Painting on canvas became common in the 16th century, as aforementioned, and has been used largely in European and American painting traditions. A canvas support expands and contracts with variations in relative humidity, but the effect is not as drastic as with wood. Canvas, however, will deteriorate with age and acidic conditions and may be easily torn. In many cases, parts of the paint and ground will lift from the surface, a condition variously called “cleavage,” “flaking,” “blistering,” or “scaling.” The traditional method to address these problems is to reinforce the back of the canvas by attaching a new canvas to the old in a process called “lining,” also referred to as “relining.” A number of techniques and adhesives have been employed for lining, but with all methods there is a risk of altering the surface texture of the painting if the procedure is not carried out with the utmost care and skill. The most frequently used technique until the mid-20th century consisted of ironing a new canvas to the old, using an adhesive composed of a warm mixture of animal glue and a farinaceous paste, sometimes with the addition of a small proportion of plasticizer. This method, though less common today, is still used, especially in Italy and France. It has the advantage that the heat and moisture help to flatten raised (“cupped”) paint and local deformations and tears in the canvas. Another method, introduced after the mid-19th century, uses a thermoplastic wax-resin mixture. Originally executed with heated irons as in the glue-paste method, it increased in popularity by the introduction, around 1950, of the so-called “vacuum hot table.”

With this table, the two canvases are coated with molten adhesive (at about 160 °F [70 °C]) and joined together on an electrically heated metal plate. They are covered with a membrane, enabling the air between the two canvases to be evacuated with a pump through holes at the corners of the table; adhesion then occurs on cooling. Excessive vacuum pressure and heat can drastically alter a painting’s texture. In addition, during this process, wax penetration can darken canvas and thin or porous paint layers. To overcome this latter defect, “heat-seal” adhesives were introduced in the late 1960s. Formulations containing synthetic resins, including polyvinyl acetate and, increasingly, an ethylene-vinyl acetate copolymer, are applied in solution or dispersion to the surfaces and, after drying, are adhered on the hot table. Ethylene-vinyl acetate copolymer adhesives are also available as dry, nonpenetrating films. More recently, cold-setting polymer dispersions in water have been introduced by using a low-pressure suction table, from which the water is removed through spaced perforations in the table surface with a powerful downdraft of air. Pressure-sensitive adhesives have also been introduced as lining adhesives but have not been widely adopted. Although all these methods are currently in use, the trend has been to move away from lining and wholesale treatments in general in favour of more refined, precise, and limited treatments that address condition problems in a more specific way.

The low-pressure suction table mentioned above and a smaller device used for localized treatment generally referred to as a “suction plate” have gained wide use at the turn of the 21st century. The more elaborate versions of this instrument are equipped with heating elements and humidification systems beneath the perforated table surface. These features make it possible to apply controlled humidity, heat, and gentle pressure to perform a variety of treatments, including tear realignment and repair, reduction of planar deformations, and the introduction of consolidating adhesives to reattach cleaving paint. The practice of edge lining (sometimes referred to as “strip lining”), which has been increasingly used as an alternative to overall lining, aims to reinforce weak and torn edges where the canvas is prone to give way. This treatment is often used in conjunction with local or overall treatments executed by using the suction table and suction plate.

In the past, paintings have occasionally been transferred from wood to canvas by a variant of the treatments described above. The reverse of this—i.e., attaching a painting on canvas to a stable rigid support (a process known as “marouflage”)—is still sometimes done for various reasons.

The ground (i.e., the inert paint layer covering the support below the painting itself) can ordinarily be regarded for conservation purposes as part of the painting layers. Occasionally, the ground may lose its adhesion to either the support or the paint layers, or the ground may fracture internally, resulting in cleavage and paint loss.

The paint layers themselves are subject to a number of maladies as a result of natural decay, faulty original technique, unsuitable conditions, ill treatment, and improper earlier restorations. It must be remembered that, whereas housepaint usually has to be renewed every few years, the paint of easel paintings is required to survive indefinitely and may be already 600 years old. The most prevalent defect is cleavage. If the loss is not total, the paint can be secured, according to circumstances, with a dilute protein adhesive such as gelatin or sturgeon glue, a synthetic polymer, or a wax adhesive. The paint is usually coaxed into place with an electrically heated spatula or a micro hot-air tool.

As painting materials became more readily available in commercial preparations in the 18th and 19th centuries, systematic methods of painting that were once passed from master to apprentice were replaced by greater individual experimentation, which in some cases led to faulty technique. Artists sometimes used too much oil, leading to ineradicable wrinkling, or they superimposed layers that dried at different rates, producing a wide craquelure as a result of unequal shrinkage, a phenomenon that occurred increasingly as the 19th century progressed because of the use of a brown pigment called “bitumen.” Bituminous paints never dry completely, producing a surface effect resembling crocodile skin. These defects cannot be cured and can be visually ameliorated only by judicious retouching.

A notable defect arising from aging is the fading or changing of the original pigments by excessive light. Although this is more evident with thin-layer paintings, such as watercolours, it is also visible in oil paintings. The palette of the earlier painters was, in general, stable to light; however, some of the pigments used, notably the “lakes,” which consisted of vegetable dyestuffs mordanted onto translucent inert materials, often faded easily. Copper resinate, a transparent green much used from the 15th to the 18th century, became a deep chocolate brown after prolonged exposure to light. After the discovery of synthetic dyestuffs in 1856, a further series of pigments were created, some of which were later discovered to fade rapidly. Unfortunately, it is impossible to restore the original colour, and in this case conservation, in its true sense of arresting decay, is important; i.e., to limit the light to the lowest possible level consistent with adequate viewing—in practice about 15 lumens per square foot (15 foot-candles; 150 lux). Ultraviolet light, the most damaging kind of light, which comes from daylight and fluorescent fixtures, can and must be filtered out in order to avoid damage.

Almost every painting of any degree of antiquity will have losses and damages, and a painting of earlier than the 19th century in perfect condition will usually be an object of special interest. Before a more conscientious approach to restoration became general in the mid-20th century, areas of paintings that had a number of small losses were often—indeed, generally—entirely repainted. It was considered normal in any case to repaint not only losses or gravely damaged areas but also a wide area of surrounding original paint, often with materials that would visibly darken or fade with time. Large areas with significant detail missing were often repainted inventively in what was supposed to be the style of the original artist. It is customary nowadays to inpaint only the actual missing areas, matching carefully the artist’s technique and paint texture. Some restorers adopt various methods of inpainting in which the surrounding original paint is not imitated completely. The inpainting is done in a colour or with a texture intended to eliminate the shock of seeing a completely lost area without actually deceiving the observer. The aim in inpainting is always to use pigments and mediums that do not change with time and might be easily removed in any future treatment. Various stable, modern resins are employed in place of oil paint to ease reversibility and to avoid discoloration. Exact imitation of the original entails close study of the painter’s technique, especially the multilayer methods, since the successive layers, being partly translucent, contribute to the final visual effect. Minute details of texture, brushstrokes, and craquelure must also be simulated.

A variety of natural resins, sometimes mixed with drying oil or other constituents, have been used to varnish paintings. Although the traditional use of varnish was partly to protect the paint from accidental damage and abrasion, its main purpose was aesthetic: to saturate and intensify the colours and to give the surface a unified appearance. Mastic and damar, the most commonly used natural resins, are subject to deterioration. Their chief limitations are that they become brittle, yellow, and less soluble with age. In most cases a discoloured varnish may be safely removed by using organic solvent mixtures or other cleaning agents, but the process is very delicate and may cause significant physical and aesthetic harm to the painting when it is done improperly. Some paintings exhibit a greater sensitivity to cleaning than others, and some varnishes may be unusually intractable owing to their formulation. In addition, many organic solvents are known to leach components of the medium from oil paint. For these reasons, cleaning should be carried out only by an experienced professional, and the frequency of the procedure should be kept to an absolute minimum.

When the varnish is in good condition but covered with grime, the conservator may, after close inspection, clean the surface with aqueous solutions of nonionic detergents or mild solvents. Choice of solvent mixture and mode of application has always depended on the skill and experience of the conservator, but modern scientific theory has clarified the procedures. Synthetic resins have been widely adopted for use as picture varnishes. They are chosen for chemical stability with regard to light and atmosphere so that they can eventually be removed by safe solvents and will not rapidly discolour or physically deteriorate. Acrylic copolymers and polycyclohexanones have been the most commonly used since the 1960s. The synthetic varnish resins may be broadly divided into two classes of high-molecular-weight and low-molecular-weight resins. The high-molecular-weight resins are judged by many conservators to lack the desirable aesthetic and handling characteristics that are found in natural resins. Low-molecular-weight resins approach the appearance and behaviour of natural resins more closely and are currently receiving more attention. Recently introduced varnishes based on hydrogenated hydrocarbon styrene and methyl styrene resin hold promise as substitutes for natural resins. Research continues, however, in order to find the “ideal” varnish, combining ease of application, chemical stability, and an acceptable aesthetic quality. Paintings that are varnished, contrary to the intention of the artist, can become permanently altered in appearance over time and become diminished in value. In the last quarter of the 19th century, certain artists, most notably the Impressionists and Post-Impressionists, began to eschew the use of varnish.