a mix containing Barium sulfate to make the walls impervious to x-rays. Moldings Plain, or unenriched, moldings are formed with a running mold of zinc cut to the required profile a process that has remained the same for over 200 years. For a cornice molding two running rules are usual, one on the wall, the other on the ceiling, upon which the mold is worked to and from by one workman, while another man roughly lays on the plaster to the shape of the molding. The miters at the angles are finished off with joint rules made of sheet steel of various lengths, three or four inches (102 mm) wide, and about one-eighth inch thick, with one end cut to an angle of about 30°. In some cases the steel plate is let into a stock or handle of hardwood. Enrichments may be moldings added after the main outline molding is set, and are cast in molds made of gelatin or plaster of Paris. Cracks Cracks in plastering may be caused by settlement of the building, by the use of inferior materials or by bad workmanship. However, due to none of these, cracks may yet ensue by the too fast drying of the work, caused through the laying of plaster on dry walls which suck from the composition the moisture required to enable it to set, by the application of external heat or the heat of the sun, by the laying of a coat upon one which has not properly set, the cracking in this case being caused by unequal contraction, or by the use of too small a proportion of sand. In older properties, hairline cracks in plastered ceilings can occur due to minor deflection / movement of timber joists which support the floor above. Traditionally, crack propagation was arrested by stirring chopped horsehair thoroughly into the plaster mix. Slabs finished with a thin setting coat or slab. In some cases, with fireproof ceilings, for instance, the metal lathing are hung up with wire hangers so as to allow a space of several inches between the soffit of the concrete floor and the ceiling. For partitions metal laths are grouted in with semi-fluid plaster. Where very great strength is required, the work may be reinforced by small iron rods through the slabs. This forms a very strong and rigid partition which is at the same time fire-resisting and of