Gum Bichromate is not an inherently difficult process. With practice and patience successful prints can be produced. Only one chemical is required to produce the light sensitive emulsion, and prints are developed in water. Nor is a light-tight workspace required; coating and developing can be undertaken under subdued household lighting conditions.
A word of caution is to highlight the differences between published sources of information on gum printing — perhaps due to the very flexibility that gum printing offers. There is no right way to print gum — different approaches and methods arise from the different results individual practioners seek.
The flexibility of gum means that you can choose to make graphic images in one colour, or you can seek to make three-colour images, or perhaps you prefer (as I do) to create layers of multiple colour on the one image. All these approaches are valid, however each will influence your working methods in different ways — the key is to find the working method that reflects your individual vision.
The information contained on these pages details my way of working, which has evolved from ten years working with gum. It is not the only way to print gum, but it is the way that works for me and the printmaking outcomes I desire. Your needs may be different and you should explore and investigate as much material as you need. I have suggested a variety of sites to visit and the resources section lists suggested books and other sources of information.
Gum printing variables
The skill in making gum prints is learning how to combine a variety of variables, which range from sourcing an appropriate choice of paper (or alternative support) and its preparation, through to the combination of gum and pigment, sensitiser proportion and exposure and development. These numerous variables can lead to frustration — but often the potential for prints of exquisite beauty. Patience and persistence will deliver results.
The sections below detail the key variables in gum printing. Understanding them will assist you to master the process.
Working with historical processes gives you the freedom to work with a broad range of papers as well as other supports such as fabric, glass and metals. My preference is to work with paper, and gum can be printed on a variety of paper surfaces. Key points to consider in choosing paper include;
- Single coated gum prints usually lack depth and contrast, with many people choosing to work with multiple coatings — this requires papers that can handle a significant degree of physical handling and multiple soakings
- Papers need a degree of dimensional stability to enable accurate registration between coats.
- As the gum emulsion is hardened from the top down the chosen paper requires enough 'tooth' to allow the hardened gum and pigment to adhere to the paper
Gum printing in common with most historical processes is a contact printing process, requiring that the negative is the same size as the final print. Unless you work with a camera producing an 8" x 10" or larger negative, working with gum will necessitate producing an enlarged negative the size of the desired finished print. There are several routes to making enlarged negatives, either working with traditional wet darkroom materials, or using a digital workflow.
Pigments & gum arabic
The colour in gum prints comes from the use of pigment which is combined with the gum arabic. Traditionally watercolour paints are used, though dry pigments can also be used. Some practioners also work with inks or gouache paints.
Gum arabic, a thick viscous syrup, can either be made from dry gum arabic or it can be purchased ready-made from art supply stores. There is much debate in gum circles about the best gums to work with. It is also possible to use PVA adhesive rather than gum arabic.
Sensitiser & development
Two different dichromate sensitisers are commonly used, ammonium and potassium dichromate. Ammonium is faster and prints with less contrast, while potassium is slower and more contrasty. Further control can be achieved through the ratio of sensitiser to gum/pigment. Choice of dichromate will also be influenced by your printing lights and negatives.
The standard development method is to use 'automatic' development, which involves developing the print for 30 mins in tepid water. However physical development, which involves using brushes, water jets and other methods can be used to alter tones, brush away details or make other changes to the print.
Printing lights & exposure
Like other historical processes, gum requires a strong UV light source for exposure. Suitable light sources include the sun, fluorescent light boxes, metal halide and mercury vapour lamps. Choice of sensitiser, negatives and emulsion mix can all have an influence on the light source most suitable for your working process.