Canvas appears to be the most suitable material for pictures of a large size. There are, however, exceptions, even among painters, who habitually used canvas for their large works. Thus Rubens' Magi in the Antwerp Museum, one of the largest as well as one of the most admired compositions of that master, is executed on an enormous wooden panel formed by several transverse sections. Rubens' choice of a panel in this case and in others is not a mere chance. It may be laid down as an axiom that everything counts in the execution of a picture, and the support is not the least important factor in the final appearance of the work. It is an even more important factor in its preservation. Too much stress cannot, therefore, be laid on the importance of seeing that the support of the painting is of the best quality.
The history of painting shows so many examples of fine works which have perished that we must take every possible precaution to prevent damage to those which may be painted in the future. If wood is used, it must be carefully seasoned, thoroughly dry and cut radially, as otherwise it will be affected by changes of temperature and the picture will warp or split. If canvas is chosen, it should be of hemp, or better still of pure, unbleached flax fibre, without any admixture of cotton; this will prevent its being too liable to suffer from changes of temperature. It will easily be understood that if the tissue is constantly expanding and con tracting the priming or the paint itself will be likely to scale off.
Preparation of the is, however, another important matter to consider; the wood or canvas may be treated with a priming or used without. The latter method, which is the simplest, may be dealt with first without regard to chronology; for though it was occasionally used by old masters, such as Rem brandt and Van Goyen, it is in the main a modern practice. Many present-day painters prefer to dispense with priming, and this often gives good results, especially in painting on wood. If canvas is used the method has certain disadvantages, for contact with the oil renders the canvas brittle, and it thus loses the flexibility which is one of its principal merits.
Canvas, therefore, needs to be covered with priming before it is used, and the priming is, of course, an important factor. It has been the subject of extensive research from the time when it was first used. The basis of all priming is either size or casein mixed with plaster or carbonate of lime, or else a thin coating of white lead over a coating of size. These simple elements can be com
bined in a great variety of ways. Experiments with different kinds of priming have been tried at all periods of the history of paint ing. The Primitives used plaster and carbonate of lime for tempera painting, and this practice was continued long after the invention of oil painting. It was principally in the i7th century that it became customary to use white lead, more or less strongly tinted with grey, red or brown. A transitional method, which the present writer regards as excellent, was to cover the canvas with a thin coat of carbonate of lime, and then to complete the priming with white lead. Later, painters contented themselves with sizing the canvas and then covering it with one or two thin coatings of white lead. This process has become almost universal at the present day, but some painters are tending to return to plaster priming. It should never be forgotten that the principal quality which should be possessed by the priming of canvases is flexibility. Consequently the first coating of size which is placed over the canvas in order to fill up the interstices in the fabric must not be too thick, and if subsequent coats of carbonate of lime or white lead, or both of those materials in succession, are put on, they also should be very thinly applied. If the canvas is to dry well, it is better to apply several thin coats of priming than one thick one. It is extremely important to make sure that the canvas has dried properly. This is not always done with sufficient care before a picture is begun, and yet nothing is more dangerous than to use a canvas which has not dried properly. The dark colours will inevitably crack sooner or later. Industrially prepared canvases are often prepared without paying pre-consideration to the artist's requirements and so expose the paintings to accidents of this kind. This can be proved by a very simple experiment ; take a piece of wood or canvas, and cover it with a coat of white lead ; then, instead of waiting several months, as would be necessary if a picture were to be painted, paint on it patches of brown, madder lake, ultramarine and emerald green. These colours will rapidly crack; the first cracks will be noticeable on the emerald green after a few days only.