but if it is present in too large a proportion the work will crack in setting. The hard cements used for plastering, such as Parian, Keene's, and Martin's, are laid generally in two coats, the first of cement and sand 1/2 to 3/4 inches thick, the second or setting coat of neat cement about 1/8 inch thick. These and similar cements have gypsum as a base, to which a certain proportion of another substance, such as alum, borax or carbonate of soda, is added, and the whole baked or calcined at a low temperature. The plaster they contain causes them to set quickly with a very hard smooth surface, which may be painted or papered within a few hours of its being finished. In Australia, plaster or cement render that is applied to external brickwork on dwellings or commercial buildings can be one or two coats. In two coat render a base coat is applied with a common mix of 4 parts sand to one part cement and one part dehydrated lime and water to make a consistent mortar. Render is applied using a hawk and trowel and pushed on about 12 mm thick to begin. For two coat, some plasterers apply two full depth bands of render (one at the base of the wall and one around chest height) which are screeded plumb and square and allowed to dry while applying the first coat over the remaining exposed wall. The render is then scratched to provide a key for the second coat. This method allows the rest of the wall to be rendered and screeded off without the need to continually check if the second coat is plumb. Alternatively, both coats can be applied with the plasterer using a t-bar to screed the final coat until it is plumb, straight and square. The first method is generally used where quality of finish is at a premium. The second method is quicker but can be several millimetres out of plumb. The second coat can be a slightly weaker mix 5/1/1, or the same as the base coat with maybe a water- proofer in the mix added to the water to minimize efflorescence (rising of salts). Some plasterers used lime putty in second coat instead of dehydrated lime in the render. The mortar is applied to about 5 mm thick and when the render hardens is screeded off straight. A wood