A response
I've been locked out of this thread since soon after my last post a couple of days ago, because I accidentally clicked on "ignore thread" when I meant to click on "ignore user." (I don't choose to continue reading posts from persons whose purpose seems to be to insult and inflame, rather than to have a dialogue with fellow forum members). But since a few reasonable people here seem to be taking this troll seriously, I think it's only fair that I should correct some of the more inaccurate pronouncements that were hurled in my direction, and since the software is suddenly allowing me into the "post reply" page for the thread, I shall try to do so as briefly as possible, just addressing the more important points that still stick in my mind from a couple of days ago and letting the sillier ones go:
1. Repeatability: Somehow it was believed that I had said that tricolor gum, or maybe just my tricolor process in particular, is not controllable or repeatable. I never said that; it's not so.
2. Accurate colors with tricolor gum: I wrote that with tricolor gum it is easy to make a color print in which the colors are read as "accurate enough" by the mind, but difficult to make a color print in which every color in the original is exactly and precisely replicated in the print. This is even more true of cmyk reproduction than of tricolor gum, so the advice to go to cmyk printing for answers doesn't provide any useful solution. As most of us know, there are many colors that can't be printed in a standard cmyk printing, because they are "out of gamut." The cmyk reproduction is read by the mind as a fairly accurate representation of the colors in the original, but all you have to do to see how far off it is, is to put the original up next to the reproduction. When I spoke of it being difficult to reproduce every color precisely in tricolor gum, I was talking about levels of accuracy way beyond what the usual cmyk reproduction is capable of. z-man recommended following the press practice of making as many as ten negatives (and ten press runs) to achieve accurate colors. I could be wrong, but I doubt there are many publishers of books and posters who could afford all those press runs, and at any rate, in tricolor gum you could probably do it with two more colors beyond the original three (making it hexacolor gum rather than tricolor gum) if someone wanted to be that precise about replicating every color exactly accurately.
3. Confusing RGB with CMYK: This is something I've never done. Perhaps the listing on my table of contents, "How RGB channels become CMY separations when inverted" may have been misinterpreted by someone determined to misinterpret, but if a person had actually clicked on that link and followed the numbers, you would see exactly how it works. As anyone knows who has ever printed color photographs in the darkroom (or adjusted color curves in photoshop), RGB and CMY exist in a complementary relationship with each other. The more red, the less cyan and vice versa. The same with the blue-yellow continuum and the green-magenta continuum. When RGB channels are inverted, they become negatives that accurately print CMY. People don't believe that until they follow the numbers through, (or just print the separations) but it's true. But to point out that observable fact is hardly to confuse RGB with CMY.
4. Photoflood: I was treated to a hail of ridicule because I print with a photoflood bulb. z-man made a point of saying, as a way of insulting me, that he can read, but he apparently didn't read my page on why I use the photoflood. I don't use the photoflood because I'm too stupid to know any better, but because it has proved to be a very good light for printing gum. I started out with it because it was a cheap and easy light source, and I wasn't sure before starting that I was going to stick with gum, so I didn't want to invest a lot of money into startup. That was more than 20 years ago, and I still don't see any reason to use anything else.
Among the many contemptuous remarks that were made on this subject, it was said that if I would get some other light, my printing times would be more like 10-15 minutes. Why would I want printing times of 10-15 minutes, when my printing times with the photoflood are 2-5 minutes? And as for the spectrum, there's very little known about the actual spectral sensitivity of different dichromated colloid emulsions. The conventional wisdom is that their sensitivities are all the same, because it's assumed that it's the dichromate that determines the spectral sensitivity. But when the spectral sensitivity of various dichromated emulsions has actually been compared, it's been shown that they aren't the same, that the colloid itself has a significant effect on the spectral sensitivity, and that the spectral sensitivity of gum is more in the visible range than in the uv range. So for gum, it may not matter that there's not much uv from the photoflood.
At any rate, I've always believed in letting the gum decide; in other words, I prefer to let observation rather than theory (or ridicule!) drive my decisions, and the photoflood works very well for gum. I don't have a lot of the problems that many other gum printers report (dichromate stain, tonal inversions, low contrast etc) and I suspect that the reason for that may be the light source I use.
5. Pigments. It was said that I should order yellow, cyan and magenta process paints from Jerry's Art-O-Rama rather than using the more lightfast pigments that are available in good artist's lines. Daniel Smith (my art supplier) doesn't even carry such process paints, and if they did, I wouldn't buy them. Generally speaking, such process paints are made of inferior pigments and aren't lightfast, and besides, there are pigments available that are close enough to true yellow, cyan and magenta, that they serve quite well, even if they fail to satisfy the literal-minded by going by those exact names.
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Additional thoughts in response to comments by PE (since I don't trust the system to let me reply more than once to a thread that I've clicked "ignore thread" for, I'll put all my responses here):
Yes, I agree that you have to look at the colors as well as whether the three colors produce neutral greys, but you have to start somewhere. Once you've got a neutral grey when the three colors are overlaid, then you can proceed to determining how well a combination of pigments reproduces individual colors, or color ranges. Generally, in my experience, if a combination of pigments reproduces green well, then it won't do so well with the purples, and so on, and that's why it may be necessary to introduce some secondary colors for precisely accurate color reproduction, as recommended by Bruce MacEvoy, the man I consult for expertise on pigments.
You mentioned that you were surprised at how a CMYK print with the K layer left off looks, but it shouldn't be surprising because it's a function of the separations and of the printing inks. Since, as I explained in the page I referred to earlier in this thread, the color information is altered to accommodate the black and the limitations of the printing inks, the CMY parts of a CMYK file aren't the same as a true CMY file generated by inverting the RGB file. Gum printers sometimes make this mistake, thinking that by choosing CMYK in photoshop and not using the K printer, they are getting true CMY separations, and are surprised at how wimpy and offcolor the result is. I don't know, but strongly suspect, that that's what's wrong with the tricolor gum prints on one of the links supplied way way earlier in this thread, where CMY is compared to CMYK: the CMY prints look very strange,, probably because the CMY prints aren't the result of a straightforward conversion to CMY, but are the CMY portion of (default) photoshop CMYK, which is a horse of a different color altogether.
Your point about the density required for true black is well taken.
Thanks for the opportunity to respond; as far as I know the thread is still unavailable to me so I won't be participating further, but just wanted to clear up some of the misconceptions I did see before I locked myself out of the thread.
Katharine