_Plastering._—The qualities of internal plaster depend upon the construction of the wall, the methods of application of the plaster, and the quality of the plastering material. [Illustration: Scratch coat is for bonding; brown coat for plasticity; finished coat for appearance] The walls and ceiling to which plaster is to be applied must be so constructed as to be practically rigid under the loads that they will carry. Since plaster is not elastic, any slight change in shape of the surface will cause it to crack. The common backings which are satisfactory for plastering are wood lath, metal lath, and masonry, such as concrete, terra-cotta tile, brick, plaster board, etc. Wood lath makes the least rigid back of all, and for this reason is not considered the best, although it is the cheapest. Unless the wood laths are wet before the plaster is applied, they will absorb the moisture from the plaster and swell, thus cracking the wall. Metal lath for this reason is superior. Masonry walls should be made rough to give the necessary key for the plaster to cling to. In brick walls the joints are raked out, in concrete walls the surface is picked, and the outside of terra-cotta tile is marked with grooves for this purpose. The best results in plaster are secured with three coats. The first coat is called the scratch coat, and is intended to form a bond between the wall itself and the plaster. It should be pressed into the apertures between the lath to secure a good bonding key, and its surface should be scratched with a tool to give the required bond between it and the next coat, or brown coat. The brown coat forms the main body of the plaster and averages about ¾ inch to ⅞ inch thick. The finished coat is then added on top of this and is intended to develop a plane surface with the desired color. Each coat should be allowed to dry out and then be wet before the next one is added. If wood lath is used, this drying and wetting will cause the lath to shrink and swell, so that cracks will be developed in the scratch and brown coats. These should be filled in before the finished coat is added. The materials which should be used in the various coats depend upon the requirements which are necessary for each one. As the most important characteristic of the scratch coat is strength, and that of the brown plasticity, and the final coat appearance, the materials must be proportioned accordingly. SCRATCH-COAT PROPORTIONS Hydrated lime 133 parts by weight Sand 400 “ “ Hair 1 part “ BROWN COAT Hydrated lime 100 parts “ Sand 400 “ “ Hair ½ part “