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How To Sensitise Carbon Tissue For Photographic Carbon Printing
Photographic carbon printing depends on the effect of light on carbon tissue (a gelatin film) which has been sensitised by treatment with potassium or ammonium diochromate. There are several alternative processes such as collotype printing, silk screen printing and photogravure which also depend on this effect.
Carbon tissue, which unfortunately is no longer commercially available, has to be hand-made. This carbon tissue, once made and dried can be stored either sensitised or unsensitised in the refrigerator/freezer until ready for use.
Storage of Carbon Tissue
As it takes several hours for the tissue to dry after sensitising it, it is sometimes more time efficient to have sensitised tissue at hand when you are ready to print. It is vital that you keep in mind that in this state the tissue is sensitive to light. Sensitised tissue must be kept in a dark container and stored in the refrigerator/freezer. Black, plastic, light tight sleeves, such as those used for silver gelatin paper, are the best storage option. This sleeve can them be put in a cardboard box in the refrigerator/freezer.
The Sensitiser for Carbon Tissue
The sensitising solution is either
6% ammonium diochromate (mixed with room temperature distilled water) or
3% potassium diochromate (mixed with hot distilled water).
Potassium diochromate tends to crystalise when it cools making it more difficult to work with. It also results in a warm tone print.
Ammonium diochromate is more soluble and therefore doesn’t crystalise when it cools. It results in a higher dmax and therefore a cooler print.
Once mixed, the diochromate is stored in the refrigerator. When ready to use, use directly from the refrigerator. If warm diochromate is used it will melt the carbon that it is sensitising.
One litre of sensitiser is suitable for 6-8 8” x 10” prints. With each successive sheet of carbon tissue sensitised the solution loses strength.
The Method of Sensitising Carbon Tissue
There are two methods of sensitising
The brush method or
The tray method
Without going into too much detail, the brush method uses a smaller quantity of solution brushed over the surface repeatedly for several minutes. I have researched and tried both methods and have settled on using the tray method which I detail here.
Pouring one litre of solution into a tray, a sheet of carbon tissue is submerged in the solution and gently soaked and rocked for 2 to 2.5 minutes at which time it is pulled from solution, drained and laid flat. Working gently with a rubber squeegee, draw off the excess solution with one stroke in each direction. (Left, right, up, down).
Once dry, interleaf with mylar (clear plastic), place in light-tight, black sleeves and store in refrigerator/freezer.
As with each step in the carbon process determining the best way to sensitise the carbon tissue requires some experimentation. Choosing the optimal diochromate for your process, the preferred method of sensitising, drying times and storage options requires a little patience. Once you have resolved how best to sensitise your carbon tissue you will find that this step becomes the quickest and easiest part of the photographic carbon printing process.