J. Laffan Fine Art Conservation

Professional Conservation and Restoration of Fine Art and Antiques, Ireland

Oil Painting Structure Explained

Support

First there is the support consisting of canvas, linen, wood panel, sheet metal usually copper or zinc. Canvas will lose its strength after time particularly around the edges of paintings. This is the main reason that paintings are lined with new canvas. This new canvas was held in place traditionally with animal glue or paste. More modern methods use beeswax and resin, synthetic adhesive wax, or Beva 371.

Ground

The second layer is the ground consisting of chalk bound traditionally with animal glue size. Rubbed down between coats a fine surface can be achieved especially when the last coats contain red or grey clays. This gives a good keyed semi porous surface. With changes in temperature and humidity the ground layer forms fine cracking, this is a distinctive feature of old oil paintings. The ground can separate from the support, this is usually the result of extremes of humidity or of heat drying the glues completely. This can cause flaking of ground and the paint on top, to remedy this problem in paintings on canvas they are lined as the adhesives used will penetrate to the ground under pressure.

Paint

Oil paint is a suspension of granulated pigments in linseed oil. The oil hardens slowly, first oxidizing at the surface to form a skin. After six months to a year a painting will be hard enough to varnish. After 30 to 40 years the bonding is tough enough to resist weaker solvents, after 80 to 100 years it is resistant to stronger solvents. This is a generalization as a lot will depend on the particular pigments and types of oil used.

Varnish

Varnish is applied to oil paintings to protect them but the main reason is to replace the characteristic wet look as oil paint dries. The varnishes used are natural resins dissolved in distilled turpentine oil. The resins release the solvent slowly, but eventually these resins also dry out and change from clear to yellow brown. After 25 to 50 years the varnish layer will have to be replaced.

Matt varnish was popular in the 1960s and 70s, it is still used by some people, it usually contains wax in the matting agent which hardens and turns white. A less glossy surface is often achieved with a spray gun, this also leaves problems, as the surface is formed from tiny particles piling up like miniature loose snow. The solvents will dry faster, the open surface traps dust which is harder to remove.