Materials through the ages: Brush strokes - paint

Materials World magazine
,
29 Oct 2012
Sistine chapel

Maria Felice examines how the materials used for painting have changed over the millennia.

Over the centuries paint, along with lots of talent and hard work, has produced many fantastic images. One of the most splendid examples is the ceiling of the Sistine Chapel in Rome, painted by Michelangelo between 1508 and 1512, making it 500 years old this year. The motivations behind paintings have changed over the millennia from story-telling in caves to portraits in stately homes and from religious paintings in churches to graffiti on garage doors. Materials used are sometimes so closely linked to a particular time in history – either because of preference or because of availability – that the age of a newly discovered piece of art is determined by studying the materials it is made from.

The pigment or dye is what gives paint its colour, and the only other essential component is the binder, also called the vehicle, which makes the paint stick and influences its gloss and durability. Solvent is volatile and does not become part of the paint film – but it is needed to dissolve the various substances and to adjust the viscosity of the paint. Finally, various additives, such as biocides to fight bacterial growth, can be included.

History
The earliest known use of paint dates back around 40,000 years to the cave paintings of southern Europe. The naturally occurring pigments used include ochre, haematite, manganese oxide and charcoal. Ochre was commonly used as a pigment, and its name is still used today to describe colours. It is derived from naturally tinted clay and chemically is hydrated iron oxide, or in geological terms – limonite.

Some 35,000 years later, the Ancient Egyptians took pigmentation a step further, mixing colours with an adhesive and applying them separately from each other. A good example of the use of colour in Egyptian culture is the walls at Dendera, on the west bank of the Nile in central Egypt, which are still brightly coloured despite 2,000 years of exposure to the elements.

Tempera paints were common in Ancient Egypt. The binder is often made from egg yolk and the result is a long-lasting colour and finish. As well as egg yolk and pigment, another material, such as water and vinegar, is needed because egg yolk cracks when dry. Milk paint, made from milk, lime and pigment, has been used for thousands of years and might be making a comeback because of its nontoxicity. However, once mixed it needs to be used quickly because of milk’s short shelf life.

Watercolour paints use a binder that is watersoluble, and it’s likely that a basic version might have been used in cave paintings, but its popularity as an artistic medium began in the Renaissance. Since the 19th Century, the binder used has been gum arabic – the hardened sap from acacia trees. (The Sistine Chapel ceiling was painted using the fresco technique that involves applying water-based paint to wet plaster – because of the plaster, no binder is needed.) Gouache is similar to watercolour paints, but benefits from increased opacity. An inert white pigment, such as chalk, is added to make the paint opaque and the ratio of pigment to binder is higher than in and the ratio of pigment to binder is higher than in watercolour. It dates back to the Middle Ages, when it was used to illuminate manuscripts.

Oil paints were developed outside Europe, possibly in Afghanistan, approximately 2,800 years ago, using walnut or poppy seed oil as the binder. They then travelled west, and since the Renaissance have been the most popular paint for fine art pieces. The oil used is typically linseed. Recent advances in chemistry have produced water-miscible oil paints that make cleaning up easier – this is achieved by altering the molecular structure of the oil.

In the 1940s, acrylic paints were produced by suspending pigment in an acrylic polymer emulsion. Once applied, they are water resistant. Acrylic paints dry very quickly, although glycol or glycerin can be added as acrylic retarders to allow time for blending and highlighting.

 

Green alternatives
Materials for painting have come full circle from natural to synthetic, and back again as awareness has increased of the risks some synthetic materials present to the environment and human health.

The pungent smell of a newly painted room will go away after a few days, but harm could still be being done. Organic compounds (VOCs), which are present in many paints (commonly in the solvent) react with other elements to produce ozone, and so can be harmful to the environment. Only half of the VOCs present are emitted in the fi rst year of the paint being applied.

Paints with few or no VOCs do exist, but these still have chemicals in them. Natural paints are available, which have safer and biodegradable ingredients, but are not suitable for all applications. For example d-limonene is a natural solvent from citrus fruits that is produced by distilling the oil that is extracted after pressing the fruit peel. Coloured clay and other natural ingredients are also making a comeback in paints for children to play with.

Graffiti artists are known to use aerosol paints – first made by Edward Seymour in 1949 who added paint to aerosol can technology, at his wife Bonnie’s suggestion. The paint and propellant components are regulated, for example by limiting the amount of hydrocarbons, but a better, quirkier alternative for graffiti artists is to use moss paint. Beer or buttermilk and a bit of sugar is added to moss, blended and then applied to the wall. Frequent misting with water is recommended to keep the artwork alive.

What’s in a colour?
The extraordinary blue of ultramarine, obtained by grinding lapis lazuli, can be seen in paint as early as the 6th Century in Afghanistan and the 10th and 11th Centuries in China and India. Its colour is thanks to electron transfer between the sulphur anions present that requires an intense absorption of light at a wavelength of about 617nm. This is the red end of the visible light spectrum, so the resulting colour that we see is very blue. In the early 1800s a more economic synthetic alternative was developed, made using pure clay with silica and alumina in the ratio SiO2:Al2O3 (kaolin), anhydrous sodium sulphate Na2SO4, anhydrous sodium carbonate Na2CO3, powdered sulphur and powdered charcoal.