Often a wall has its strength estimated by the amount of power necessary to crush the bricks, instead of by the forces or influences that will render the mortar unfit for its purpose. The mortar should be made from the very best materials that can be obtained, as practically the strength of the mortar determines the strength of a brick structure.

The lime should always be freshly burnt, as stale lime loses the power of setting firmly. For the very best lime mortar, hydraulic lime should be used, stone or grey lime being used in cheaper mortars. Hydraulic limes should be finely ground, otherwise they are liable to slake when they have been built in the work, and the swelling which ensues will crack and spoil the wall in which they have been used. Also hydraulic lime mortars must be used immediately they are made, as they set rapidly as compared with the stone or grey lime mortars. Chalk lime should never be used for building purposes, except in small sheds where cost prohibits the employment of a better lime. Chalk limes must not be used in making mortar lor dwelling-houses. All limes before being mixed with sand should be thoroughly slaked. This is generally done by measuring out the required quantities of lime and sand, and forming with the sand a ring in which the lime is placed, water being added in sufficient quantities to slake the lime, and care being taken not to add more than is necessary.

The slaking commences by the lime absorbing the water, and the swelling of its bulk, accompanied by hissing and giving off of steam; the purer the lime the more violent is the slaking process; hydraulic limes sometimes take hours to commence, while chalk limes start immediately. The sand is shovelled over the slaking lime, and the whole mass is left for a sufficient time, after which the lime and sand are thoroughly incorporated, making the required mortar. The sand used must be free from all earthy material, pit sand being considered the best; if the sand does contain organic or clayey matter, it should be washed before use. The proportion of sand and lime used informing mortar are stated on p. 89.