Oil painting
Supports for oil painting

The earliest oil paintings were almost all panel paintings on wood, which had been seasoned and prepared in a complicated and rather expensive process with the panel constructed from several pieces of wood, although such a support has a tendency to warp. Panels continued to be used well into the 17th century, including by Rubens, who painted several large works on wood. The artists of the Italian regions moved towards canvas in the early 16th century, led partly by a wish to paint larger images, which would have been too heavy as panels. Canvas for sails was made in Venice and so easily available and cheaper than wood.

Smaller paintings, with very fine detail, were easier to paint on a very firm surface, and wood panels or copper plates, often reused from printmaking, were often chosen for small cabinet paintings even in the 19th century. Portrait miniatures normally used very firm supports, including ivory, or stiff paper card.

Traditional artists' canvas is made from linen, but less expensive cotton fabric has been used. The artist first prepares a wooden frame called a "stretcher" or "strainer". The difference between the two names is that stretchers are slightly adjustable, while strainers are rigid and lack adjustable corner notches. The canvas is then pulled across the wooden frame and tacked or stapled tightly to the back edge. Then the artist applies a " size " to isolate the canvas from the acidic qualities of the paint. Traditionally, the canvas was coated with a layer of animal glue (modern painters will use rabbit skin glue) as the size and primed with lead white paint, sometimes with added chalk. Panels were prepared with a gesso, a mixture of glue and chalk.

Modern acrylic " gesso " is made of titanium dioxide with an acrylic binder. It is frequently used on canvas, whereas real gesso is not suitable for canvas. The artist might apply several layers of gesso, sanding each smooth after it has dried. Acrylic gesso is very difficult to sand. One manufacturer makes a "sandable" acrylic gesso, but it is intended for panels only and not canvas. It is possible to make the gesso a particular color, but most store-bought gesso is white. The gesso layer, depending on its thickness, will tend to draw the oil paint into the porous surface. Excessive or uneven gesso layers are sometimes visible in the surface of finished paintings as a change that's not from the paint.

Standard sizes for oil paintings were set in France in the 19th century. The standards were used by most artists, not only the French, as it was—and evidently still is—supported by the main suppliers of artists' materials. Size 0 (toile de 0) to size 120 (toile de 120) is divided in separate "runs" for figures (figure), landscapes (paysage) and marines (marine) that more or less preserve the diagonal. Thus a 0 figure corresponds in height with a paysage 1 and a marine 2.

Although surfaces like linoleum, wooden panel, paper, slate, pressed wood, Masonite, and cardboard have been used, the most popular surface since the 16th century has been canvas, although many artists used panel through the 17th century and beyond. Panel is more expensive, heavier, harder to transport, and prone to warp or split in poor conditions. For fine detail, however, the absolute solidity of a wooden panel has an advantage.
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