Here's an understatement: dye transfer printing is time-consuming and expensive. Making the first dye print from a negative costs me over $100 in materials and several days' time, and demands extraordinary skill, understanding, and good artistic judgment. It's no wonder that few printers make dye transfers.
A dye transfer picture is not created chemically in paper; it's more analogous to the mechanical printing process that magazines use. Instead of plates engraved with a screened halftone image for each of the colors, though, dye transfer uses three continuous-tone sheet film plates called matrices. The matrices are soaked in water-based cyan, magenta and yellow dyes. The matrices are rinsed clean of excess dye and squeegeed against a sheet of gelatin-coated paper, much like regular photographic paper but without the silver compounds. The gelatin absorbs the dye from the matrix. The result is a continuous-tone dye photograph on paper.
The details are a wee bit more involved...
First, I need to separate out each of the individual color images that comprise a color negative. I use red, green and blue filters to enlarge the individual color negative images onto sheets of Pan Matrix Film. I process that to produce a relief image: the thickness of gelatin left on the film base after processing is proportional to the amount of light that hit the film.
Now I can work by room light. I soak the matrices in "acid-fixing" dyes. Gelatin will absorb just so much acid-fixing dye out of the dye bath. That's what makes dye transfer work. The thickness of the matrix relief image is proportional to the original exposure, so the amount of dye picked up by the matrix is also proportional. More light means more gelatin, which means more dye in the print.
The red-, green- and blue-exposed matrices go into cyan, magenta and yellow dye baths, respectively. I can adjust the contrast of each dye image by changing the acidity of that dye bath. I have sets of different contrast dyes, much as black-and-white printers have different grades of paper. Sometimes, I even use different contrasts for the different matrices to correct for some unusual color balance problem, like aerial photographs that are too blue in the shadows.
The matrices absorb all the dye they can in five to 10 minutes. I rinse each matrix in 1% acetic acid to remove the excess dye from the surface; because the dyes are fixed (held in place) by acid, they don't wash out of the gelatin. Incidentally, the large amounts of dilute acetic acid used in the process leave the prints smelling faintly of vinegar for months-to-years after printing.
I take the first rinsed matrix and use a roller squeegee to press it emulsion-to-emulsion against a sheet of dye transfer receiving paper. The dye molecules transfer to the paper's emulsion. After 5–10 minutes, I peel off the matrix; I now have that primary color's dye image in the receiving page. I wash the matrix in hot water, after which I can dry it or dye it for another print. I repeat this for all three matrices. Voila, a full-color dye transfer print.
It probably won't be right. It'll be too light or dark, too contrasty or flat, and/or off-color. The shadows may not have the right amount of detail, or the highlights may need a little more sparkle. Or etc., etc.
Before Photoshop, we had dye transfer for the ultimate print control. Dye transfer printing provides me with a vast (even daunting) number of ways to control the appearance of the print. I can use masks when making the matrices to adjust the color balance, color rendition and tone scale of the print. I can control the density and contrast of each matrix individually during exposure and processing. I can adjust the acidity of the dye baths to change the image contrast and density (which looks different from changing the matrix contrast).
I can lighten or darken an image by changing the composition of the rinse baths; I can even add dye selectively to shadows or remove it from only the highlights. I can make additional transfers from each matrix to fine-tune the image. Each primary color image adjusts completely independently of the others, using any combination of these tools. Learning how to judge what corrections will take me from a first print to that perfect final one is what's really demanding about dye transfer.
Of course, my technique must be meticulous. If I don't process the matrices with perfect evenness, I get uncontrollable color shifts and blotches. Wet matrices are fragile beyond belief and demand careful handling. If I roll a print too lightly, I'll get dye bleeding and patchy, unsharp prints. If I roll too hard, I'll stretch the paper and get color fringes. Any air bubbles or dust between the matrix and paper prevent the dye from transferring at those spots, making for much retouching and spotting later. I must do everything right while handling large sheets of wet, floppy film and paper.
But, it's like getting to Carnegie Hall. Thirty years of practice, practice, practice and I'm there! Believe me: the results are worth all the effort.