Oil paintings

By Bettina Ebert, Paintings Conservator and Researcher, Asiarta Foundation

Traditional oil paintings on canvas are a multi-component system consisting of many different materials combined together in a layer structure. This usually consists of an auxiliary support, support, ground and paint layers, and a varnish layer.

Example of a strainer

The auxiliary support is usually a wooden frame onto which the canvas is stretched. This holds the canvas under tension, thus reducing movement of the fabric. There are two common types of auxiliary support. Strainers are rigid frames which have fixed corners that are not expandable. Stretchers are frames which have expandable corners that allow adjustment of tension.

Fabric has commonly been used as a painting support for many centuries in Europe. Religious paintings with extensive gilded backgrounds became less popular over time. With the rise of realistic oil painting techniques, it was therefore no longer necessary to paint on a rigid support.  In Venice, one of the centres of painting in Europe around the 1500s, textiles were soon favoured as a painting support. This was due to the fact that the damp conditions in the city often caused extensive moisture damage to wall paintings. Numerous other practical reasons favoured the increased popularity of canvas as a painting support. For example, textiles are easy to transport as the flexibility of the material allows it to be rolled. The cost of creating paintings was greatly reduced with canvas, as less technical skill was required in the preparation of the textile support over a wooden support. Several pieces of fabric could simply be stitched together to create a larger support, allowing for large works of art. These days, canvas is much more easily available for the artist than a high quality wooden support, as it can be bought in all sizes ready for use in art supply shops.

A simple woven fabric

When making fabric, the intercellular matter in fibres is removed. The fibres are then carded, combed and spun. Thread or yarn is made by twisting fibres together into a strand. These can then be woven into fabric. In a woven piece of fabric, warp is the term given to the lengthwise threads which are held parallel under tension in a loom during the weaving process. The weft refers to the thread which is passed in and out among the warp threads to create the woven fabric. The finished edge of a piece of fabric created by the weft threads is referred to as the selvedge.

Historically, linen was favoured as a painting support, perhaps because the flax fibre was the most important vegetable fibre used in textiles. Occasionally, there is evidence of the use of hemp or jute, though this tended to be of coarser quality. With the import of cheap cotton from the Americas in the 18th century, mechanised weaving of cotton yarn was developed towards the end of the century. It was only 50 years later that machine-woven canvases began to be produced. Due to its cheapness, cotton canvas was soon introduced around 1900, and began to be used commonly by the 1950s.

Linen is made from the bast fibres of the flax plant, Linum usitatissimum, and consists of about 70-80% cellulose. Individual fibres are 25 to 150 cm in length. Cotton, on the other hand, is made from the seed hair of the cotton plant Gossypium, and has a 90% cellulose content. Cotton fibres are much shorter, typically 2 to 3 cm in length, leading to a weaker fabric. Polyester is a synthetic polymer, polyethylene terephthalate (PET). Despite the fact that synthetic fabrics have become extremely popular in the clothing industry, this has not been the case with polyester as a painting support.

Fabric is subjected to tension, or load, when it is stretched onto an auxiliary support. However, fabric is also elastic, which means that it is able to regain its original shape and size after the removal of tension that led to its deformation. As a result of cellulose degradation, canvas exhibits rapid and extensive strength loss due to chain scission. In general, it is possible to estimate that the life expectancy of a canvas painting is around 200 years. After this time, it may require treatment to provide additional structural support. In addition to strength loss, there are colour changes which accompany the degradation process of canvas paintings. This is usually a darkening and yellowing, and is more rapid in cotton than linen. This may be problematic as the white colour associated with cotton has occasionally been utilized by the artist, and any darkening will alter the tonal relationships in the painting.

Water stains on a canvas

The cellulose that makes up cotton and linen canvases is an organic compound of the formula (C₆H₁₀O₅)_n . Cellulose is a straight-chain polymer consisting of glucose units linked together, and has a high molecular weight. Cellulose consists of many hydroxyl groups, which give it a high affinity for water. Cellulose absorbs moisture from the atmosphere up to the equilibrium point. The hygroscopic nature of cellulose fibres results in significant interaction with moisture from the environment. Cellulose responds to an increase in moisture by swelling and expanding as it absorbs moisture in order to reach its equilibrium moisture content at a given relative humidity.

When a woven textile is subjected to a great increase in moisture, it shrinks despite swelling of the cellulose. This results from an increase in crimp within the fabric, which is a result of its woven nature. As the individual yarns swell, the opposing fibres have to travel a greater path around the swollen fibres. In order to accommodate for this, the weft is pulled closer together, leading to fabric shrinkage. Generally, the more tightly woven the textile and the greater the crimp, the greater will be the shrinkage of the textile in response to increased moisture. Therefore, it can be generalised that commercially prepared artists’ canvases from the 19th century onwards have a high shrinkage potential. Nevertheless, moisture, when used prudently, may be used successfully in treating canvas paintings. For example, some of the defects that may be treated with moisture include cupping of the paint layer, planar distortions of the textile, consolidation of flaking paint, regeneration of the size layer, corner distortions resulting from extensive keying out, and plastic deformation of the canvas in the form of bulges or other distortions.

Canvas swells and shrinks differentially from other layers in the painting, and its mechanical properties change as a result of changes in moisture content. This may lead to shear and tensile stresses, cracking and delamination of paint. Cellulose degrades by means of oxidative or hydrolytic processes. It is relatively stable to auto-oxidation, but the primary hydroxyl groups can be progressively oxidised to carboxylic acid groups, especially under the influence of light. Even a small amount of oxidation or hydrolysis which results in chain breaking has a disproportionate effect on physical properties, since only one such break per chain halves the degree of polymerization. Cellulose is hydrolysed by aqueous acids at random points along the chain to give smaller cellulose units and then to glucose itself. On ageing, canvas becomes brittle and discolours, and loss of strength occurs fairly rapidly.

In order to act as an adequate support for painting, canvas requires stretching and, due to its absorbent nature, preparation with layers of size and ground. Size is a penetrating liquid that is employed to fill the porous surface of the support, isolate coatings and make surfaces suitable to receive coatings. It usually consists of some form of animal glue or proteinaceous material. Sizing seals the canvas by penetrating into the fabric, thus reducing the absorbency of the textile. This prevents the canvas from soaking up the adhesive from the subsequent ground layers. Oil paint should never come into direct contact with the cellulose fibre, or the textile support will become brittle and degrade from interaction with the acids in the oil medium.

Animal glue is a proteinaceous material. Collagen is a fibrous structural protein of the connective tissue in animals and fish. This includes the skin, muscle, bone and hide, and is traditionally used to make glue. When boiled in water, collagen is hydrolyzed irreversibly and turns into gelatin. Gelatin is a mixture of peptides and proteins produced by partial hydrolysis of collagen. In water, gelatin forms a high viscosity solution which sets into a gel on cooling.

Size applied to a canvas is very responsive to moisture, swelling or shrinking depending on the relative humidity. In humid environments, the size layer can also be a breeding ground for mould or fungus. Gelatin is generally stable to photo-oxidation. Most of the effects noted in relation to the size layer are the result of its interaction with moisture. Hence, atmospheric moisture in the form of relative humidity has the greatest impact. Generally, the size layer softens or gels in high humidity, while embrittlement is an issue in low humidity. The response of the size layer to changes in humidity can cause cracking in the paint layers above, which may not be able to contract or expand to the same degree as the size layer.

The terms ground, primer, priming, and gesso all refer to a pigmented preparatory layer that is applied to painting supports. With the increased popularity of canvas over wood as a support, oil-based grounds gained in popularity, since they are less brittle than the traditional gesso (animal glue and chalk or gypsum) used on wooden supports. A traditional oil ground consists of a drying oil such as linseed, together with pigment, often lead white. Nowadays, artists can choose to prime their own canvas, or purchase commercially primed canvas. Usually, modern grounds consist of acrylic emulsion, oil or alkyd and titanium white, as well as fillers such as calcium carbonate.

The ground supplies a colour and uniform texture with sufficient tooth for the subsequent application of paint. It also acts as the intermediate structural layer between the support and paint, and provides the necessary degree of absorbency. The ground has a significant effect on the resulting texture and colour of the finished painting. Grounds can be coloured or white, depending on the desired effect.

Paint consists of pigments suspended in a binding medium. Additionally, there may be extenders, which are cheap pigments usually with a low refractive index that add bulk to the paint, as well as drying agents or siccatives, which modify the drying time of the paint. Tube paints generally also contain fillers, extenders and stabilizers. For example, waxes such as stearates and palmitates are often found. In the past, egg tempera was traditionally used, while modern paints include oil, PVA, alkyds or acrylics. More information on modern paintings is available under the section on the conservation of acrylic paintings.

Artist's pigments

Pigments may derive from mineral, vegetable, animal, or synthetic sources. Earth pigments include the ochres, umbers and sienas. Some natural pigments are ultramarine, malachite, azurite and orpiment. Examples of manufactured inorganic or mineral pigments are cadmium yellow, lead white, vermillion and zinc white. Organic pigments that are of vegetable origin include gamboge, sap green, indigo, madder lake and saffron, while examples of organic pigments of animal origin are Indian yellow and cochineal. Modern synthetic organic pigments have been manufactured, and include phthalocyanine blue, alizarin and Hansa yellow among others. Some examples of modern inorganic pigments are titanium white and cadmium red. Lakes are pigments that are made by precipitating a dye upon an almost colourless pigment such as alumina hydrate, clay or barytes.

Pigments are chosen by artists based on many different factors. This may include their refractive index in a given medium, which has an impact on the opacity or translucency of the paint. Tinting strength or hiding power varies between pigments, as does their lightfastness or chemical stability. Some pigments may be incompatible with certain binding media or other pigments. Light has an effect on many pigments, initiating chemical changes or causing fading as a result of photo-oxidation, which reduces the extensive conjugation in organic molecules, leading to the formation of smaller colourless molecules. Pollutants such as sulphur dioxide can also affect pigments.

An artist's palettes

Oil paint incorporates drying oils such as linseed, poppyseed and walnut oils. Oils are glycerides of fatty acids; drying oils have at least 65% polyunsaturated fatty acids. Their C=C double bonds enable polymerisation and oxidation reactions which lead to the formation of a three-dimensional cross-linked network. Oxygen, light and metallic salts act as catalysts for the reactions, and the drying process is initiated by free radical formation. Oxidation reactions are also involved in the degradation of oil paint. Acidic degradation products are formed, which increase the acidity of the oil. An excess of free fatty acids leads to their eventual migration to the surface, where they are occasionally visible as bloom. Yellowing is the result of the formation of chromophores. As oil paint ages, it hardens and loses flexibility, leading to embrittlement and cracking. Moisture or high humidity leads to the hydrolytic breakdown of bonds, which results in the gradual weakening of the paint film.

The following articles provide more information on conservation of acrylic paintings, lacquer paintings and paper.