Cutting Edge: Philip Shaw

Middlesex University has harnessed ancient wisdom to solve the problem of supplying plant dyes for screen printing inks

In the autumn of 1994, I found myself facing the prospect of yet another year of diminished funding. The screen printing area for which I had responsibility was becoming increasingly difficult to run on the budget and jokingly I suggested that if the cuts continued, we might have to make our own ink. This remark turned out to be the start of six years of research leading to the application of an ancient technology to a very modern problem.

Screen printing was first introduced about a century ago. It developed during the 1950s and 1960s to a point where it began to rival lithography, particularly for poster work. For much of that time the industry used predominantly oil-based inks, requiring copious quantities of organic solvents for thinning and cleaning. The welcome introduction over the past decade of water-based screen inks has all but eliminated the use of solvents, as well as reducing the industry's reliance upon oil. So I was surprised to discover that the pigmentation of these water-based inks remained essentially "non-renewable", being largely synthesised from mineral compounds, coal tar and petroleum products. I already knew that colour could be obtained from plant material, but could plant colours be made into screen printing inks? The idea that it might be possible to develop genuinely renewable ink pigments capable of challenging the synthetic product began to take root.

This realisation led to the more precise aim of investigating the possibility of using vegetable colours to replace the "process" or "CMYK" inks used in the reproduction of photographic images. The term "CMYK" stands for cyan, a slightly greenish blue; magenta, a reddish-violet; yellow; and black. When these colours are selectively over-printed, they can be made to reproduce the appearance of the original colour photograph. The initial task therefore was to find plant colours which most closely resembled these four colours.

The literature on plant dyes proved invaluable in identifying possible sources of colour. Of the hundreds of dye plants listed, 76 were chosen as potential sources of the CMYK range and after extensive testing a short-list of 11 were subjected to further tests designed to determine both their suitability as inks and their stability in light.

The most famous plant dye, indigo, proved too dull, though its brilliance could be improved by dissolving the indigo pigment in sulphuric acid. I was relieved to discover a more light-fast and environmentally friendly form of indigo. "Maya blue", prepared by heating indigo and hydrated silica in an oven, dates from the great Mayan civilisation of the Yucatan peninsula and, remarkably, this was to prove the most effective cyan substitute.

The best source of yellow was found to be a mixture of weld and Persian berries, again historic European dye plants. Weld seeds have been found at Neolithic sites dating back 10,000 years. Madder root, another ancient dye, provided the best substitute for process magenta. Logwood, a native tree of Central America, produced excellent blacks. With the exception of logwood and indigo, these sources are all viable as crops in the UK and Europe and were all grown in the Cat Hill "Ink Garden" at Middlesex University.

The potential of plant pigments does not stop at printing inks. Researchers around the world are now working on applications ranging from ink-jet colour production to paint manufacture and specialised textile dyes.

Philip Shaw is senior lecturer at Middlesex University.