Carbon on Glass (Back Exposure).
This process relies on the back exposure and requires no transfer. It is a carbon printing process. Pigmented gelatin layer on glass is made and sensitized similar to a carbon process. Because of the exposure through the back of the glass the image is formed at the glass / gelatin interface. That means that the image is built with light exposure from the “bottom”, leaving the unexposed layer on top. At the end of exposure the glass plate is simply immersed in hot water, which dissolves unexposed gelatin away, leaving the image behind. The hardened gelatin sticks to a clean glass very well. No transfer or other image processing is needed. In this process the negative and the light sensitive emulsion is separated by the thickness of the glass and therefore a collimated light source is needed. Direct sun work very well. Starting exposures of 1-2 minutes are recommended. Overcast days are not good. BL tubes do not give sharp shadows, so they will result in a very blurry image. Plate burners perhaps can be used, but I have not tried one.
Negatives should have the density suitable for a regular carbon printing.
Glass.
I use cheap plate window glass washed and scrubbed with cleaning powder. Do not touch with bare hands after washing.
Gelatin.
Food grade is good just as higher bloom photographic grades.
Pigments.
I have used tube watercolors and powdered pigments. Winsor and Newton watercolors DO NOT WORK. They have a preservative that spontaneously hardens the gelatin as it dries. Easy to test. Just cover a glass plate with some gelatin mix and allow to dry for a day. Immerse in hot water and see if gelatin dissolves completely. Daniel Smith and Daler Rowney brands worked for me. Test your pigments before making a lot of plates. On the other hand you can just scrub gelatin with some Chlorox bleach and reuse the plates.
Sample recipe:
2 packages of food grade gelatin (14 grams), 6 grams of sugar, 4 drops of Jet Dry, 140 cc of water. Jet Dry is a detergent used in automatic dishwashers to prevent spots when drying. It is a wonderful aid in making carbon emulsions.
Soak gelatin in cold water, heat to about 40-50 degC, add jet dry. Add watercolor to a mortar and mix with little of gelatin mix, adding more as it is mixed.
Placed the glass on a level surface and pour about 40 cc for a 11x14” image. The gelatin is spread quickly with a comb. Once is sets it is dried for a day. Can be kept dry for months (that is pigment dependents I think) or forever frozen.
Sensitizing is done in yellow light. I use ammonium dichromate, and about 1% is a good starting point. Try 30second to 1 minute in a tray with constant agitation. As there is no paper to soak short sensitization times work well. I do not chill my sensitizer bath. Use rubber squeegee to remove excess and let dry in a dark room.
Place the negative on the glass side, not a gelatin side. Place another glass plate on top of negative to keep is flat and a black mat on the bottom (facing gelatin) to protect it from stray light. I use spring clamps to hold the sandwich together. Cover on top with another black mat. Walk outside, remove the top black cover and expose. 1-2 minute of direct sun is a good start for testing.
Develop in a hot water bath (40—60) degC. The gelatin dissolves quickly, leaving the image on the glass plate. It is quite magical to see the carbon print develop if you are new to the process.
This is really a carbon process and carbon manuals should be consulted for more details on contrast, sensitization, pigment concentrations, recipes, etc.
Marek
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