Pigments and gum are crucial elements in creating successful gum prints and are a common topic of discussion in gum printing circles. It is after all the hardened gum, combined with pigment that forms the image on the paper. This is therefore an area to which gum printers need to invest some time and research.
Traditionally gum printers have used either watercolour paints or dry pigments. My own choice has been to work with watercolour paints — I like the convenience and ease of use - so my comments are only relevant to those who chose to work this way. A key element to working with watercolour paints is to understand their characteristics — to be familiar with permanency and lightfastness, as well as the relative transparency and strength of different pigments.
An excellent source of information on pigments and watercolour paints is Bruce MacEvoy's online guide handprint.com, I also recommend you view the detailed section on pigments at Katharine Thayer's website.
Some basic pointers on choosing watercolour paints
- Buy artist grade paints — avoid cheaper student grades, which often use inferior materials and can lead to pigment staining of your paper
- If you want your prints to last you need to know how permanent and lightfast your pigments are. Don't trust the manufacturers ratings though, use an independent source such as handprint.com
- Aim to understand the different levels of transparency between paints; for example yellow ochre tends to be more opaque than burnt sienna. This is useful knowledge when you consider the order in which you choose to print different colours. Katharine Thayer provides an excellent discussion on the characteristics of watercolour paints on her website
- Start with just a few primary colour pigments, reds, blues, yellows, and greens — as you become more experienced you can broaden the selection of pigments you use.
Combining pigment & gum
The proportion of pigment to gum is a vital control in gum printing — too much pigment and you will 'stain' your paper, creating a veiling of the highlights with pigment; too little pigment however will produce weak washed out colour.
To understand more about suitable levels of pigment concentration you can utilise the infamous pigment test — described in 'The Keepers of Light' and other publications. There is much debate in gum circles as to how relevant this test is as it does not include dichromate and is not exposed and hardened on the paper. However what this test will allow you to do is to become familiar with the relative strength of your chosen pigments, i.e. how much do you need to provide the strength of colour that you want without causing staining of the paper.
Essentially the test involves combining pigment and gum at a reasonably high concentration to begin with, and then adding progressively more gum to the mix. At each change of gum concentration a small amount of the gum/pigment mixture is brushed on the chosen printing paper. Once you have made a suitable number of step changes in the gum dilution of the pigment, the paper is immersed in water (as you do when you develop gum prints) for 30 mins. At the end of the test the strongest concentration of gum/pigment that washes out completely from the paper will indicate the highest pigment to gum ratio you can use for that gum/pigment/paper combination without staining.
There is no hard and fast rule for how you should measure your pigment - some weigh their pigment, other squeeze a 'worm' of pigment to a certain length, and some mix by eye. Weighing pigment offers the most precision and repeatable results.
Gum arabic is the sap from the acacia tree, which when combined with water produces a thick viscous colloidal syrup. Gum arabic can be purchased either dry or in pre-mixed liquid form. Over the years I have used a variety of pre-mixed gum arabic with success. The current one that I use, made by Roberson & Co Ltd in the UK is made with Kordofan grade gum arabic without any additional additives (apart from a preservative - which needs to be added if you make your own).
An alternative to gum arabic is to use a PVA adhesive such as 'Gloy'. This is combined in the same way with pigment as the gum arabic. Working details for it use are included in 'Spirits of Salts' by Randell Webb and Martin Reed.