----- Original Message -----
Sent: Saturday, June 28, 2008 1:31
PM
Subject: Carbon on glass with back
exposure
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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