2. _The preparation of the Wall._ The greatest obstacle to the permanence of fresco painting is damp: hence, if the wall to be painted is covered with old mortar, the ingredients of which are unknown, this coat should be entirely removed until the solid brick or stone is laid bare. The rough coat then applied is composed of river-sand and lime, and of such thickness as is generally used in preparing the walls of dwelling-houses. The surface of this coat should be rough, but not uneven. Thus prepared, the wall should be suffered to become perfectly dry and hard; the longer it remains in this state the safer it will be, especially if the lime used was in the first instance fresh. In that case two or three years should elapse before the process of painting is commenced. The preparation and seasoning of the lime is one of the essential conditions of fresco painting. At Munich it is made and kept as follows:--A pit is filled with clean burnt limestone, which is slaked, and then stirred continually till it is reduced to an impalpable consistence. The surface having settled to a level, clean river-sand is spread over it to the depth of a foot or more, so as to exclude the air, and, lastly, the whole is covered with earth. It is allowed to remain thus for at least three years before it is used, either for the purposes of painting (lime being the white pigment) or for coating the walls. The last preparation for painting on the mortar, is as follows:--The surface is wetted with pure water, till it ceases to absorb. A thin coat of plaster is then spread over that portion only which is to be painted: the surface of this coat should be moderately rough. As soon as it begins to set (_i. e._, in about ten minutes or so, according to the temperature) a second thin coat is laid on, and the surfaces are smoothed with a wooden trowel. Some painters like to work on a perfectly smooth surface, in which case the last coat is polished by applying a piece of paper on the surface, and passing the trowel over it. When a small amount of roughness is required, a dry brush, or a piece of beaver nap attached to the trowel, is passed over the plaster in all directions. 3. _The process of Painting._ The wall being properly prepared, the outline of the figures is to be traced with a sharp point on the plaster, as before described. The artist commences his work when the surface is in such a state that it will barely receive the impression of the finger, and not so wet as to allow the colours to run or to be liable to be stirred up by the brush. If the wall has been previously well wetted, it will in general not dry too rapidly; but if in warm weather the surface becomes too hard to imbibe the colour properly, a small quantity of water is from time to time sprinkled over the surface.