Whatever the subject and whatever the sur face on which it is painted, it may be handled in either of two ways: either so as to recog nize the hard and impenetrable wall or vault which it adorns, or so as to ignore it and, as it were, make the spectator forget its exist ence or at least its impenetrability. In the first kind, the colors are more or less flat, the figures are formally disposed and appear to be as nearly as possible in one plane, and there is little effect of light-and-shade (chiaroscuro) or perspective. Such are the engraved paint ings on Egyptian temple-walls and columns, nearly all medival paintings and, in a measure, some of those of the modern Puvis de Cha vannes (q.v.). One feels that these are paint ings on a wall. To the second class belong many Pompeiian paintings and nearly all those of Renaissance and modern times, in which perspective, both geometric and aerial, and all the resources of realistic representation, of distance and the third dimension, combine to create an illusion of actuality and space; to "break through the wall" as the painters say. But even in such paintings a certain formality of composition and an evident recognition of the shape of the space occupied may serve to distinguish them as real mural decorations rather than easel pictures framed and hoisted into their places on wall or ceiling.
Methods and Four processes or methods of producing mural paintings have predominated in the history of the art: Fresco, Tempera (or painting in distemper), Encaustic and Oil painting.
Fresco was the process most generally em ployed in the Early Renaissance, in the Middle Ages and to some extent in antiquity. In fresco painting the pigment, mixed with water, is applied to a specially prepared surface of wet plaster, which it penetrates so that when the plaster sets the picture is incorporated in its substance and to some extent encased in a thin coating of carbonate of lime. Only so much of the picture can he painted at a time as will cover the plaster before it hardens, after which the unpainted remainder of the plaster must be cut away. Thus each day's work must be complete in itself, there can be no retouching or correction on that portion ex cept in distemper, which is less permanent. q3uon fresco° is pure fresco, untouched with tempera, and such are thegreatest works of the later masters in fresco. The mediaeval and early Renaissance painters (and probably also the Greco-Romans) used tempera retouching quite freely. But even with tempera retouching fresco-painting is a difficult process; and the work must be done in place, not in the studio, and to paint a damp ceiling while lying on one's back is no easy task. Oil painting has al most completely driven out this most permanent and monumental style of painting, in which most of the greatest works of the Italian Re naissance were executed. See PAINTING.
Tempera, or painting in distemper, is a process of painting on dry plaster with pig ments carried by some slightly glutinous me dium, like white of egg or honey, or gum, mixed with water. The work must be done in place, like fresco, but is easier of execution and permits of retouching and correction ad libitum. It is far less permanent than fresco, being likely to scale, peel or powder off. It was, however, much used in antiquity, and the so called ((frescoing° or painted decoration on walls and ceilings of ordinary modern houses and apartments is done in distemper.
Encaustic is painting on a surface prepared with wax, and employs either distemper pig ments •which after painting are covered with a coat of melted wax, or pigments mixed with melted wax on a hot palette. It is a difficult and meticulous process, which was used by the Romans for certain details of their mural a coat of melted wax, or pigments mixed with occasionally by modern artists, F. C. Crownin shield among them, but has no decided advan tage over oils sufficient to counterbalance its difficulties.
Oil painting, which employs linseed oil as the carrying medium of the pigment and tur pentine as a dryer, appears to have been first developed in Flanders in the 15th century and thence to have been carried to Italy, where it finally supplanted all other processes, first for easel pictures and altar-pieces, and later, in the hands of the great Venetian painters, for mural decorations of the greatest magnificence. Oil painting has many advantages; it can be executed in the studio, upon wood, copper or canvas, the latter being usually employed; it is the most brilliant of all kinds of painting and lends itself to the greatest variety of technic, and can be indefinitely corrected and retouched. For these reasons it is the preferred process of nearly all modern muralists. Its disadvan tages are that it cannot be executed directly on plaster, even specially prepared for it, with any prospect of permanence; and execution in the studio often leads to disappointing results when the work is in place; moreover it is liable to darken with age and to suffer from deteriora tion of the canvas. Yet many of the great Venetian masterpieces are as brilliant to-day or nearly so — as when painted over 300 years ago. Sometimes the painted canvas, instead of being mounted on a stretcher, is °frilled° to the wall by the use of thick white lead and oil. This method is called °Tnarouflage° by the French. It can only be used upon walls abso lutely protected from dampness.