Historical Context
Oil printing was introduced by G.E.H. Rawlins in 1904, putting into practice Alphonse Poitevin's 1855 observation that dichromated gelatin accepts greasy ink according to how much light it has received. It became the direct precursor to bromoil, which adapted the same principles to silver gelatin prints. During the Pictorialist era (1890s-1920s), oil printing was prized for its painterly qualities and the extensive control it offered photographers.
The process uses dichromated gelatin sensitised on paper, exposed to UV light, then inked by hand. The differential hardening of the gelatin allows ink to adhere to shadow areas while being repelled by the swollen, unhardened highlights.
Chemistry
Gelatin Sizing Solution
| Component | Amount |
|---|---|
| Photographic gelatin (250 bloom) | 50 g |
| Distilled water | to 500 ml (a 10% solution) |
Let the gelatin rest for at least 48 hours before coating, and keep it no hotter than 50 °C (120 °F). It keeps up to three months sealed, but one month is better.
Sensitiser
| Component | Amount |
|---|---|
| Potassium dichromate | about 3 g |
| Distilled water | 100 ml |
AlternativePhotography.com gives about 3% potassium dichromate, or 2% for the more sensitive ammonium dichromate. A dichromate-free variant sensitises the gelatin with ferric ammonium citrate (about 5 g in 50 ml water and 50 ml acetone, with 0.6 g oxalic acid) and develops the image in dilute hydrogen peroxide (100 ml of 3% in 900 ml water).
Procedure
Paper Preparation
-
Size the paper: Soak the paper in water for 5 to 10 minutes, scald it for 10 to 20 seconds with boiling water, then coat it with gelatin at about 38 °C (100 °F; never above 54 °C) at 50-60 ml per A4 sheet. Let it set for at least 15 minutes, then dry it flat for about 48 hours at 60-65% humidity.
-
Sensitise: Float dried, sized paper on the dichromate solution until evenly coated, then hang to dry in darkness. Sensitising time is not given in the sources used here, so test.
- Dichromate-free option: brush on about 10 ml of the ferric ammonium citrate sensitiser per A4 sheet and dry it in a darkened room for at least 2 hours (the source prefers 4-5 hours, and not more than 24 hours before exposure).
-
Storage: Use sensitised paper soon after it is dry: dichromated gelatin keeps hardening slowly in the dark.
Exposure
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Contact print: Place sensitised paper under negative in contact frame.
-
Expose: Expose to UV light or strong sunlight (the ferric ammonium citrate variant took about 1.5 to 5 minutes at 365-385 nm; under 90 seconds gave weak blacks). The image will appear in yellow-brown tones as the dichromate hardens the gelatin in the shadow areas.
-
Judgment: Exposure is complete when a faint image is visible. Over-exposure hardens too much gelatin; under-exposure leaves too little.
Inking
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Wash out: Wash the exposed print in cold water until the yellow dichromate has cleared, which can take up to an hour. Then soak it in water no warmer than 28°C for around 10 minutes; longer soaking lowers contrast and density. The unhardened gelatin in highlights swells while hardened shadow areas remain firm.
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Surface preparation: Blot excess water but keep paper damp.
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Apply ink: Using stiff brushes, stipple lithographic ink onto the damp print. The hardened shadow areas accept ink; the swollen highlight areas repel it.
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Build density: Work gradually with repeated applications. The image builds through successive inkings.
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Dry: Allow to dry completely between inking sessions if multiple applications needed.
Inks and Tools
Recommended Inks
- Lithographic or etching inks (Charbonnel, Graphic Chemical)
- Oil-based relief inks
- Custom mixtures with linseed oil for consistency control
Brushes
- Stiff hog bristle for stippling
- Badger blenders for smoothing
- Various sizes for different image areas
Comparison with Bromoil
| Aspect | Oil Printing | Bromoil |
|---|---|---|
| Starting point | Sized paper | Silver gelatin print |
| Sensitiser | Dichromate | Silver image (bleached) |
| Negative | Full-size negative, contact printed | Any size, via an enlarged silver print |
| Dichromate | In the sensitiser (a dichromate-free FAC variant exists) | In the usual bleach (potassium dichromate) |
| Preparation | More steps | Simpler |
| Image quality | Potentially higher | Dependent on original print |
Troubleshooting
| Problem | Likely Cause | Solution |
|---|---|---|
| Ink won't adhere | Paper too wet; under-exposure | Blot more; increase exposure |
| Ink in highlights | Paper too dry; over-exposure | More water; reduce exposure |
| Uneven gelatin | Poor sizing technique | Multiple thin coats |
| Low Dmax | Insufficient inking | More ink; stiffer ink |
| Image washes off | Under-exposure; weak gelatin | Increase both |
Safety
| Chemical | Hazard | Precautions |
|---|---|---|
| Potassium 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 |
| Ammonium dichromate | Same hazards as potassium dichromate | Same as above |
| Gelatin | Non-toxic | Standard precautions |
| Acetone (dichromate-free variant) | Highly flammable; vapour causes drowsiness; irritates eyes | No flames or sparks; ventilation |
| Oxalic acid (optional, dichromate-free variant) | Harmful if swallowed or in contact with skin; a regulated poison in the UK above 10% w/w (licence needed for the public) | Gloves and eye protection; may be omitted |
| Hydrogen peroxide (3%, dichromate-free developer) | Mild irritant at this strength; concentrated grades are corrosive oxidisers | Use pharmacy-grade 3% only; gloves |
| Lithographic ink | Skin irritant | Gloves; ventilation |
Oil printing traditionally uses dichromates, which contain hexavalent chromium and are classified as carcinogenic, mutagenic and toxic to reproduction (all Category 1B). 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. Bromoil is not a dichromate-free alternative, because its usual bleach also contains potassium dichromate. The dichromate-free option is the ferric ammonium citrate and hydrogen peroxide sensitiser described above.
Gum Oil Variant
Some practitioners combine gum bichromate with oil printing:
- Print a gum bichromate layer first (for colour)
- Apply oil printing layer on top
- Results in multi-layer, multi-colour prints
Pictorialists did build multi-layer prints (gum over platinum was common), but the modern "gumoil" process is a different technique: Karl Koenig devised it around 1990, exposing a gum layer through a positive transparency and rubbing oil paint into the areas the gum leaves bare.
Further Reading
Bertram Cox, The Oil Pigment Processes (Autotype Company, c. 1920) Historical reference from the process's peak period.
Luis Nadeau, History and Practice of Oil and Bromoil Printing (Atelier Luis Nadeau) Comprehensive modern reference covering both processes.
Robert Demachy and Constant Puyo's work Pictorialist masters who used oil printing extensively. Their prints demonstrate the process's artistic potential.
Sources
- The Rawlins oil print process, AlternativePhotography.com
- Introduction to Oil and Bromoil printing, AlternativePhotography.com
- An introduction to the bromoil process, National Science and Media Museum
- ECHA CHEM substance record: Potassium dichromate, European Chemicals Agency
- UK REACH Authorisation List (Annex 14), Health and Safety Executive
- The gumoil process, AlternativePhotography.com