Where possible these should be placed outside of the house and as near to the sink as may be; and, vice versa, the sink should be placed on the wall of the kitchen nearest to the grease-trap. It is usual to construct such traps of brick and cement. This is, however, objectionable, both on account of the sharp corners resulting from such a construction, and on account of its liability to develop cracks and leakage in the joints of the brickwork through irregular settlement. Moreover, the mason seldom possesses the requisite knowledge to give the trap the best possible form in all its parts, and a poorly formed grease-trap is rather the rule than the exception when it is constructed of masonry.

Much the best material for grease-traps is glazed earthenware, and the best form, in the writer's opinion, is that shown in Fig. 66. It is made in sections, the lower forming the trap proper, and the upper the extension pipe, to bring the trap below the reach of frost. One or more extension pieces should be used according to the depth of the frost line in any given locality. The clean-out cap sets in the flange of the extension pipe as shown, and may be made of cast iron with a lifting ring in the centre. The size of the trap should depend upon the size of the establishment which it serves and the length of time it is designed to hold the grease in the intervals between cleaning out. The earthenware traps range from eighteen to thirty inches in diameter. The inlet and outlet openings should be large enough to receive four-inch iron pipes. The trap should be ventilated by Y branches in one or both of these pipes, according to circumstances, depending upon the system of sewerage and house drainage adopted. The inlet pipe should enter about six inches above the outlet pipe in order to give a slight fall to the entering water and room for the floating grease to rise somewhat without obstructing the flow. The parts should be carefully put together with good cement, except at the junction of the iron top with the earthenware, where rubber may be used, or sand in a groove may receive a downward projecting flange or ring on the iron.

Fig. 06. - Earthenware Grease-trap.