An introduction to the materials and construction of easel paintings

Paintings are made of organic and inorganic materials which are put together by an artist to create a specific image. They are of a basically simple construction consisting of an image or paint layer and a support for that layer. This, however, simplifies the fact that the structure of a painting can be very complex within these two general layers. Supports can themselves be supported. There can be additions or changes made by the artist or subsequently. There may be many complex layers of ground and paint, surface coatings, restorations, dirt, and so on. With careful observation a trained eye should be able to detect many of these elements unaided. The materials found in and on paintings are best considered layer by layer. The painting construction discussed here will be confined to easel paintings, defined as paintings not attached to an immovable object and therefore portable (albeit often with difficulty). Wall paintings and other works which form part of an architectural setting are not the subject of this paper.

Supports

The image layers of paintings are painted on any substrate which will support that paint. In general artists have used a limited number of materials to support their work and these are therefore not difficult to identify.

Canvas

This is the most common support for paintings. It has a simple weave and a strong thread and is most commonly made of linen or cotton but is now sometimes replaced by modern materials with a polyester content. The merits of a cloth support over a rigid one, that it is light and flexible and easily transported and prepared were recognised by the ancients, but it came into wide use from the 15th century when the most common material was linen. Prior to the introduction of mechanically produced close, tight weaves a hand loomed square weave of coarse single strands was used. This often produced a canvas with a very open weave. Cotton canvases came into use after the commercial production of cotton in the 19th century. At this time a smooth canvas with a diagonal or twill weave was also in favour, especially for portraits.

Linen is a cellulose material woven from fibres of the flax plant (Linum usitatissimum) and it is distinguishable from cotton in that it is has a darker linen colour (cotton is white or very pale) and by the bold character of its weave. Canvas is normally impregnated with a sealant known as size which is usually rabbit skin glue in older paintings, and can be other synthetic materials in modern paintings. This is applied to the canvas to prevent the ground or paint layer soaking into the canvas. It is also to protect the canvas, as oil paint in direct contact with the canvas will cause it to become weak and brittle. The canvas itself is generally attached to a stretcher or a strainer but may remain unsupported, or be stuck onto some sort of rigid support. Stretchers and strainers are generally made of wood (most commonly pine or ash) and usually with tongue and groove joins, mitred at the corners and bevelled away from the canvas toward the inside. The terms stretcher and strainer are often used interchangeably, but should differentiate between a framework which has no method of opening out the joins to tighten the canvas (strainer), and one which does by means of wedges or keys (stretcher). Recently new methods of creating a more even tensioning have been developed using metal inserts in the wood which enlarge the joint evenly through each member of the stretcher. Large paintings require the stretcher itself to be further supported. This is provided by cross members or cross bars.

The canvas is traditionally attached to the stretcher by tacks (copper or iron)and more recently by metal staples.

Both the canvas and the stretcher might have numerous labels which give some indication of the painting’s history. Many commercially prepared canvases (especially 19th century) bear a colourman’s mark identifying the artist’s supplier or manufacturer of prepared canvases. This can be of great help in identifying and dating the painting.

Panels

The term panel generally refers to paintings on a wooden support. Various woods have been commonly used in the support of paintings for the past 5,000 years. Generally the wood will reflect the region from which the painting came, thus poplar for Italian Renaissance paintings, oak for British paintings, pine for southern German paintings etc. This is not, however a rule and should not be relied upon, for example many 19th century European paintings are on mahogany panels. and the movement of oak for panels from the Baltic States to Holland is the subject of much research at the moment. The Dutch and Flemish panel makers also had a mark by which to identify their work. Panels are often constructed of more than one piece of wood, each plank being termed a member. These generally have the grain running in the same direction, but occasionally may have a complex construction with the grain of some members running in different directions. The way in which the wood members have been cut from the tree are of great importance for the stability of the panel. They may be either a radial cut (most stable) or a tangential cut (prone to warping). The various members of the panel are most commonly joined by a simple abutment of the planks using animal glue to adhere them, however various other methods of joining panels are used (tongue and groove, ziz-zag, overlapping half way) and these are often an indication of date and place of the panel’s construction.

Panels are often provided with an ancillary support on the reverse in order to attempt to restrain the panel from warping. Generally this has been added by a restorer or conservator but may have been applied in the first construction of the panel. This is most commonly a cradle or cradling which consists of wooden bars attached to the back of the painting running along the grain through which are inserted slats which go across the grain. These slats should be free to move, allowing for the change of size of the panel im response to changes in relative humidity.

Other supports

Many other supports for paintings have been used. Metal supports have been known since the 14th century, the most common being copper, but tin, zinc and silver are not unknown. These provide a smooth surface and give a certain amount of luminosity. They and are relatively stable and durable.

Cardboard, millboard, hardboard, have been used more usually since the 19th century but card (cardboard) is found supporting earlier paintings.

Stone (marble, slate.) is occasionally found as a support material.

Paper is a generally unsatisfactory support for oil paintings, but has been used quite commonly for oil sketches.

Composite wooden structures such as plywood and fibre board are also used as supports.

Vellum and ivory, can be found as painting supports but are relatively unusual except for portrait miniatures where they are common.

Glass and leather are also materials which have found favour with some painters.

Ground Layer

The function of a ground layer is to prepare the surface of the support for the superimposed paint layers. It also provides texture and colour for the proposed paint. Grounds are not always used by artists and in some cases the paint layer is applied directly to the support. Many different types of materials have been used for the ground layers but the most common in older paintings is gesso. This is chalk in the form of calcium carbonate or calcium sulphate bound with rabbit skin glue or lead white pigment bound in oil. Modern painters tend to use synthetic materials.

The grounds of paintings are frequently coloured. This gives tonality to the whole painting, a white ground giving a cool appearance and a red ground giving a warm one. Although these are the most common colours for grounds other colours have been used, for example a dark grey is also quite common.

Paint or Image Layer

A paint film consists of a range of organic and inorganic pigments bound in a medium which holds them together. This medium dries and hardens with time. Many different media have been used in the past and these will be discussed individually. The artist may draw or sketch out his intended design before painting. This is often done using charcoal or black paints made from pigments derived from carbon. The use of carbon has allowed the conservator to use a method termed infra-red reflectography visually to cut through the paint layers to see the underdrawing. When with time the paint becomes increasingly transparent it can allow the viewer to see this underdrawing or changes in the image. This is called a pentimento (or pentimenti if used in the plural). X-rays are also useful in determining any changes made by the artist as any metallic pigment (usually a light colour) blocks the x-ray. This allows any image made with these pigments to show on the x-ray film even though it is covered by the later paint. The texture of the paint also has an influence on the way a painting looks. Many painters have used this as part of the effect of the image. Where the paint is in high relief this texture is called impasto.

Pigment

These are finely ground coloured substance which imparts colour to the medium. They are derived either from inorganic substances, (earths, calcined earths or minerals and metals), or from organic substances, (vegetable or animal), or synthetic products. Dyes (coloured substances which can dissolve) are also used by precipitating them onto a base (usually alumina hydrate). This makes a transparent colour called a lake. This is used in glazes but is often susceptible to fading. Many pigments in used in the past were very expensive and difficult to acquire. Their history is fascinating and can be very romantic. True ultramarine for instance, is made from ground lapis lazuli and indian yellow was from the urine of cows fed on mangos in India, a practice which has been banned as it harms the cow. Red lakes come from the a secretion of the females and eggs of the cochineal beetle and dragons blood was long thought to be a mixture of dragon and elephant blood. It is, in fact, a dark resin from an eastern asian tree (Calamua draco).

Medium

By far the most common binder for pigment is oil. This was known to painters of the 14th century and earlier but was not widely adopted for use until about 1400. By the middle of the 16th century it was fully in use as the main form of paint medium. This medium leaves paintings with a well saturated rich tonality to the colours.

Oil paint is made from a drying oil, a vegetable oil which dries to a tough hard film by oxidation through absorption of oxygen from the air. The oil undergoes a series of chemical reactions which alters the physical and chemical properties of the oil. Numerous different oils are used in paints, however the most common is linseed oil made from the pressed seeds of the flax plant. Walnut and poppy seed oils are also commonly found used as paint binders.

Pigment can also be bound in other materials.

Tempera paint is an emulsion of an aqueous (water) liquid with an oily, fatty waxy or resinous substance. It is thought that tempera made with egg yolk was used as far back as the ancient Greeks but it was in general use in Italy around the time of Giotto and continued to be the main medium for easel painting until oil paint was developed. Many 15th century painters used egg tempera with the addition of oil. Egg Tempera paintings are characterised by their brilliant luminous crispness (but more pale than oil paintings) and their lack of yellowing with age. (If varnished they may appear yellowed as the varnish may have yellowed.)

Other mediums include casein, derived from milk; wax, usually beeswax; gum arabic; natural glue (size), and glair, egg white emulsion used mostly in illuminated manuscripts.

Encaustic paintings are made using hot wax as the medium.

Modern synthetic colours are now widely used. Polymer colours are made by dispersing the pigment in an acrylic emulsion.

Paintings are generally made up of several layers of paint with different colours overlaying each other. Many painters use a transparent glaze or wash layer over the paint to change the underlying colour subtly. This generally results in a very rich effect.

Varnish Layer

Varnishes are used to enhance the colours and to protect the paint from dirt and atmospheric pollutants. Their application is not universally intended by the artist (especially with some modern and impressionist painters) and a painting which was not painted to be varnished will alter significantly if varnished. This will probably be an irreversible action. Varnishes were traditionally made from natural resins such as dammar, collected from trees (Shorea and hopea) from Malaysia, Borneo, Java and Sumatra, and mastic which is taken from a Mediterranean tree (Pistachia lentiscus.)

Other natural varnishes not commonly used on easel paintings include sanderac, from the alerce tree (Calitrus quadrivalis) from North Africa; shellac, from the lac insect in India, and copal, resin of trees from the Phillipines.

Since the 1930’s synthetic varnishes have been used. Acrylic resins are now being researched which will have the same optical properties as dammar but which will not yellow as do the natural resins. Many different acrylic varnishes are in use by artists and restorers today.

Additional Layers

Further layers can be found on paintings. These will generally be varied arrangements of additional varnish, dirt, pollution, restorer’s paint, more varnish etc. It is these layers which the restorer wishes to remove when cleaning a painting. It is of course not always as simple as that. Many artists have been known to go back to a painting years later to rework the image. This layer is easily confused with the unintended additions. The employment of cross sections to analyse the layer structure of a painting is a technical tool which is of great benefit to the restorer in determining not only the original structure of the painting but also in ascertaining which layers are not original to the painting.

There are many other further analytical methods at the disposal of the conservator to determine the composition of a painting. These generally require elaborate scientific equipment but can be of great value to the conservator and owner alike in making judgments about the origin and date of the work and in determining whether additions are intended by the artist.