Medium and Message: Anatomy of paintings
Before I delve into painting media, it’s helpful to establish the terms I’m going to use. Although at their simplest paintings consist of a fairly flat and even surface with a layer of paint on it, in practice most have a more complex anatomy.

Earliest surviving paintings, such as these from around thirty-six millennia ago, in the Chauvet-Pont d’Arc Cave in the Ardèche, were simple applications of pigment to a stone surface. It’s a miracle that any have survived, given the lack of bonding between pigment and stone.

Fast-forward over thirty-five thousand years, and in 1570-72 Giorgio Vasari created something similar in his painting of Perseus and Andromeda consisting of pigment bound in a drying oil, applied to slate. The big difference is in his paint, which was developed over several centuries to form a strong bonded layer adhered to the surface of the rock.
Stone and walls make a robust support for a painting, but aren’t exactly portable unless they’re as small as Vasari’s sheet of slate. Alternatives were constructed from wood in panels, and could be painted on an easel rather than up a scaffold, hence these portable works of art are widely known as easel paintings, and use wood panel supports.
The next step in development was to apply a primer to the wooden panel to form a smooth surface to which the paint layer would bond better, forming the ground.

When Leonardo da Vinci abandoned his Adoration of the Magi as he left Florence in about 1482, he had applied little paint to its ground, on which he had already made extensive underdrawings and prepared tonal modelling for the paint layer he never completed.
Our easel painting now has a physical support, a purely mechanical task, and on that the ground on which the paint is applied. Those roles are distinct, even when a single surface such as a wall or sheet of paper fulfils both. Most traditional paintings by professional painters made in the last few centuries have been on supports of stretched canvas, which have been sized to protect their fibres, then had a white or tinted ground such as chalk bound to them.
Support and ground form the receiver, on which colour in the form of pigments has to be bound. Although it’s possible to get pigment to adhere to a suitably rough ground, part of the principle behind painting in pastels, the result isn’t durable, and clients and patrons are likely to be wary about paying for a painting that literally crumbles into dust in front of their eyes.

Not all paints contain pigments: some prefer soluble dyes instead. Generally dyes, often derived from plants, aren’t as durable. Pigments consist of large insoluble particles, as seen in the cross-section above, to protect them from physical damage, chemical reactions, and most importantly the adverse effects of exposure to light, which can cause their intensity to fade.
The goal of many paint systems is to trap these pigment particles in a solid layer formed by a chemical binder which is liquid when applied but hardens by the chemical process of polymerisation into a solid. In the case of oil paints, the binder is an oil that undergoes slow oxidation to form the polymer, a drying oil such as linseed obtained from the common flax plant.
The final component involved in a painting is one that should vanish during the process of applying the paint: a diluent or solvent used to thin the paint, and clean wet paint from brushes and the other tools used in the process. Diluents are often confused with binders, but they are usually opposites: an ideal diluent should evaporate quickly, leaving no residues and a robust if thin paint layer behind, for the binder to turn it into a strong, enduring and faithful record of what the artist intended.
In traditional oil painting, typical diluents are organic solvents such as turpentine and white spirit, used to spread a thin layer of drying oil binder and pigment particles. Remove most of the drying oil and use largely diluent, though, as in peinture à l’essence, used by Degas, and you can end up without any proper paint layer at all, with powdery pigment trying not to fall off.

Degas’ Danseuse dans sa loge (Dancer in her Dressing Room) (c 1879) is one of his experimental paintings using both pastel and peinture à l’essence applied to canvas.
In this respect, watercolours are strangely named. Oil paints rely on drying oils as their binder, but in watercolours the water is the diluent not the binder, which is gum arabic; many other painting methods also rely on water as diluent, but aren’t called watercolour as a result.
We finally end up with a rigid support, on which is the ground, that provides an adherent base for the paint layer, composed of pigment particles in a binder, that was originally thinned using a diluent. In most older paintings, those are then coated with layers of varnish intended to protect the surface of the paint layer.

Just like Giorgio Vasari with his oil painting on slate, there’s always someone who breaks conventions. This painting of Dandenongs from Heidelberg was made by Charles Conder on a sheet of wood from a cigar box. Conder was a member of a group of Australian artists who held a ‘9 by 5’ exhibition of paintings they had made on wooden cigar-box panels of 9 by 5 inches (23 x 13 cm) in size. Similar panels had been used extensively by Georges Seurat for his oil sketches, for which he had invented the term croquetons. They also resembled the small wood panels used by the Macchiaioli for their plein air oil sketches.
In thirty-six thousand years we have progressed from painting the walls of caves to sketching on a wooden cigar box.
Key terms
- easel painting, on a portable support rather than a wall or ceiling;
- support, a more or less rigid surface for a painting;
- ground, a priming layer applied to the support to form an adherent base for paint;
- paint layer, consisting of pigment particles in a binder, thinned during painting with a diluent;
- varnish, to protect the surface of the paint layer.