Negatives for Carbon Printing
Carbon printing is quite flexible and changing the strength of the dichromate sensitizer allows for the use of negatives with widely varying density ranges, from as low as log 0.9 to as high as log 2.7 and even higher. In my experience, however, the best negatives for carbon are those with a high density range which maximize the very long scale and straight-line characteristics of the carbon process. This type of negative also maximizes the relief potential of the medium when combined with the right type of tissue. The optimum negative will have a DR of about 1.8 (with a range from about .30 to 2.10) and exposure should be sufficient to get as much shadow detail as possible off the toe and into the straight line of the film’s characteristic curve. As a general rule negatives for carbon printing exposed under normal lighting conditions should be developed about 1.5 times as long as would be required for silver negatives exposed under similar lighting conditions. Since increased development times also increases shadow density the EFS (effective film speed) of the film should also be increased slightly, generally about one-fourth to one-half over the manufacturer’s ISO rating of the film.
Much of my present work is with digital negatives. The use of digital negatives allows one to standarize procedures and can greatly increase productivity. In making digital negatives I use Mark Nelson’s Precision Digital Negative system and print on Pictorico OHP with an Epson 2200 printer. The quality of my work with digital negatives closely approximates the quality I get from in-camera negatives.
Carbon tissue sensitized with dichromate has its maximum sensitivity in the UV and violet at about 350-420 nm. Its sensitivity extends through the blue but falls off very rapidly in the green to about 515nm. It has virtually no sensitivity to orange and red light. In practice carbon tissue is exposed with a source rich in UV light, such as the sun, mercury vapor or metal halide lamps, banks of BL (black-light) fluorescent tubes, and carbon arc, metal halide, pulsed xenon and continuous wave xenon plate burners made for the graphic arts. Each system has its advantages and disadvantages, but when used properly all are capable of good results. For the handy, there are a number of relatively inexpensive artificial light sources that can be put together or assembled from materials available at your local garden and home supply store. Consult the references for more information.
Sensitized carbon tissue should be exposed with a contact printing frame capable of exerting very firm pressure, or better, with a vacuum easel. The dry tissue can be very stiff and requires a lot of pressure to maintain good contact with the negative. Mask around the edges of the negative with lithographic tape to create a safe edge of about 1/4”. Failure to mask the negative may result in frilling during warm-water development.
If the final image is to be made with the single transfer process the negative should be reversed for correct orientation of the final image. To reverse the negative, just sandwich it with its base side in contact with the emulsion. In double transfer the final orientation of the print will be correct if the emulsion of the negative is placed in contact with the emulsion of the tissue. If it is necessary to reverse the negative some loss of sharpness should be anticipated, greater with a diffuse light source such as a bank of UV tubes, least with semi-collimated and point-light sources.
Exposure times in carbon printing will vary significantly according to tissue type and light source. I expose carbon with two different light sources, a UV bank of twelve 48” BL fluorescent tubes spaced at about 4” from the printing frame, and an AmerGraph ULF-28 Continuous Wave Xenon light source. Under these conditions, typical printing times for well-exposed and developed negatives range from about three to fifteen minutes, with the major variable being the tissue itself. The AmerGraph ULF-28 prints on average slightly more than twice as fast as the bank of BL tubes.
After exposure the carbon tissue must be transferred to a final or temporary support for development in warm water. The support on which the carbon image is developed must be somewhat larger than the tissue. Paper supports should be soaked for a minute or so in a tray of water at about 60-65°F before mating with the sensitized and exposed tissue. After exposure, place the carbon tissue in the tray of cool water, along with the final or temporary support, and allow it to soak for about a minute. The tissue should be brought into contact under water with the final or temporary support, then the sandwich is removed from the water and allowed to drain for a few seconds. Place the sandwich on a sheet of plate glass, tissue uppermost, and squeegee out the excess water, applying gradually increasing pressure with the squeegee. Blot off the water from around the edges, place the sandwich on a sheet of clean blotting paper, cover with a sheet of plate glass and let stand under pressure for about 20-30 minutes.
The next step is development in warm water. Transfer the tissue/support sandwich to a tray of warm water at about 110-120°F. In a minute or so the soluble gelatin will begin to melt and start to ooze out at the edges of the tissue. You should now strip the tissue from the support: starting at one corner, lift the tissue and gently pull it off the support with an upward motion. Discard the tissue.
At this point you will not see an image but a mass of melting, oozing pigment. To clear the image agitate it in the warm water with a gentle rocking and shaking motion, occasionally lifting it from the water to drain for a few seconds. After about 4 minutes most of the insoluble gelatin will have washed away and you will have a good view of the image.
Continue to agitate as before, lifting the print from time to time to drain. The print should be fully developed in 6-10 minutes, at which point the drain water should be clear.
There is no precise ending point to development with carbon printing so if the print is too dark after 6-10 minutes continue development in the warm water until it looks right to the eye. With extended development of 30-60 minutes it is possible to further reduce the density of the print by as much as 1/2 to one full stop.
Carbon prints have some dry down, though relatively speaking not nearly as much as Pt./Pd. Therefore, continue development until the highlights are just a bit lighter than they should appear on the final print.
After development is judged complete transfer the print to a tray of cool water (50-60°F.) and continue agitation for a minute or so, lifting it out of the water several times to drain. Leave the print in the cool water for 2-4 minutes to allow the gelatin to set, then hang it up to dry.
Veiled margins and other pigmented gelatin residue left around the borders of the print may be removed from the print by carefully rubbing these areas with a clean sponge.
Prints developed on a final paper support will have an overall yellow stain from the dichromate sensitizer. To remove this stain, soak the print in a 3% solution of sodium bisulfite or potassium metabisulfite for 5-10 minutes, or until the dichromate stain clears. Always allow the print to dry before clearing because the relief image, which is extremely delicate following development, is further softened by the clearing agents, and blistering or reticulation of the image is possible. After clearing, rinse the print in running water at 60-70°F. for 5-10 minutes. The print is now chemically stable and ready to be mounted for presentation, unless some retouching is required.
Carbon reliefs developed on plastic with the double transfer do not require clearing as the dichromate has no way of leeching into the plastic support.
When printing with the double transfer procedure we must transfer the relief on its temporary plastic support to a final support. The most reliable support for the double transfer procedures is a paper support sized with a layer of relatively soft gelatin, as previously described. To make the transfer, first soak the final paper in water at about 65-70°F for 5-10 minutes. When the gelatin has swollen, which will be recognized by a soft, slimy feel to the paper, it is time to make the transfer. Now, soak the image on its plastic temporary carrier for 30-45 seconds in the same tray of water, then bring the two into contact under water. Transfer the sandwich to a flat surface, paper support uppermost and gently squeegee the back of the paper support to remove all water. Place blotting paper or a clean towel over the paper and cover it with a sheet of plate glass. After about 10-15 minutes remove the sandwich and place it on a drying rack, or hang it to dry. Depending will take several hours, or with heavy final support papers, even overnight. When dry the paper, which now carries the image, should peel easily from the plastic. In fact, it may do this on its own as it dries. If all has gone well there should have been a complete transfer of the image from the temporary plastic support to the final paper support. You should re-soak the image in cool water for 5-10 minutes to remove the gloss, then hang to dry.
The thick gelatin layer of carbon photographs can cause heavy curling and wrinkling of the print. A dry mount press can be used to flatten out the print. To flatten, warm up the dry mount press to about 180-200° F and place the print in the press between two sheets of clean board and leave for 1-2 minutes. Remove the print from the press and place it under pressure for 20-30 minutes to cool and then store it flat under light pressure. Another solution is to just place the print between two flat surfaces for a week or so, with light pressure applied with some type of weight.
If any retouching is needed I recommend the use of tube watercolors. Tube watercolors are available in a wide range of colors and unless you are using a very unusual color it should not be difficult to match the color of the tissue.
Most problems in carbon printing can be traced and corrected without great difficulty. Good record keeping is a great asset in figuring out what went wrong.