Lathing nails are usually of iron, cut, wrought or cast—and in the better class of work they are galvanized to prevent rusting. Zinc nails are sometimes used, but are costly.
Materials for Plaster.—The lime principally used for internal plastering is that calcined from chalk or other nearly pure lime stone, and is known as fat, pure, chalk or rich lime. Hydraulic limes (see BRICKWORK and MORTAR) are also used by the plas terer, chiefly for external work. Perfect slaking of the calcined lime before use is very important as, if used in a partially slaked condition, it will "blow" when in position and blister the work. Lime should therefore be run as soon as the building is begun, and at least three weeks should elapse between the operation of run ning the lime and its use.
Hair is used in plaster as a binding medium, and gives tenacity to the material. Ox-hair, which is sold in three qualities, is the kind usually specified ; but horsehair, which is shorter, is some times substituted in its stead or mixed with the ox-hair in the lower qualities. Good hair should be long, strong, and free from grease and dirt, and before use must be well beaten to separate the lumps. In America, goats' hair is frequently used, though it is not so strong as ox-hair. The quantity used in good work is one pound of hair to two or three cubic feet of coarse stuff.
Manila hemp fibre has been used as a substitute for hair. As a result of experiments to ascertain its strength as compared with that of other materials, it was found that plaster slabs made with Manila hemp fibre broke at 1951b., plaster mixed with Sisal hemp at 15o1b., jute at 1451b., and goats' hair at 14421b. Another test was made in the following manner. Two barrels of mortar were made up of equal proportions of lime and sand, one containing the usual quantity of goats' hair, and the other Manila fibre. After remaining in a dry cellar for nine months the barrels were opened. It was found that the hair had been almost entirely eaten away by the action of the lime, and the mortar consequently broke up and crumbled quite easily. The mortar containing the Manila hemp, on the other hand, showed great cohesion, and required some effort to pull it apart, the hemp fibre being apparently quite uninjured. Sawdust has been used as a substitute for hair and also instead of sand as an aggregate. It will enable mortar to stand the effects of frost and rough weather. It is useful some times for heavy cornices and similar work, as it renders the mate rial light and strong. The sawdust should be used dry.
Some remarks are made on the ordinary sands for building in the articles on BRICKWORK and MORTAR. For fine plasterer's work special sands, not hitherto referred to, are used, such as silver sand, which is used when a light colour and fine texture are required. In England this fine white sand is procured chiefly from Leighton Buzzard.
Stucco is a term loosely applied to nearly all kinds of external plastering, whether composed of lime or of cement. The principal varieties of stucco are common, rough, trowelled and bastard. Common stucco for external work is usually composed of one part hydraulic lime and three parts sand. The wall should be suffi ciently rough to form a key and well wetted to prevent the moist ure being absorbed from the plaster. Portland cement is used for all but the commonest external work. White Portland cement, made in the United States and France, is very successfully used with white sand to give an excellent stucco closely resembling stone and very durable.
Rough stucco is used to imitate stonework. It is worked with a hand float covered with rough felt, which forms a sand surface on the plaster. Lines are ruled before the stuff is set to represent the joints of stonework. Trowelled stucco, the finishing coat of this work, consists of three parts sand to two parts fine stuff. A very fine smooth surface is produced by means of the hand float. Bastard stucco is of similar composition, but less labour is expended on it. It is laid on in two coats with a skimming float, scoured off at once, and then trowelled. Stucco may be executed in colours, the desired tints being obtained by mixing with the lime various oxides. Black and greys are obtained by using forge ashes in varying proportions, greens by green enamel, reds by using litharge or red lead, and blues by mixing oxide or carbonate of copper with the other materials.