Historical Context

Gum printing became popular with pictorialist photographers in the 1890s-1900s, who prized the process for its painterly qualities and ease of manipulation. Unlike most photographic processes, gum prints can be extensively hand-worked during development.

Chemistry

Gum Solution

Pre-mixed gum arabic solution:

Component	Amount
Gum arabic powder	30 g
Very warm distilled water	90 ml (or less)

Refrigerate it (Blackburn). Or use a bottled, pre-dissolved commercial gum arabic solution.

Sensitiser

Component	Amount
Ammonium dichromate	29 g (near saturated)
Very warm distilled water	100 ml

Or potassium dichromate, which is far less soluble: a near-saturated solution is about 13 g per 100 ml, and many printers use around 5%. It is slightly less sensitive than ammonium dichromate.

Working Emulsion (typical ratio)

Component	Amount
Gum arabic solution	6 ml
Dichromate sensitiser	6 ml
Watercolour pigment	0.5 g, or 2-3 cm of tube colour

These are Photographers' Formulary's figures for an 8 x 10 in print (equal gum and dichromate; Blackburn's 3 eyedroppers of each plus 2-3 cm of tube colour is the same idea). Formulary's pigment-to-gum ratio runs from 0.5 g to 4-6 g per 12 ml of gum depending on the pigment and the saturation wanted.

Ratios can be adjusted:

More gum = higher contrast, but too much makes the coating flake during development
More dichromate = faster printing and generally lower contrast
More pigment = denser image but longer exposure

Procedure

Paper preparation: Size paper with gelatin hardened with glyoxal (a suspected mutagen and skin sensitiser; or, historically, formaldehyde, a Category 1B carcinogen), use a synthetic size such as AKD, or use a well-sized watercolour paper. For multiple layers, pre-shrink the paper by soaking it in very warm water for at least 10 minutes (Formulary: 30 minutes in 60 °C / 140 °F water) and drying it before the first coat. Photographers' Formulary sizes with 3.25 g gelatin in 125 ml water plus 5 ml glyoxal as hardener.
Register system: For multiple layers, establish registration marks. Common systems include pin registration or corner marks.
Mixing emulsion: Under subdued light, combine gum, pigment, and dichromate. Mix thoroughly.
Coating: Apply emulsion with brush (hake brush or foam brush). Brush out evenly—brushwork will be visible. Dry in darkness.
Exposure: Contact print under UV light. Gum is relatively slow:
- Sunlight: Blackburn gives "from 1 to several minutes", with about 3 minutes as a first test
- UV lamps and tubes: no standard time. Make a test strip
Exposure depends on the light source, its distance, negative density, pigment-to-gum ratio, and gum thickness.
Development: Develop in plain water. Unhardened gum dissolves. Development can be manipulated:
- Gentle agitation for soft development
- Spray or flow water for more aggressive clearing
- Brush gently to locally remove gum
Use three trays of clean water at 20 °C (68 °F), face down, for about 10 minutes each (Formulary); Blackburn gives 5-15 minutes in each and aims to finish within 20-30 minutes.
Drying: Dry flat. If building multiple layers, allow to dry completely between coats.
Multiple layers: Blackburn recommends multiple coats. Clear the dichromate stain only once, after the last layer: a brief soak in 5% (or weaker) potassium metabisulfite (Formulary sprays 5 g in 500 ml and washes for 10 minutes).

Multi-Layer Gum (Colour)

Traditional tricolour gum uses three (or more) layers:

Cyan layer: Phthalo blue or similar cool blue pigment
Magenta layer: Quinacridone magenta or similar
Yellow layer: Hansa yellow or similar

Layer Order

Yellow first (least visual impact from slight misregistration)
Magenta second
Cyan last

Each layer requires its own appropriately adjusted negative.

For gum layers over a cyanotype or platinum base print, see gum over platinum and cyanotype.

Contrast Control

Dichromate concentration: Higher = faster, generally lower contrast
Pigment load: More pigment = longer exposure, richer shadows
Negative adjustment: Adjust curves per layer
Development manipulation: Aggressive development = lower contrast

Pigment Selection

Good starting pigments:

Colour	Pigment	Notes
Black	Lamp black, ivory black	Neutral, high tinting
Brown	Burnt umber, raw sienna	Warm tones
Blue	Phthalo blue, Prussian blue	Strong tinting
Red	Quinacridone, cadmium red	Varies widely
Yellow	Hansa yellow, cadmium yellow	Use sparingly

Avoid fugitive pigments (check lightfastness ratings).

Troubleshooting

Problem	Likely Cause	Solution
Image washes off completely	Underexposure; too much gum	Increase exposure; reduce gum ratio
No detail in shadows	Overexposure; too much dichromate	Reduce exposure; lower dichromate
Staining	Insufficient development	Develop longer; clear with metabisulfite
Grainy appearance	Pigment not well dispersed	Grind pigment better; use tube watercolours

Safety

Chemical	Hazards
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.
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.
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 (EU harmonised). Read the safety data sheet that comes with your chemicals, and dispose of them in line with local regulations.

Chemical	Hazard	Precautions
Ammonium/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
Gum arabic	Low toxicity	Standard precautions
Watercolour pigments	Varies by pigment; some contain toxic metals	Check pigment safety; cadmium and cobalt pigments require care

Hexavalent Chromium Warning

Dichromates contain hexavalent chromium. 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 gloves and eye protection, avoid raising dust, and work in well-ventilated areas. Dichromate-free sensitisers such as DAS (diazidostilbene) work for carbon and casein printing, but practitioners report poor results in true gum printing, so there is no established substitute for gum yet.

Sources

An introduction to the gum bichromate process, Peter J. Blackburn, AlternativePhotography.com
The 19th century gum bichromate process in 21st century concept and techniques, AlternativePhotography.com
Gum Bichromate Kit instructions (07-0100/07-0105), Photographers' Formulary
ECHA CHEM substance record: Ammonium dichromate, European Chemicals Agency
ECHA CHEM substance record: Glyoxal, European Chemicals Agency (via the Silverlog chemicals database)
UK REACH Authorisation List (Annex 14), Health and Safety Executive
Using DAS to replace potassium dichromate in the gum bichromate process, Photrio (practitioner discussion)

This guide combines established practice with community experience. Always follow safety precautions and cross-reference with primary sources before mixing chemicals.

Gum Bichromate

At a glance

Image characteristics

Key chemicals