Painting conservation techniques

By Bettina Ebert, Paintings Conservator and Researcher, Asiarta Foundation

Below are brief descriptions of some of the conservation treatments that may be undertaken on a canvas painting.

Surface dirt removal

Dust and dirt has a tendency to collect on the backs of paintings, settling between the canvas and stretcher bars, and becoming engrained in the exposed canvas. This can be removed by gentle brushing and vacuuming. Dirt deposition on the painting surface has a tendency to give the painting a grey, dull appearance. Insect droppings react with the paint if left on the surface, etching or corroding the paint. Dirt may consist of dust from the environment, as well as particulates from pollution in the air. Deposits from nicotine usually appear yellowish.

A severely soiled painting during dirt removal

Generally, aqueous cleaning systems are very effective at dirt removal. However, certain paints and pigments may be sensitive to moisture. Water may be modified by altering its pH. Often, a higher pH is more effective at dirt removal, although it also poses a greater risk to the paint layer. The conductivity of the cleaning solution may be modified to match the conductivity of the surface to be cleaned.

Further modifications may include the addition of surfactants or detergents, as well as chelating agents. Chelators sequester metallic ions in the dirt, allowing them to be solubilised and carried away by the water. Aqueous cleaning solutions may also be gelled, allowing greater contact time between the cleaning solution and the surface to be cleaned. Surfactants are molecules with polar and non-polar ends. The polar end is attracted to water, allowing the surfactant to remain in solution. The non-polar ends are water-repellent, or hydrophobic, and have a tendency to aggregate together into micelles. Surfactants lower the surface tension of water, thus improving its wetting ability. Micelles solubilise dirt, allowing it to be washed away by water. Material that cannot be solubilised is suspended within the hydrophobic centres of the micelles, forming little dirt droplets within the water. Any modification of the cleaning system means that subsequent clearance is very important, since not all additional components are volatile and may remain on the paint surface, possibly causing adverse reactions.

Painting detail during varnish removal

Varnish removal

On ageing, varnish may become very yellow. This discolouration will alter the tonal balance within the painting, making it look significantly different from how the artist intended. This may necessate the removal of the varnish layer, and replacement with a new varnish layer that has better ageing properties. Varnish removal is usually achieved with the use of solvents. However, the use of solvents on a paint surface carries many dangers. Soluble components may be leached out of the paint film when solvents are applied to the surface. Solvents also have a significant swelling effect on paint. Some paint surfaces may be sensitive to solvents and will dissolve – acrylics are a very good example of this. Pigments vary in their sensitivity to solvents, while degraded oil paint may also be affected.

Canvas paintings are multi-component, biaxially stressed laminar composites, and all of the components within the structure will respond to different degrees to changes in relative humidity. Alteration of the moisture content leads to changes in dimension and stiffness. Environmental changes may lead to cyclic, repetitive stresses within the structure of the painting which may be severe enough to result in cracking of the paint layer directly, or through fatigue mechanisms.

Such cracks allow for direct moisture exchange between the front and back of the canvas support. Moisture may accumulate between individual paint layers, at the paint and ground interface, or between the ground and support, leading to loss of adhesion and layer separation. Moisture may soften or dissolve the size layer, while shrinkage of the canvas can result if a canvas becomes very wet.

Tented paint

Paint flakes may lift up in a tent-like shape if the support has shrunk and can no longer accommodate the paint. Flakes may also curl or lift at the edges of cracks, leading to concave flakes – this process is termed cupping. Severe degradation of the binding medium and failure of the binding agent may result in loss of adhesion – the paint becomes powdery and friable, and pigment loss can occur.

Flaking paint should be treated as soon as possible, before paint loss occurs. This is achieved by the application of a consolidant (an adhesive or binding agent) in order to readhere the flakes. Many different consolidants are available, including aqueous and solvent-based consolidants.

The application of heat and pressure may help in the readhesion of detached paint flakes, although the paint must first be tested for sensitivity.

Tear in a canvas support

Structural treatments

Planar distortions or deformations may occur in the form of bulges or dents in the support. These may be treated with the application of moisture, heat and pressure. A canvas support may also be damaged structurally in the form of tears, holes or punctures. The turnover edge of a tacking margin may split from excessive tension and a weak, deteriorated support. In the past, lining the painting (adhering it to a secondary support) was often undertaken even for small structural damages. Nowadays, alternative treatments to lining are preferred, with lining seen as a last resort for a very deteriorated canvas.

Thread by thread reweaving of the tear may be possible in certain cases. Where individual threads are lost or no longer join together, threads may be taken from the canvas edge or a new or degraded fabric of similar thread type and thickness. These threads can then be attached to the torn threads prior to reweaving under the stereo-binocular microscope. Fabric inlays may be required when there is a hole in the canvas. The right choice of adhesive and fabric is very important, as the original support may otherwise exhibit deformation where the patch is adhered.

Fabric inlay

When the tacking margins of a support are very deteriorated, strip lining may be opted for. This involves the adhesion of fabric strips to the tacking margins in order to strengthen them. In some cases, a strip lining is necessary so as to widen the narrow tacking margin of a painting that requires re-stretching, or when the tacking margin has previously been cut off, usually prior to lining.

Filling and retouching of losses

Losses to the paint layer are usually restored by filling them with putty composed of a binding agent and filler. The putty may be toned with pigments where necessary. The surface of the fill is levelled and structured to blend with the surrounding paint layers. Subsequently, retouching is undertaken, which involves applying paint to the fill so as to visually reintegrate defective areas with the remaining original paint. For this purpose, pigments are combined with a stable binding medium and applied in fine brushstrokes to the fill, matching the colour to the surrounding paint. Fills and retouchings must be reversible and should not discolour on ageing, as that would make them stand out immediately.

Filled and retouched losses, visible as dark patches in ultraviolet light