I mentioned elsewhere on the forum that I have fallen into the hole of carbon printing as well. About two weeks after starting with this endeavor, I am capable of producing prints that are sort of passable. I use digital negatives because this approach gives me more freedom in terms of image size, as well as more consistency in my approach. For your entertainment, inspiration or critique, I'll share my process parameters:
Digital negatives:
Epson 3880 with Jon Cone's ConeColor Pro inkset (not ideal for digital negatives, but I like to use one printer for all my printing purposes)
Color density +50
Advanced B&W mode; yellow +75, photo black
Esselte 57161 A4-sized inkjet transparencies
Front feed to prevent pizza wheels (works like a treat too!)
I use a 3mm printed safe edge.
Glop recipe (amounts indicated per liter; I generally mix 150ml - 400ml):
Gelatin 250 bloom - 100g
Sugar - 100g (low humidity here in this time of year; heavy curling)
Pigment is Amsterdam brand lamp black acrylic paint - 160g (yes, that much! I found it the only way to get sufficient contrast in the prints from the weak digital negatives I make)
No other additives. Haven't tried adding glycerin to deal with the curling, but I found that using more sugar and storing the tissue flat under a pile of books helps enough with that.
I pour to a height of 0.5-1mm on cheap 200g/sqm A5 size sketching paper, ca. 20ml for a 17x13cm size (6.8" x 5.2"); drying time is ca. 36 hours.
I found that 'pouring' goes particularly well using a large 80ml syringe with a long, blunt needle (no coincidence these are the exact items that are shipped with the ConeColor Pro ink set...). Easy dosage, virtually no bubbles, very even thickness.
Sensitizing:
Spirit sensitizer, 6% ammoniumbichromate in water, diluted with acetone to 0.75% to 2% working solution, 4ml per print for the indicated size (= 30mg to 80mg ammoniumbichromate per print)
Brushed on with a 1" soft brush or a 3" foam brush; I see little difference in terms of application with particularly weaker dilutions being prone to brush marks regardless of the brush used.
I tried diluting with denatured alcohol, which worked fine (no fogging or other unwanted chemical interactions), but dried *much* slower than acetone.
Exposure:
Philips face tanning unit HB172 with 4x15W fluorescent tanning tubes.
Distance between light source and tissue on exposure is 30cm (12"). This prevents the tissue from heating up and getting sticky. I could probably get away with 15cm/6", but tolerances on exposure would be smaller as well.
As a printing frame, I use a cheap photo frame. I place a kitchen towel under the tissue in order to keep it flat; I found this helps tremendously in getting good contact between the tissue and the negative.
Exposure times vary between 6m50s to 7m30s, depending on desired contrast, dmax, etc. Most of the contrast control is done by varying the sensitizer concentration.
Transfer and development:
I transfer to Schut A5 sized 160g/sqm sketching paper, which I also use for cyanotypes. I size it three times with a 3% gelatin + ca. 1% chrome alum solution.
I noticed no differences in the transfer process or success rate with transfer water temperatures varying from ca. 16C to 30C (ca. 60F - 85F).
I let the tissue-paper sandwich rest for ca. 30 minutes under the pressure of a couple of books.
'Development' is done in water of 40C - 70C (ca. 105 - 160F); again, I noticed no real differences other than hotter water works a bit faster (but not very much) and is obviously harder on the paper.
After development is complete, I gently scrape away the unwanted borders of tissue outside the image area and rinse briefly in cold, running water. Hang to dry.
I'll probably make changes as I continue experimentation, but I find that the method described above works pretty well at this point for me. YMMV.
Digital snapshot of my girlfriend (EOS7D with 35/2IS); slightly overexposed during printing (visible in the safe edge). 0.75% sensitizer, 7m30s.
http://www.koraks.nl/galleries/prints/CPT_141_E.jpg
Some bubbles and pigment clumps due to errors during transfer and hot water development (scratched the unwanted tissue from the edges in the hot water bath; not a good idea, it turned out).
Photo from a shoot with a friend's band a couple of years ago (EOS20D with 24-70L; no longer in my possession due to theft...). 1.75% sensitizer, ca. 7m
http://www.koraks.nl/galleries/prints/CPT_141_S.jpg
You may or may not like the pretty extensive dodging and burning in digital post processing; either way, they have little to do with the printing process. Just so you know.