Historical context
Gum printing was a favourite of the pictorialists in the 1890s and 1900s (see gum bichromate). Laying one or more gum layers over a print made by another process is a combination-printing practice. We have found no reliable source for who first put gum over platinum or cyanotype, so we give no inventor or year. The two modern practitioner accounts we rely on are Sam Wang's tri-colour gum over cyanotype (2003) and Clay Harmon's gum over platinum (2004).
What it is and how it differs
- Two or more processes on one sheet. A first print (cyanotype, or a platinum or palladium print) is made and cleared. One or more pigmented, sensitised gum layers are then coated over it and exposed through new negatives.
- A base that is not just paper. The base print supplies the structure, and the gum adds colour. In Wang's tri-colour method the cyanotype is the cyan layer; gum yellow and magenta follow.
- Registration is the main difficulty. Every gum layer must be lined up with the base print, which has already been through water and may have changed size.
- Sensitive to the base. Pigmented dichromate gum over a metal print has to be well made: Harmon's advice is to print the base with correct highlights and slightly weak shadows, using a lower-than-normal contrast mix, to leave room for colour.
For the base processes see cyanotype and platinum/palladium; a contemporary printing-out version is in platinotype print-out.
Materials and chemistry
Dichromate is the hazardous ingredient. Read the Safety section before buying or mixing it.
Cyanotype base (Wang)
| Component | Amount |
|---|---|
| Stock A: ferric ammonium citrate | 50 g in 250 cc water |
| Stock B: potassium ferricyanide | 25 g in 250 cc water |
| Working sensitiser | 2 parts A to 1 part B |
Gum layers
| Item | Wang (tri-colour) | Harmon (over platinum, first coat) |
|---|---|---|
| Gum arabic | 1 part gum to 2 parts water, then diluted 1:1 for the yellow and magenta layers | 10 ml gum |
| Pigment | Hansa yellow, then red or magenta, added by test brush | 0.4 g burnt umber and 0.2 g Indian yellow |
| Ammonium dichromate | About 0.3 g per 10 cc of gum-pigment mix | 10 ml of a 30% solution |
Harmon's second coat uses 0.5 g burnt umber, 0.1 g green gold, 0.1 g perylene maroon and 8 ml dichromate (strength not stated).
Procedure
- Prepare the paper. Wang does not size the paper: he soaks it in hot water for about an hour to pre-shrink it, then dries it thoroughly. Harmon wants a body-sized hot-press paper of about 130 lb with a neutral to acidic pH. Whatman Watercolor needs only a brief dip in 1.5% oxalic acid, Fabriano Extra White a good five-minute soak, and Rives BFK at least two hot-water soak-and-dry cycles before the oxalic treatment.
- Make the base print. For cyanotype, coat, dry, expose through the cyan negative and develop. Wang adds a little vinegar to the first wash and develops only until the yellow has gone, usually no more than a couple of minutes. For platinum, print and process the base as normal.
- Dry completely. Damp paper can make dichromate bleach silver emulsions (Wang).
- Mix the gum in tungsten light. Brush on with a hake or roll on with a clean roller (Harmon). Let it set a few minutes, then dry in the dark, with a fan if necessary.
- Register. Wang aligns the print against the negative on a light table by eye and secures it with short strips of removable tape. Harmon does the same, lays glass or a negative sleeve on top to flatten the negative, and tapes one or two edges with soft-release clear tape.
- Expose under UV. Wang uses test strips and gives no fixed time. Harmon's rule of thumb for the first coat is 80% of the palladium exposure time (about zone VI to VII), and 30% for a second, darker coat.
- Develop. Wang places the print in slightly warm water (about 29 °C) for a minute or two, then floats it face down in water at 21 to 27 °C for 5 to 10 minutes. Harmon slides it face up into water at 21 to 24 °C, then lays it face down for 30 minutes, and rinses for 5 to 10 minutes in cool water once the highlights have cleared.
- Dry until just damp and then on a screen. Do not lay a sopping wet print directly on the screen, because pigment pools in the middle (Harmon).
- Repeat for further layers. Wang's tri-colour order after the cyan base is yellow, then magenta.
Contrast and colour
- More dichromate gives more speed and lower contrast; more pigment gives less speed and more contrast (Harmon).
- Yellow, brown and orange pigments slow the exposure, and blue and violet pigments speed it up (Harmon).
- For colour in the highlights, Harmon halves the pigment, uses more dichromate and exposes for 150% of the platinum time. Results vary.
- Wang notes the colour balance may be off after the magenta layer and extra printings may be needed.
Troubleshooting
| Problem | Likely cause | Remedy |
|---|---|---|
| Layers do not line up | Paper changed size in water; imprecise alignment | Pre-shrink with hot soak-and-dry cycles; align on a light table and tape (Wang, Harmon) |
| Sizing yellows | Glyoxal-hardened size left more than two days | Gum within two days; one coat of gum cures it (Harmon) |
| Pigment pools in the middle | Wet print laid on the drying screen | Hang until just damp first (Harmon) |
| Wrong colour depth | Too much pigment or dichromate | Adjust as above and test-brush before coating |
| Cyanotype base fades or bleaches | Alkali attacks Prussian blue | Use unbuffered paper; keep the wash water non-alkaline (Ware) |
| Some pigment washes away | Normal during development | Accept as part of the process (Wang) |
Safety
Ammonium dichromate is a Category 1B carcinogen, mutagen and reproductive toxin, fatal if inhaled and corrosive. It is on the REACH Authorisation List (Annex XIV), carried over into UK REACH, so it is hard to buy lawfully for hobby use. Wear nitrile gloves and eye protection, never raise dust (buy ready-made solution if at all), and dispose of it as hazardous waste. Dichromate-free sensitisers such as DAS work for carbon and casein printing, but practitioners report poor results in true gum printing, so there is no established substitute (see the gum bichromate guide).
| Chemical | Hazards |
|---|---|
| Ammonium dichromateDanger | May intensify fire; oxidiser.; May cause cancer.; May cause genetic defects.; May damage fertility. May damage the unborn child.; Fatal if inhaled.; Harmful in contact with skin.; Toxic if swallowed.; Causes damage to organs through prolonged or repeated exposure.; Causes severe skin burns and eye damage.; May cause allergy or asthma symptoms or breathing difficulties if inhaled.; May cause an allergic skin reaction.; Very toxic to aquatic life with long lasting effects. |
| Potassium dichromateDanger | May intensify fire; oxidiser.; May cause cancer.; May cause genetic defects.; May damage fertility. May damage the unborn child.; Fatal if inhaled.; Harmful in contact with skin.; Toxic if swallowed.; Causes damage to organs through prolonged or repeated exposure.; Causes severe skin burns and eye damage.; May cause allergy or asthma symptoms or breathing difficulties if inhaled.; May cause an allergic skin reaction.; Very toxic to aquatic life with long lasting effects. |
| Potassium ferricyanideWarning | Causes serious eye irritation.; Toxic to aquatic life with long lasting effects. |
| Ferric ammonium citrate (green or brown)Warning | Causes skin irritation.; Causes serious eye irritation. |
| Oxalic acidWarning | Harmful in contact with skin.; Harmful if swallowed. |
| DAS (diazidostilbene)Warning | Flammable solid.; Harmful if inhaled.; Harmful in contact with skin.; Harmful if swallowed. |
| GlyoxalWarning | Suspected of causing genetic defects.; Harmful if inhaled.; Causes skin irritation.; Causes serious eye irritation.; May cause an allergic skin reaction. |
Classifications from the European Chemicals Agency (official EU classification where one exists, otherwise the one agreed by REACH registrants or most often notified). Read the safety data sheet that comes with your chemicals, and dispose of them in line with local regulations.
- Dichromate classification. Both dichromates carry H350, H340, H360FD, H330 and H314 (chemicals database, CLP harmonised classification). Wang calls ammonium dichromate "very toxic" and a known carcinogen, and warns it is combustible when dry.
- Oxalic acid is harmful in contact with skin and if swallowed (H312, H302). Harmon's 1.5% bath is well below the UK 10% w/w threshold for regulated sale.
- Glyoxal sizing. Harmon sizes with gelatin hardened with glyoxal. We do not give the recipe. Glyoxal is in the hazard table above (a suspected mutagen, a skin sensitiser and harmful if inhaled, under its EU harmonised classification); the gum bichromate guide discusses sizing options.
- Pigments. Check each pigment's safety data; cadmium and cobalt pigments need care.
- Silver negatives. Dichromate and silver gelatin negatives are incompatible (Harmon).
Further reading
Christina Z. Anderson, Gum Printing: A Step-by-Step Manual (Routledge). The definitive modern gum reference, including multi-layer work.
Pradip Malde and Mike Ware, Platinotype (Routledge, 2020). The printing-out platinum and palladium base.
Sources
- Tri-Color Gum over Cyanotype, Sam Wang, Unblinking Eye (2003)
- Gum Over Platinum, Clay Harmon, Unblinking Eye (2004)
- Simple Cyanotype workshop notes, Mike Ware
- UK REACH Authorisation List (Annex 14), Health and Safety Executive
- ECHA CHEM substance record: Ammonium dichromate, European Chemicals Agency
- ECHA CHEM substance record: Glyoxal, European Chemicals Agency (via the Silverlog chemicals database)