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	300 g
Distilled water	700 ml

Or use commercial pre-mixed gum arabic solution (14-16 Baumé).

Sensitiser

Component	Amount
Ammonium dichromate	26 g
Distilled water	100 ml

Or potassium dichromate (20 g per 100 ml)—slightly less sensitive.

Working Emulsion (typical ratio)

Component	Amount
Gum arabic solution	12 ml
Watercolour pigment	As needed
Dichromate sensitiser	12 ml

Ratios can be adjusted:

More gum = softer image, more manipulation possible
More dichromate = higher contrast
More pigment = denser image but longer exposure

Procedure

Paper preparation: Size paper with gelatin (5-10% solution) or use pre-sized watercolour paper. Paper must be pre-shrunk by soaking and drying before first use.
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: 2-10 minutes
- UV tubes: 5-20 minutes
Exposure depends on pigment density, dichromate concentration, 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 warm water for faster development
Development time: 15-60 minutes or longer.
Drying: Dry flat. If building multiple layers, allow to dry completely between coats.
Multiple layers: For full tonal range, apply 3+ layers.

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.

Contrast Control

Dichromate concentration: Higher = more 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; warmer water
Grainy appearance	Pigment not well dispersed	Grind pigment better; use tube watercolours

Safety

Chemical	Hazard	Precautions
Ammonium/potassium dichromate	Carcinogenic; toxic; skin sensitiser	See carbon transfer safety notes
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

Same cautions as carbon transfer apply. Dichromates contain hexavalent chromium which is carcinogenic. EU restrictions apply. Always wear gloves and work in well-ventilated areas.

Gum Bichromate

Image characteristics

Key chemicals