Historical Context
Carbon printing was developed in the 1850s-60s, with Joseph Swan introducing a practical commercial process in 1864. Carbon prints are among the most permanent photographs possible—the image is composed of pigmented gelatin rather than metallic silver, and properly made carbon prints can last for centuries.
Process Overview
Carbon printing involves creating "tissue" (pigmented gelatin on a temporary support), sensitising it with dichromate, exposing through a negative, transferring to a final support, and developing in warm water to dissolve unhardened gelatin.
Chemistry
Carbon Tissue Formula ("glop", per litre, from Sandy King)
| Component | Amount |
|---|---|
| Distilled water | 900 ml |
| Gelatin (Bloom 175-250; plain Knox works) | 80 g |
| Plain white sugar | 50 g |
| Pigment (Sumi or India ink, ivory black, lampblack) | 15-20 g for a first try |
| Glycerine (optional, for arid climates) | 5-10 ml |
| Isopropyl alcohol (surfactant) | 50 ml |
| Thymol, 30% in isopropyl alcohol (preservative) | 1 ml |
Top up to 1 litre.
Preparation
- Mix the glop and hold it at about 46-52 °C (115-125 °F) for at least an hour before coating
- Coat onto a temporary support such as vellum or Yupo, in a room at 20-22 °C (68-72 °F); about 50 ml covers an 8 x 10 in tissue coated freehand
- Dry the tissue flat (King gives 3 to 24 hours)
Sensitiser
| Component | Amount |
|---|---|
| Potassium dichromate (tray sensitising) or ammonium dichromate (spirit sensitising) | About 0.25% to 4-6% |
| Distilled water | to volume |
Dichromate strength is matched to the negative's density range: stronger solutions give lower contrast, so they suit negatives with a long density range. King's usable range of about 0.25% to 4-6% lets carbon match negatives from about 0.9 to 2.7 in density range. Spirit sensitising (a stock at three times the target strength, mixed 1 part to 2 parts acetone or alcohol, brushed on) must use ammonium dichromate; potassium dichromate is for tray sensitising only.
Procedure (Single Transfer)
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Sensitise tissue: Under subdued tungsten light, immerse dried tissue in a cool potassium dichromate bath for about 3 minutes (King); it then dries in 1-2 hours in darkness. Expose within 5-10 minutes of it drying, at around 50% relative humidity, to limit dark reaction.
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Exposure: Contact print sensitised tissue under UV light, tissue emulsion facing light, with negative placed emulsion-to-emulsion.
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Mating: After exposure, soak the tissue in cool water (about 18 °C) for 45-50 seconds. Squeegee it onto the final support, cover with blotting paper and plate glass, and leave for 20-30 minutes (King).
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Development: After mating, place the sandwich in water at about 40 °C (105 °F). The temporary support will separate. Gently agitate—unhardened gelatin dissolves, leaving the image.
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Clearing and drying: Allow print to cool, clear any remaining dichromate stain in a weak sodium bisulfite or metabisulfite bath, wash, and dry. (An alum bath hardens the gelatin but does not clear the stain.)
Single vs Double Transfer
Single Transfer
- Tissue transfers directly to final support
- Image appears reversed (laterally flipped)
- Produces maximum relief effect
- Simpler process
Double Transfer
- Tissue transfers to temporary support first, then to final support
- Corrects image orientation
- Allows use of textured final papers
- More complex but more flexible
Making Your Own Tissue
While commercial carbon tissue is available, many practitioners make their own:
- Choose pigment: Lampblack for neutral tones, iron oxides for warm tones, or any lightfast pigment
- Disperse pigment: Grind pigment with a small amount of water to form paste
- Mix with gelatin: Add dispersed pigment to warm gelatin solution
- Pour and dry: Pour onto a release layer (King uses vellum or Yupo) and dry flat
The ratio of pigment to gelatin affects both the colour density and the printing characteristics of the tissue.
Troubleshooting
| Problem | Likely Cause | Solution |
|---|---|---|
| Image too light | Underexposure; overdevelopment | Increase exposure; lower water temperature |
| Image too dark | Overexposure; underdevelopment | Decrease exposure; raise water temperature |
| Frilling (edges lifting) | Poor tissue/support adhesion | Better mating; ensure good contact |
| Mottle | Uneven sensitisation or development | Improve technique; consistent agitation |
Safety
| Chemical | Hazard | Precautions |
|---|---|---|
| Potassium/ammonium dichromate | Danger: may cause cancer, genetic defects and harm to fertility or the unborn child (Carc./Muta./Repr. 1B); fatal if inhaled; toxic if swallowed; causes severe skin burns; skin and respiratory sensitiser; oxidiser; very toxic to aquatic life | Nitrile gloves and eye protection; avoid raising dust (buy ready-made solution if possible); never spray; dispose of as hazardous waste |
Hexavalent Chromium Warning
Dichromates contain hexavalent chromium (Cr VI). They are classified as carcinogenic, mutagenic and toxic to reproduction (all Category 1B), and as skin and respiratory sensitisers. Potassium, ammonium and sodium dichromate are on the REACH Authorisation List (Annex XIV): since 21 September 2017 they may not be used in the EU without an authorisation, and the same listing was carried over into UK REACH.
Always:
- Wear nitrile gloves
- Work in well-ventilated areas
- Avoid skin contact—chromium sensitisation can be permanent
- Dispose of waste according to local hazardous waste regulations
- Consider a dichromate-free sensitiser: DAS (diazidostilbene) can be mixed into the tissue gelatin, and practitioners report workable carbon prints with it, though published data are limited and DAS powder is itself flammable and harmful
Further Reading
Sandy King, Don Nelson & John Lockhart, Carbon Transfer Printing: A Step-by-Step Manual, Featuring Contemporary Carbon Printers and Their Creative Practice (Routledge, 2019) Part of the Contemporary Practices series. Covers single and double transfer, colour carbon, carbro process, and hexavalent chromium safety. 175 photographs from contemporary practitioners. The definitive modern reference for carbon transfer.
Christopher James, The Book of Alternative Photographic Processes, 3rd Edition (Cengage Learning, 2015) The definitive comprehensive text on alternative processes, with extensive coverage of carbon printing.
AlternativePhotography.com Extensive free articles on carbon transfer, including practical troubleshooting guides and contemporary practitioner profiles.
Sources
- The Carbon Transfer Process, Part 1, Sandy King, Unblinking Eye
- The Carbon Transfer Process, Part 2, Sandy King, Unblinking Eye
- ECHA CHEM substance record: Potassium dichromate, European Chemicals Agency
- ECHA CHEM substance record: Ammonium dichromate, European Chemicals Agency
- UK REACH Authorisation List (Annex 14), Health and Safety Executive
- Sensitizers in Carbon Transfer Printing: What to Know, Allan Chemical Corporation