U of S | Mailing List Archive | alt-photo-process-l | Re: Gum Color Gamut & Beyond CMYK?

Re: Gum Color Gamut & Beyond CMYK?

On Sep 21, 2008, at 9:41 AM, Dirk-Jan Treffers wrote:

hi guys..

I'm just wondering: If you're gonna use six layers, you're gonna need 6 different negatives as wel, right? Is there any (cheap) way of doing this, without having to buy programms like QuakrXpress, or having every digital image converted for me by my local printer?

Is it for example an idea to use photoshop, and not only make RGB separation for cyaan, magenta and yellow, but also CMYK separations for Red, green and blue (and if you like a 7th layer (makes you go to heaven ;-) ?) a black layer? Or is this totally wrong of me to think this way?
Umm.... yes, sorry. RGB and CMY separations both print cyan, magenta and yellow (although the default CMYK space in Photoshop doesn't do this well for our purposes; the straight RGB inversion is a much more straightforward way of converting RGB to CMY that doesn't lose color information, as the default CMYK conversion does). But at any rate, it's not CMY plus RGB that are the six colors, but CMY plus orange, purple, green. The flickr example used Hexachrome separations --as per Jacek's suggestion-- to make the separations for the secondary colors orange and green. I'm not sure how (and am not terribly interested in trying to figure out how) you would do the separations any other way, or how you would make a separation for purple. My understanding from Bruce McEvoy's explanation is that because of the way the practical mixing wheel differs from the theoretical color wheel, it's much easier to make vibrant oranges from three primary colors than to make vibrant purples or greens (although in my experience you can do both fairly easily with judicious choice of pigments) so it seems like you'd need purple as a secondary more than you'd need orange. But since I've never actually done this six-color thing, only read about it, I can't speak from my own observations on this.

To tell you the truth, I don't know how you'd make the

kind regards,

2008/9/21 Katharine Thayer <kthayer@pacifier.com>
Just a clarification: since this was a response to my post rather than to Jacek's or Loris's, I should perhaps emphasize that this isn't exactly what I (per Bruce McEvoy) am talking about on my tricolor page. What McEvoy means by using six colors to produce the secondaries and tertiaries accurately is the three primaries plus the three secondaries. What the fellow on flickr, and Jacek, mean by a six-color process is CMYK plus two secondaries. Not the same thing.

On Sep 20, 2008, at 11:16 PM, davidhatton@totalise.co.uk wrote:


Take a look at this photostream in flickr


It's exaccerlly what you're talking about


David H

On Sep 21 2008, Katharine Thayer wrote:

See my page on tricolor gum printing


for a brief consideration of this topic (scroll down the page to the
subheading, "Is it possible to reproduce the entire spectrum using
three color layers?" )

The answer, from Bruce McEvoy, is no, you do need to add the
secondary colors (in other words, in gum printing, use six layers
including the secondary colors in addition to the primary colors, in
order to produce accurate colors throughout the spectrum, and I
suppose some sort of profiles as Jacek suggests might be the way to
generate the color separations for the secondary colors. But this
seems to me an enormous effort and possibly a misplaced effort to
boot; if you're looking for this kind of color precision, gum may not
be the right process for you. Not that I would discourage anyone
from dedicating themselves to this kind of study if it appeals to
them, just that for me and my house, I don't quite see the point.

However, the purpose of the extra colors and separations, as I
understand it, is not to extend the gamut so much as to refine it.
Depending on the pigments you choose as primaries, the layered color
mixtures will produce different palettes of greens, oranges and
purples; generally when printing tricolor we pick three primaries
that give us the kind of secondary mixtures we want. For example, I
generally prefer ultramarine or prussian blue for tricolor because I
don't like the greens that pthalo produces with almost any yellow,
they look unnatural to me, and since I print a lot of landscapes with
trees/leaves, I want a more natural looking palette in the greens.
Adding the secondary colors to produce six layers will theoretically
make it less necessary to pick and choose primary pigments to produce
the kind of secondaries you want, because you can choose the
secondaries to your liking. But like I said, this seems a heck of a
lot of work, when three colors, chosen carefully, will almost always
give us a palette that gives us a reasonable enough approximation of
realistic color, with special attention to producing the secondaries
that we're most interested in.


On Sep 20, 2008, at 9:11 PM, Jacek Gonsalves wrote:

> Hi all,
> Just continuing what Loris was talking about in regards with using
> a predefined set of pigments and building a profile for Gum
> printing. If one could build the profiles based on the CMYK
> pigments, can we say that this holds a certain color gamut. Lets
> say we want to increase the color gamut?
> There are other methods as Pantone Hexachrome method of 6 color
> separations CMYKOG, adds orange and green separation. Adding these
> additional separation would therefore increase the color gamut.
> That is if the image you have includes those additional colors.
> I'm sure there are methods to make additional separations in PS,
> though it might be complicated? If someone knows how to do this
> please chime in..
> Though it excites me to think that one could increase the color
> gamut from finding what pigments fall further than the usual inkjet
> printers do these days.
> Regards
> Jacek