Feature

The History of Colour in Paint

Dr Garth Cambray

The tie between art and science has always been strong, with science producing chemicals and materials which can be turned into objects of beauty and value. Paint, that simple product of chemistry has an interesting history that goes back to the birth of human intelligence.

Many people say a picture is worth a thousand words. To paint a picture we require paints with colours, and herein lays a scientific catch which has made life difficult for artists in ancient times to express themselves - how to make paints colourfast.

If we look at a simple colour like red - red could easily be obtained by using blood, which gives a red colour due to the iron which is specially bound in the protein haemoglobin. However, as blood dries, the conditions surrounding these proteins change and so the shape of the protein changes and eventually, the iron is released from the state that makes it look red and instead gives a rusty brown dry blood colour. So, for an artist trying to capture the bright red of a rose, another source of colour would have to be found to carry the memory of that rose to eternity.

Chemistry has advanced in recent years to such an extent that there are now thousands of pigments that can be synthesised to give almost any colour imaginable. To use modern paint on an ancient artwork which is being restored, is however not acceptable as this alters the character and value of the painting.

This article, part of a two part series will look at aspects of the history of colour paint and how, prior to the modern chemistry driven era, people got colour in their art to last. In the next month's edition we will look at the modern science of colour and how pigments are designed and synthesised today.

Blue

Lapis lazuli

One of the most expensive colours ever produced was ultramarine blue, produced using the semi precious stone lapis lazuli. Lapis lazuli has, since early times, been mined in the mountains of Afghanistan. For hundreds of years ultramarine blue made using lapis lazuli has commanded prices pegged to that of gold. To make proper ultramarine blue, the semi precious stones are ground to a fine powder and mixed with resin, then kneaded in special linen bags. The bags are soaked in water and the pigment leaches out over a period of weeks.

Red

The colour red is another story. The not so bright, Natural Red Earth colours are relatively simple pigments. As with blood, the colour is largely given by the presence of the metal iron, as iron oxide. Depending on the natural source, various other secondary colour-giving chemicals may be present, meaning that an artist could have used Natural Red Earth colours from a number of different deposits to give a range of similar reddish brown colours. Because this is such a simple pigment, it has been used since time immemorial - as cave paintings around the world show.

Natural Red Earth colours are not however bright enough to duplicate the vivid reds found in the world. In ancient times, more complicated pigments were also produced using rudimentary chemistry. The Madder plant, Rubia tinctorum, can yield a number of different pigments which share a lot in common with the synthetic pigments of today.

Rubia tinctorum

Madder plants were harvested and the roots piled in heaps, dried, crushed, separated from the bark and then ground to a fine powder. The next step to extract colour involved a fermentation of the glycosidic (chemical with strings of glucose attached to it) precursors to yield the dye alizarin. This fermentation was a slow process and dyes were generally aged in barrels for a number of years to ensure that the fermentation was complete and maximum colour brilliance was obtained. Depending on the acidity of the paint and other additives, various shades of red could be produced.

To produce yellow, a number of different options were available. If lead and tin oxides are combined and heated, the chemical Pb₂SnO₄, can be produced. This chemical has a yellow colour, and, depending on the method of production, the colour can range in intensity from yellow, to yellow green. This is a simple pigment, and is stable for long periods of time.

Green

Malachite

The pursuit of the colour green has caused many deaths. Certain greens were made from arsenic compounds. Others from the less poisonous (but still dangerous) malachite rock deposits found around the world. Malachite is made by grinding rocks containing the chemical CuCO₃.Cu(OH)₂ and then using them as pigments. It is a relatively robust pigment and has been used all around the world in ancient artworks. The various colour pigments produced above were then mixed into binders and oils to produce paints. 

Acacia Senegal - one of the sources of gum Arabic.

Linseed oil was the most commonly used oil as it is relatively stable and not excessively expensive. Gum Arabic - the exudates of plant saps of certain Acacia trees, was often used as a binder which sets to form a surface which is hard and shiny.

Many other pigments were produced from exotic sources, such as animals, plants, precious minerals and metals, but the problem remained - colour was expensive. In next month's edition, find out how colour has been made accessible by modern chemistry.

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