the same print, one of which was printed with
> > > lampblack and one with some combination of your choice of cyan, magenta,
> > > and yellow, which show that the color is identical. Also try scanning
> > > the print and see what Photoshop says is in each of the CMYK channels.
> >
> >
> > If we wanted color and effects identical to commercial materials, why
> > wouldn't we just do C-prints or similar "color photography"?
> >
> > Judy
Received on Wed Jul 14 10:28:07 2004