A binder is the substance that holds pigment particles together and adheres them to a surface. From animal glue to linseed oil to acrylic, learn binder history and types.
In the Stone Age caves of southern France and northern Spain, painters mixed ochre with animal fat to make paint. The fat was the binder. It held the ochre particles together and stuck them to the cave wall. Forty thousand years later, some of that paint is still there. The pigment has not faded because iron oxide is chemically stable. The binder has not disappeared because animal fat, in the dry, stable environment of a cave, can last for millennia. This simple combination of pigment and binder is the foundation of all painting. Every paint ever made, from the caves to the latest acrylics, consists of pigment plus binder. Change the binder, and you change everything about how the paint works, how it looks, and how long it lasts.
In painting, a binder (also called a medium) is the material that holds colored pigment particles together to form a cohesive paint film and acts as a glue to bind the paint to the surface. The binder is the non-volatile component of the paint vehicle. It is what remains in the dried paint film after the solvent has evaporated or the oil has oxidized. Without a binder, pigment is just colored dust. It will not stick to anything, it will not form a film, and it will not survive handling. The binder is what makes paint paint.
This entry covers the history of binders from prehistoric animal fat to modern synthetic polymers, the different types of binders used in painting, how binders work, and why the binder is often the most important factor in the longevity of a painting.
A binder is the component of paint that physically holds the other components together. Along with pigment, it is one of the two absolutely necessary components of any paint. Solvents, fillers, and other additives may be present, but they are optional. Pigment and binder are not.
The binder works by surrounding each pigment particle and forming a continuous film as it dries or cures. This film serves two functions: it holds the pigment particles together in a coherent layer, and it adheres that layer to the painting surface. The way the binder forms this film depends on its chemical nature. Some binders dry by physical drying, where a solvent evaporates and leaves the solid binder behind. Others dry by chemical drying, where smaller molecules combine through a chemical reaction (such as oxidation) to form larger, more solid molecules. Some binders, like beeswax, are heated for application and harden as they cool.
The National Gallery London's glossary defines binder simply: "In paintings, a binder (or medium) is the material that holds the coloured pigments together to form paint. In paintings at the National Gallery, this is often a drying oil, but may also be egg yolk or animal glue."
The choice of binder affects every aspect of the paint. It determines the working properties (how the paint flows, how fast it dries, whether it can be reactivated). It determines the optical properties (how transparent or opaque the paint appears, what surface quality it has when dry). It determines the aging properties (whether the paint yellows, cracks, or becomes brittle over time). And it determines the conservation requirements (what solvents can be used to clean or remove the paint).
Animal glue is probably the most widely used binder throughout human history. It is also the most easily obtained, made by boiling the connective tissues or skins of animals. Glues made from local animals were major media in ancient Egyptian painting, in Chinese and Japanese painting, and in many other cultures throughout world history. In Europe, animal glue was the binder for distemper and glue-size painting, and it was the binder for the gesso grounds used on panel paintings. Rabbit skin glue, in particular, was the standard size for canvas preparation until the 20th century.
Animal glue is a protein-based binder. It dries by physical drying, as the water in the glue evaporates and the protein molecules form a hard, brittle film. Animal glue is water-soluble, which means it can be reactivated with water. This is useful for making gesso that can be cleaned up with water, but it also means that paintings bound with animal glue are vulnerable to humidity and water damage.
Egg yolk was the primary binder for European panel painting from the Middle Ages until the early 16th century. Egg yolk is an emulsion of water and oil, containing proteins and lipids. It dries by a combination of physical drying (water evaporation) and chemical drying (oxidation of the lipids and polymerization of the proteins). The result is a hard, durable, water-resistant film that is less soluble than animal glue.
Egg tempera paint dries quickly and cannot be reactivated once dry. It is built up in many thin layers of small, cross-hatched strokes. The surface quality of egg tempera is distinctive: smooth, matte, and luminous, with a slight satin sheen. Italian Renaissance painters from Giotto to Botticelli used egg tempera as their primary medium. Botticelli's The Birth of Venus (c. 1485, Uffizi, Florence) is an egg tempera painting on panel. See our entry on tempera for details.
The technique of painting with drying oils emerged in Europe in the late Middle Ages. Linseed oil, extracted from flax seeds, was the primary oil used, at least in Europe, because of its light color and availability. For pure whites or particularly light blues, painters also used walnut oil or poppy oil, which yellow less than linseed oil. Around 1450, painting with pure oil paints developed, and by the 16th century, oil had replaced egg tempera as the primary binder for easel painting in Europe.
Drying oils work by chemical drying. The oil reacts with atmospheric oxygen in a process called oxidation and polymerization. The oil molecules cross-link, forming a solid, durable, water-resistant film. This process is slow, taking days to weeks depending on the thickness of the paint and environmental conditions. The slow drying time is one of the main advantages of oil paint: it allows the painter to blend colors on the surface for extended periods. See our entries on linseed oil and oil painting for details.
The Cambridge University Press article on binders in paintings notes that "availability and tradition can influence the choice of binders made by artists." In many cases, a variety of natural binders were available, and additional factors influenced the choice. Different media have highly variable properties that affect how they are used: solubility, transparency, handling, and drying time all vary depending on the binder.
Gum arabic, a natural gum from the acacia tree, has been used as a binder for watercolor and ink since ancient times. It is water-soluble, dries by physical drying (water evaporation), and can be reactivated with water. Gum tragacanth is used as a binder for pastel. These natural gums are still used today in commercial watercolor and pastel production. See our entries on watercolor and pastel for details.
Beeswax was used as a binder in antiquity. The encaustic technique, in which pigment is mixed with molten wax and applied hot, was used for the Fayum mummy portraits in Egypt (1st-3rd centuries AD). Wax dries by cooling, not by evaporation or oxidation. The resulting paint film is stable, water-resistant, and luminous. Encaustic fell out of use for centuries but was revived in the 20th century by artists like Jasper Johns. See our entry on encaustic for details.
Since the middle of the 20th century, synthetic binders have been on the market and have become increasingly popular. Acrylic polymer, introduced for artist use in the 1950s, is the most significant. Acrylic binder dries by physical drying: the water in the acrylic dispersion evaporates, and the acrylic particles coalesce into a continuous, flexible, water-resistant film. Acrylic paint dries quickly (minutes to hours), is water-soluble when wet, and becomes water-resistant when dry. It is flexible, which means it can be applied to flexible supports like canvas without cracking. See our entry on acrylic painting for details.
Other synthetic binders include PVA (polyvinyl acetate), used in some commercial paints and adhesives, and alkyd resins, used in modern oil primers and some painting media. The Romoe Network's guide to binders provides an overview of both historical and synthetic binders.
Joe Singer, in his book How to Paint Portraits in Pastel, wrote that "it is often the binder and not the pigment that is the main cause for the deterioration of paintings, especially oil." This is a critical observation. Pigments, particularly inorganic ones, can remain chemically stable for centuries. But binders age. Linseed oil yellows over decades, becoming darker and more transparent. Animal glue becomes brittle and can crack. Egg tempera is remarkably stable but can be damaged by moisture. Acrylic is too new to have a long-term track record, though conservation scientists are studying its aging behavior.
The binder also determines what conservation treatments are possible. Oil paintings can be cleaned with solvents that dissolve the varnish without dissolving the oil binder. Watercolor paintings cannot be cleaned with water, because the gum arabic binder is water-soluble. Egg tempera is resistant to water but sensitive to some solvents. The binder is what conservators work around, and understanding the binder is the first step in any conservation treatment.
The binder is one of the two essential components of paint, the other being pigment. The binder is the non-volatile component of the vehicle, the liquid that carries the pigment. For specific binders, see our entries on linseed oil (oil paint binder), egg tempera, watercolor (gum arabic binder), acrylic painting, encaustic (wax binder), and casein (milk protein binder). The binder is applied to a ground on a support. For more on how binders affect painting practice, read our post on oil painting techniques.
You can see the effect of different binders by comparing paintings in different media in any major museum. The smooth, luminous, matte surface of an Italian Renaissance egg tempera painting at the National Gallery in London is visibly different from the glossy, blendable surface of a Baroque oil painting in the same collection. The transparent, luminous washes of a watercolor at the Tate are a product of the gum arabic binder, which allows the pigment to sit on the paper surface in a thin, even film. The flat, matte, plastic-like surface of an acrylic painting at the Museum of Modern Art is a product of the acrylic polymer binder.
For more on the materials of painting, read our entries on pigment and vehicle, or explore our post on oil painting techniques to see how the oil binder affects the painting process.
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