What Is Gum Bichromate Printing? A Beginner’s Overview
Gum bichromate printing is a nineteenth-century photographic process in which paper is hand-coated with a mixture of gum arabic, pigment and a light-sensitive dichromate salt, then exposed to ultraviolet light under a negative. Where light hardens the gum, the pigment stays; elsewhere it washes away in water. The image is built from color layers rather than silver.
The result is soft, painterly and different with every print, which is why the method has stayed popular with artists who want a handmade look.
How the process works
The chemistry rests on a simple fact: dichromate salts harden colloids such as gum arabic when exposed to light. The steps, in outline, are:
- Prepare the paper. A sturdy watercolor paper is often pre-shrunk by soaking it in hot water, so it does not distort during repeated wetting.
- Mix the emulsion. Gum arabic solution, a dichromate sensitizer and artists’ pigment, such as watercolor, are combined.
- Coat the paper. The mixture is brushed evenly onto the sheet and dried in low light.
- Expose. A negative the same size as the final print, usually a digital or film transparency, is placed in contact with the coated sheet and exposed to ultraviolet light, either sunlight or a UV unit.
- Develop. The sheet is floated or submerged in water. Unhardened gum and pigment dissolve away, leaving the image.
- Dry and repeat. For a full-color image, the process is repeated with fresh coatings for each color, aligning each layer with the previous one.
Exposure time depends on the light source, the paper and the density of the negative, so printers keep test strips and notes.
A short history
The light sensitivity of dichromate salts was reported by the Scottish scientist Mungo Ponton in 1839. Later in the 1850s, the French chemist Alphonse Poitevin developed processes based on dichromated colloids. The gum version became widely used in the late nineteenth and early twentieth centuries, especially among Pictorialist photographers who wanted images to look more like drawings or paintings than documents. Names often linked to the method in that period include Robert Demachy and Alfred Maskell, though histories of photography discuss several others.
When the Pictorialist movement faded, gum printing declined. It returned in the later twentieth century as artists reclaimed hand processes, a story told in many surveys of alternative photography.
What gum prints look like
Common characteristics include:
- Soft edges and limited fine detail
- A visible brush texture and uneven coating
- Translucent color layers that mix in the eye
- Each print being slightly different, even from the same negative
Some printers see these traits as flaws to control; others see them as the point.
Layers, color and combining methods
A single gum layer gives a monochrome image. Multi-layer prints use separations, where the image is divided into color channels, for example cyan, magenta and yellow, each printed with a matching pigment. Printers also combine gum with other processes, for instance printing gum over a cyanotype or a platinum print, to add color or depth.
Some artists use the process with photographs on unusual supports, or add drawing, painting and collage. The portfolio page on this site shows work in gum prints, woodcuts and Polaroids as examples of how hand processes mix with other media, and the biography page provides background on one artist working in this area.
Safety
Dichromate salts are hazardous. Potassium and ammonium dichromate are toxic, can cause allergic skin reactions and are classed as carcinogenic with long exposure. Basic precautions include:
- Wear nitrile gloves and eye protection.
- Work in a ventilated space and avoid breathing powder.
- Keep chemicals away from food and children.
- Follow local hazardous-waste rules for disposal; do not pour solutions down the drain.
Always read the safety data sheet from the supplier. Some printers use alternatives to dichromate, though most standard instructions still rely on it. Check current guidance from a recognised chemical safety authority.
What you need to try it
- Watercolor paper, sized or pre-shrunk
- Gum arabic solution
- Tube watercolors
- A dichromate sensitizer, handled with the care above
- A flat brush and a tray
- UV light source and a contact frame
- A negative the size of the print
Frequently asked questions
Is gum bichromate printing the same as gum printing?
Yes. The names refer to the same process; “gum bichromate” is the older, more technical name.
Do you need a darkroom?
No. The coated paper is only sensitive to ultraviolet light, so you can work in dim, indoor lighting. A conventional darkroom is not required.
Why do some prints need many layers?
Each layer adds color and tone, and a single pass rarely gives full depth. Printers repeat the process, adjusting exposure each time.
Is it safe?
Dichromate is hazardous. With gloves, ventilation and proper disposal, many printers work with it, but it should not be treated casually.
For more on how artists use hand processes alongside other media, return to the home page.