The basic structure of an oil painting on canvas

1. The Stretcher is designed to expand by virtue of the corner joint design and the use of wedges which can be hammered in as the canvas stretches and warps over time.

2. The Canvas is usually linen or flax (later cotton) which has been primed with glue,(rabbit skin or later PVA etc.) which reduces its absorbency in preparation for the primer.

3. The Primer is the ground most suitable for the painting medium. Gesso was used extensively up until Victorian times having the advantage of being capable of filling the texture of the canvas for a smooth finish – lead white pigment was used later and can achieve a similar finish and is ideal for the reception of oil paint which binds well with it. More recently acrylic substitutes have been developed with the advantage of faster drying times.

4. The Painting medium and the method of application used by the artist will determine the longevity of the art work. Most techniques involve layers of pigment thinned in a variety of ways to achieve the desired effect. Artists have always experimented and often use materials which deteriorate (like bitumen) or cause reactions with other chemicals.

5. Varnish can be used as a final coat which brings all the previous layers together tonally and as a protection, or as a way of separating some layers – known as retouching varnish. Turner often used a layer of varnish to allow the application of watercolour for fine details (rigging of ships) after which another layer would be applied, then more oil glazes before the final coat. Resin suspended in turpentine is the most common form although there are many variants. Often after the painting had reached a ripe age and accumulated surface dirt a way of "freshening up" the image would be to apply another similar varnish coat, thus most paintings of any age will have several layers of dirt trapped between varnish layers.

The illustration left shows what can be achieved when an oil painting has been severely neglected. The canvas support had been torn in several places and around them large areas of primer and pigment were lost.

The first stage to save such a situation from continued deterioration is the application of a "facing" tissue which is applied to the surface of the painting to protect it and to temporarily hold any loose pigment areas or canvas tears in place.

The painting is then taken off its stretcher and laid down onto the lining table. A heat sensitive glue is pasted onto the old canvas from behind and a new canvas is bonded onto the old, suspended on its own stretcher or loom.

When heat is applied to the new canvas, the "glue" passes through the canvas layers into the primer and finally into the pigment layers securing the entire structure. Further consolidation of tears and areas of primer and paint loss is then administered from the front.

The materials used have been developed specifically for this purpose and are well tested for longevity.

Most oil paintings of any age have at some time been lined and as techniques have progressed old linings must sometimes be replaced with new.

The "tempera" painting opposite by John Duncan measures 15 feet by 10 feet (4.52 mtrs by 3.048 mtrs). It had been hanging in a very damp, disused school in Edinburgh for many years and had deteriorated as a result. The logistics of moving an object of such a size demanded that the painting was "faced", removed from its stretcher and rolled onto a specially manufactured 3-foot diameter cardboard tube in order to move it through an 8-foot door on its way to the restoration studio.

The painting was "strip-lined", surface dirt removed, flaking paint consolidated, retouching varnish applied, retouched and varnished. The restoration work took 5 months after which it was rolled onto its tube, transported to its final destination, restretched, a new frame fitted then hung by a team from Simon Blackwood Fine Arts.

John Duncan "Children's Allegory" tempera on canvas. Now at the Gillis Centre Edinburgh.