Paintings

Structure of paintings

In order to discuss the possible damage to paintings and to take steps to reduce that damage, it is important to know something of the structure of paintings and the range of materials which can be used to produce them.

Paintings consist at the very least of two layers:

• the support layer on which the image layer rests—this can be canvas, wooden panelling, or Masonite; and

• the image layer—oil paint, acrylic paint or paint in combination with other materials.

If the support and the image layer are not securely bonded, then any movement in the support will damage the paint layer.

Most paintings are more complex than this and have many more parts in their structure. A traditional painting on canvas will usually have:

• a sized support—in many cases canvas sized with skin glue;

• a priming or ground layer;

• the paint or image layer;

• a varnish layer; and

• an auxiliary support which provides physical support for the support layer.

Supports

The term 'support' refers to the layer which carries or supports the paint or image layer.

Paintings can be produced on any type of support.

Traditionally, most supports have been made from linen canvas or wooden panels.

This painting of the Destitute Asylum in Adelaide has been removed from its stretcher for treatment. The canvas support can be clearly seen around the edges of the image. Photograph courtesy Artlab Australia, reproduced with permission of the Historic Trust of South Australia

This icon is painted on a wooden panel. Photograph courtesy Artlab Australia, reproduced with permission of Mr Kostya Prosylis

In the 20th century, linen canvas has often been replaced by cotton duck, and wooden panels with compressed particle board such as Masonite.

Artists are creative beings and there are a wide range of materials which have been used in the name of art! They include:

• rigid wooden supports such as particle board products like chipboard or Masonite or the traditional wooden panels;

• rigid supports made from a range of other materials such as glass or metal;

• lightweight cottons or Nylon loosely stretched, which some artists use to give a feeling of fluidity;

• paper glued onto canvas;

• canvas.

The priming and ground layers

Priming and ground layers are used to:

• provide a good physical support for the paint layer; and

• provide a surface to mask the texture of the support. If there are no priming and ground layers, it may be possible to see the texture of the support through the paint.

A good ground layer physically keys in the paint layer as it is slightly porous.

The ground layer, however, should not be very absorbent. It must be slightly resistant to the paint, otherwise brushstrokes will not be clear and will sink into the ground.

The support is sized, usually with rabbit-skin glue; and then the ground layers are applied.

Works on canvas usually have two ground layers, although having one or three is not uncommon.

If the ground layers are not well bonded to the support, then movement of the support may lead to a delamination or cracking of the ground.

In addition, if the ground layers are not properly prepared or do not provide a secure base for the paint layer-they may not be porous enough to hold the paint for example-then problems with the paint layer will occur.

A traditional ground was usually made from lead white or a chalk gesso. Acrylic grounds are now common. While grounds are generally white, some artists, John Constable for instance, favoured coloured grounds.

The layers of size and ground can be very reactive; and if they are wet they will cause severe damage to the paint layers.

The paint layer

The paint layer or image layer can be made up of paint and a number of other materials, including paper or found objects in collage.

Oil paint is the traditional paint medium, however, in more recent times synthetic materials such as acrylics and alkaloid resins are common.

Oil paint dries by evaporation, and then by a chemical crosslinking process. This means that it becomes less flexible as it ages.

The varnish layer

Varnishes are applied on top of the paint layer. They are applied as liquids and dried to produce clear, tough films.

They protect the paint layer—to a degree depending on their composition—from physical damage and chemical attack.

Varnishes also have an aesthetic function: they smooth out the unevenness of the paint surface so preventing light being scattered when it is reflected. This gives the colours in the work a more saturated appearance-the colours appear darker and have greater depth.

It is important to note that further paint layers and transparent coloured layers known as glazes may be applied over the varnish layer. This technique produces an illusion of depth.

A range of materials have been used as varnishes. Among the most stable are:

• Dammar dissolved in turpentine—this is an example of a traditional varnish made from natural resins dissolved in solvents; and

• acrylic resins dissolved in petroleum spirit.

Auxiliary supports

Traditionally, paintings on canvas have been attached to auxiliary supports—usually a stretcher or a strainer—using staples or tacks.

The purpose of the auxiliary support is to secure the canvas and keep it taut. It is important to keep the support as taut as possible—loose supports will undergo far greater dimensional change in response to fluctuations and so are much more vulnerable to damage.

A stretcher differs from a strainer in that the corners of a stretcher can be keyed out, thereby tightening the canvas.

The corners of the stretcher are adjustable, enabling the dimensions of the stretcher to be enlarged to tighten the canvas. This is done by pushing the keys further into the keyholes, and expanding the corners.

A stretcher—note the keys in the corners. Photograph courtesy of Artlab Australia

A strainer is a wooden frame which does not have adjustable corners. Therefore if the canvas becomes loose over time, it cannot be made taut again without being re-stretched-this is a job for a conservator.

A strainer. Photograph courtesy of Artlab Australia

Examples of other auxiliary supports include:

• cradles placed on the backs of panel paintings; and

• wooden frames used to secure Masonite supports.