additive or subtractive ?

hi

i was thinking of trying tri color gum prints but i am a little confused and was hoping people who know what they are doing can answer a simple question for me. before i ask outright, i have made color separation negatives with RGB filters before, and stacked them in PS because i don't do color printing &c and gotten faux nouveau tri chromatic images so i am not a stranger to RGB filtration &c ... but here is my question ... do i use CMY layers of gum or do i use RGB layers of gum ?
i get so confused about this stuff. thinking out loud i am thinking i use RGB negatives and because CYM is the negative of RGB i use CMY, otherwise i will get a negative color image..

hope my thinking is right cause as you can tell i am about 99% guessing and 1 % clueless

john

Some people use the RGB separations but I have never tried it because the CMYK color system is the color system used for printing (subtractive). In painting primary colors are Red/Blue/yellow - one may try to make Red/Blue/yellow separation, as well, because the colors used in gum printing are not different of the colors in painting (watercolors).

The negative for each color layer is a B&W negative illuminated with the same UV light. It means that it makes no sense, IMO, to think in terms of CMY as the negative of RGB. This makes sense in the case of traditional color printing where the components of each color are sensitive to their, different, color of light. The components of the gum printing layers are all sensitive to the same UV light. They are, essentially, three or four monochrome layers - CMY or CMYK and you make the negatives for each of them as you make a negative for a B&W print. No inversion of color is needed.
Think of it this way: if you want to make a print of the Cyan layer, for instance, you will separate Cyan in the image and make a B&W negative for it, not for its opposite color, because it is, essentially, a B&W print (...well, monochrome).

However, using only CMY won't give you deep blacks and that's why I'd recommend a black layer, too. An initial palladium print as a black layer would give very nice results. But you may also like the less contrasty look of the CMY layers, without black. It's a matter of personal taste.

Regards,
Dan

Try the test of converting cmy or rgb & you will see that the info on the cmy is much better for details/tones & colour density in the B&W pos/neg.
As Dan mentioned when printing pigments cmy is the correct way (subtractive).

jnantz said:

do i use CMY layers of gum or do i use RGB layers of gum ?

Click to expand...

On printed media you always use subtractive, so cmy or optionally cmyk for laying down the layers. You simply can't have additive filtering on a reflective medium.

thanks for your helps !

Traditionally, you'll use RGB filters and CMY pigments. Thought, and especially with gum, you can do it however you want. It's not uncommon for gum printers to substitute red for magenta or blue for cyan and still get pretty good results. You can even make color photographs by using red and green filters and pigments (just use the red filter to print the green layer and vice versa). This was done with some of the first color movies, like the chariot scene in the original Ben Hurr. It's also possible to use CMY filters and RGB pigments. As long as the filter is the opposite on the color wheel of the printed pigment (or close) it'll work to some degree. And it helps if the all of the filters and pigments are evenly spaced along the color wheel, maximizing their efficiency.

The reason RGB and CMYK are used most often is biological. The human eye has red, green, and blue receptive cones. So those are the only colors we are sensitive to. All other colors we see are combinations of those. Cyan and yellow are the opposites of red and blue respectively. Green is in the middle of the visible color spectrum, and magenta is how our brains interpret a mixture of the extreme ends of the spectrum. It acts as greens opposite in our minds, though it's not a "real" color. That's why monitors project in RGB, and printing is done in CMYK. Black is just used to give the image more depth, and can be created by printing an unfiltered negative at a reduced density.

So the RGB/CMYK method has become the standard, not because it's the only one that works, but because to our eyes, it's the one that works most efficiently. Though there are better combinations, like CMYKOGV printing that can provide a wider color gamut, but they require special software to integrate properly, and can't be easily done with normal filters and standard software. They don't overlap linearly, and require special curves to take full advantage of the extended color gamut.