Color Dye-Transfer Prints for the Modern Hobbyist - *Imbibition for For the People!*
HISTORY of the PROCESS
In 1994 Kodak discontinued the "Kodak Dye-Transfer Process". Since that time, only those who have hoarded those materials or have been enterprising enough to manufacture their own silver-halide matrix films (Jim Browning) have been able to make dye-transfer prints.
However, this is a curious & unfortunate state of affairs! The earliest dye-transfer (or dye imbibition) prints were made by exploiting the same mechanism that modern day carbon & gum printers use; the ability of potassium or ammonium dichromate to harden gelatin upon exposure to UV light.
The 2 most common ways to a make a dye-imbibition print both rely upon a gelatin matrix.
In recent times (Kodak) the preferred method was with a silver-halide emulsion on a clear base. A negative is exposed through the base and developed in a tanning developer which hardens the gelatin in situ with the developed silver. This is then "etched" in hot water, which removes the gelatin that remains untanned (soluble) to reveal a relief image that clings to the base. This gelatin relief consists of varying depths that correspond directly the shadow density in the photograph.
The other method is very similar, but instead of using a silver-halide emulsion, a gelatin layer is sensitized with dichromates and exposed (again, through the base) under a negative and etched in hot water. It is this variable gelatin relief that absorbs dye to varying degrees, thus continuous tone is possible.
An obvious difference between these 2 methods is that the silver-halide emulsion can be exposed under an enlarger whereas the DCG (dichromated gelatin) requires an enlarged negative for contact printing. This is one important reason for the success of the silver-halide method.
However, the use of DCG matrices has many precedents, most notably the Pinatype process; outlined here in this British Journal of Photography article from 1907. However, the first application of this property that gelatin will imbibe & transfer a dye was disclosed by Charles Cros, a Frenchman, who called it the Hydrotype process in the late 1800's. Another process was Jos Pe. It should be noted that the Pinatype uses a slightly different type of matrix (one that is not etched = planographic, as opposed to relief) and correspondingly uses a different class of dyes and is exposed under positives, interestingly enough. No need to go into this in depth at the moment however....
So, to make a dye-transfer print, a relief matrix is soaked in a bath of an acid-dye, the dye imbibes (or migrates) into the gelatin relief image and this is then rolled into contact with a sheet of receiving paper. The dye transfers from the matrix to the receiving paper.
If 3 matrices are prepared from color separation negatives, and dyed in the appropriate secondary dye colors (CMY), and finally transferred in register to the receiving paper, a full color image is formed. This is the process in a nutshell.
Although my work on this technique is far from complete, I want to at least disclose and compile all the information so far gathered, so that other people who are interested may join the effort.
- PREPARING THE MATRICES
First, the modern worker needs a suitable clear support. This is found in Photo Formulary's Estar Melinex (mispelled on their website as 'melenex'). This is a thin, clear support that is subbed to accept emulsions. This subbing cannot imbibe any dye itself, which is ideal becaue otherwise you will get stained highlights. Although I have not tested any other materials, I'm curious what else might be suitable. Overhead transparency paper perhaps? Whatever the substrate is, it must be treated so that it "grabs" the gelatin, otherwise it will easily peel off. Search "corona discharge" for more on this, or APUG member hrst's work with substrates. Fortunately, there is an existing product at the moment.
The melinex scratches easily, and care should be taken to keep it as unblemished as possible. Vaseline or some kind silicon oil (PE) might be able to hide the scratches when it comes time for exposure (remember, we're exposing through the base).
As a side note, another option would be to do a "carbon transfer" of the clear relief image (or pigmented relief image for that matter) and this could become the matrix. This would eliminate the need for exposing through the base and a special substrate for the matrix.
2nd, you must coat the melinex with a gelatin layer. This is effectively no different than pouring carbon tissues (sans pigment). I have chosen 6% gelatin with 1% sorbitol, though there is nothing yet to suggest that this is superior to any other concentration, and not enough testing has been done to see what effect the gelatin composition will have upon the matrix characteristics. This gelatin solution pours nicely and makes a thin coating; this has been my only criteria so far. Each worker will probaly find their preferred way to achieve this, just like in carbon pouring.
3rd, the matrix blank must be sensitized. Having very little experience with this personally, I can only say that it will be necessary to have the back surface clean of any splotches before exposure, and also the deepest layers of the gelatin must be sensitized. It has been suggested (Vaughn) that this is not really a concern.
Fortunately, the rigid densitometric requirements of traditional dye-transfer printing can likely be thrown out the window when using DCG matrices. Traditional AgX matrix films had but one contrast grade, necessitating strict control of contrast in the sep negs. Thanks to the contrast control available from dichromate sensitization, considerable leaway should be had.
Chalk one up for DCG-imbibition!
4th, UV exposure under negatives. The ins & outs of making color separations are best left alone in this discussion, but tri-color gum printers will probably find their negatives excellent for preliminary testing. I am exploring the route of making enlarged negatives on mammography x-ray film (with an intermediate panchromatic step), which so far has proved promising.
5th, etching in hot (120°F) water, just like carbon. Due to the nature of the hardening by dichromates, control should be exercised to ensure consistency. This difficulty is eloquently (if not somewhat discouragingly) described in this post.
6th, at this point, one should probably clear (sodium sulfite?) the matrices and perhaps harden them. However, I can't recommend anything at the moment. Some hardeners, like chrome alum, might act unfavorably as mordants in the matrix, where we don't want them. Will advise.
As it stands now, obtaining appropriate dyes is the single biggest roadblock. Recently, a couple boxes of Kodak dyes came through eBay and sold for over $100 a pop. Not a sustainable solution.
Jim Browning has listed the following dyes in his PDF for Dye Transfer Materials: Acid Blue 45 (or 25; bluer and transfers slower), Acid Red 80 (or 289*), Acid Yellow 11 (or 23*) [*brighter colors at the expense of light-fastness].
J.S. Friedman, author of the monumental "History of Color Photography", lists a number of textile dyes that may be suitable for the process and I would encourage people to explore that route. Although the color demands might not be perfectly met, textile dyes are easily obtainable and might be perfectly satisfactory for the imaginative printer. These textile dyes, known as "Acid Fast", will likely work and are available very cheaply. I suspect that their light-fastness is good if they are well regarded by textile folks.
Many sellers on eBay have large quantities of dyes, usually being liquidated from old labs, etc. All dyes have multiple names, so consult a book like Sigma-Aldrich's Handbook of Stains, Dyes & Indicators (easily available from most University libraries) to find out alternate names, chemical information, lightfastness and color quality.
The best option might be to find a manufacturer that offers samples of their dyes. A sample is likely to be sufficient for many, many prints, or alternatively, you might wish to see what their minimum quantity is.
When all is said and done, availability of dyes will be the key to truly making modern-day dye-transfer a reality. I am fortunate to have a wide variety of old dyes given to me by a chemist who researches dye-transfer, in addition to synthesizing his own dyes. So, perhaps there is hope that in the future we can provide better options in this regard. In the meantime, I will use these dyes to test my materials and will be simultaneously exploring alternative dye options, such as textile dyes.
A word about dyes: acid dyes happen to have an affinity for gelatin, that is, they will stain it. This affinity is increased with lower pH (more acid), and thus the receiving paper is soaked in an acid solution to encourage the dyes to leave the matrix and transfer more completely. Logically enough, to clear the matrix after transferring, a dilute ammonia solution will liberate the dye from the gelatin. A matrix can be reused many times, another reason why this process was so successful (ultimately superceding carbro in the industry).
There have been other methods to make prints with different classes of dyes like Pinatype above, which used "Pina" dyes... a.k.a. a proprietary dye (though Friedman lists the probable candidates). Also, there have been methods to use basic dyes, which are exceptionally brilliant & pure, but are very quick to fade. Since these dyes react differently with gelatin, different materials & methods are required. See Friedman if interesetd.
But acid dyes are the preferred class because they are quite good in color, and are the most permament of dyes. Other dyes that will act in the same manner as acid dyes are "direct" and "reactive" dyes, though I am far from an expert on these matters.
For an excellent paper on the requirements & testing of dyes, please follow these links -> Part 1, Part 2, Part 3, to Dyes For Imbibition Printing by Colton & Thronson.
Any information on dye-transfer printing, such as that given by Jim Browning, or Ctein, or Kodak's publication E-80 will give you a solid idea of what is necessary for transferring the print, as this function is independant of the kind of matrix used. At the moment, this is my least studied area of the process so I'd hate to give misinformation. It is however, pretty straight forward.
Basically you soak the matrices in their respective dye baths until they reach equilibrium (that is, the time it takes to absorb all the dye that they can hold). Then, you transfer them to a dilute acetic acid bath of about 2%. This rinses the excess dye off. At this point, in the acid bath, the dye will not exit the relief, and if the 3 matrices are not registered yet, they can be visually aligned in this bath and punched. The color image will be visible at this point.
Each matrix is then rolled onto the receiving paper, one after the other. The time that it takes for the dye to exit the matrix and migrate to the receiver paper will be a function of the dye, and will likely vary (as it did in the Kodak process). Standard practice is to transfer each color twice.
Another important aspect is the receiving paper itself and the pre-treatment bath. Kodak supplied a pre-bath for the receiving paper, but at the moment I couldn't tell you exactly what is in it. It stands to reason that it would be an acidic bath, and indeed, a sloshing of vinegar (not very scientific, nor ideally probably) helped a down & dirty test succeed while testing with a food dye. See here.
So as you can see, considerable room for experimentation is present. For initial testing, fixed out photo paper will be more than sufficient and actually the hardeners present in the emulsion, or from some fixers, will act as mordants in their own right.
An idealized receiving paper will have a dedicated mordant. F.E. Ives first suggested this method in U.S. patent #1,121,187, 'Photographic Printing Process', from December 15th, 1914. Basically, the mordant increases the paper's affinity to hold onto the dye (from the French word mordere, "to bite").
F.E. Ives has many earlier patents describing the use of DCG matrices for imbibition printing. Friedman doesn't spend much ink in discussing dye-transfer matrices of this type, instead simply saying that F.E. Ive's patents on the subject can be considered the authoritative text. Indeed, I would recommend anyone interested to seek them out, and I'll be more than happy to include the patent #'s. (p.s. They are listed in Friedman, and that book is available on Google books).
So as you can see, the work is just beginning. But all the information is out there and nothing is beyond the scope of the hobbyist. It just isn't formalized as of yet, so there is a lot of original & novel work that must be done before we can start "spreading the gospel" and telling people exactly how to do it.
I hope this will encourage a few people to give it a go, and remember that if tri-color seems daunting, there's certainly nothing wrong with monochrome prints. That would greatly expand the list of possible dyes for the process.
More to come in the future...
- Chris Holmquist, holmburgers, 2011 -
A Few Corrections & Ammendments
E. Sanger-Shepherd and O.M. Barlett were perhaps the 1st to propose the use of a relief matrix to produce dye-imbibition prints in 1902. US Patent #728,310
One of Ives' most important contributions to the art, one that was used in all subsequent schemes, was the effect of the pH of the dye-bath on contrast. Since acid raises gelatin's affinity for the dye, lower pH will cause more dye to enter the matrix, thus, higher contrast will be obtained. To have 3 or 4 dye-solutions of each color, each with differing acid contents, would be an excellent method to have contrast control in the transferring stage. This is described in the British Journal of Photography, Vol 46 (1899), p. 409, 470 & 537.
Some of Ives' Patents are: USP1,106,816 & USP1,160,288, Ger.P.308030, Eng.P. 15823/13 & 15823/14, Fr.P. 463,737.
My explanation of why the dye transfers into the receiving paper might've been somewhat misleading The main reason for the transfer, according to Friedman, is that the matrix is necessarily very hard (tanned) comapared to the receiving sheet, which will have softer gelatin, and thus the migration.
The acid rinse after soaking the matrices in their dye baths should be a 10% solution of acetic acid, not 2%. This highly acidic bath will remove the excess dye on the matrix, but will cause the gelatin in the matrix to adhere tightly. The matrices can be left in this, registered if necessary, until ready for transfer. This prevents the possibility of them drying out between transfers.
A recommendation for testing textile dyes is a 2% dye solution in 5% acetic acid. However, this was for the purpose of making transparencies, so for prints the 2% concentration of dye could be reduced significantly.
Mat or Semi-Mat fixed out photo papers will work best as receiving papers.
A UV-restraining dye (yellow) has been added to matrices historically as standard practice to limit the depth of the relief. This might be something to explore in the future. Unfortunately, the K-dichromate stain is not sufficient to have a great affect, according to Friedman.
Lastly, if anyone is confused or needs clarification on something, it would be my pleasure to talk about it.