199. Mortar, unless very thin, will not adhere to a dry, porous brick, because the brick robs the mortar of its moisture, and therefore prevents the proper setting. On this account, brick should never be laid dry, and in very hot, dry weather the bricks should be as wet as possible. When porous bricks are used this is of great importance in obtaining a strong wall.

200. Brickwork should never be laid in lime mortar when the thermometer is below 32 degrees, as freezing lime mortar unfits it for useful purposes, and work can never be done as economically in cold weather as at other times. Lime mortar is damaged when it alternately freezes and thaws. The sun shining on one side of the wall may cause the mortar to disintegrate, while that on the other side will be frozen. This may cause serious damage by causing the wall to buckle. In constructing large buildings in winter, if one-fifth cement is added to the lime mortar, it will not be damaged by freezing. It is necessary that the surface of the brick be clean and free from frost, snow, and ice, when they are laid, or the mortar will not adhere to them.

Sometimes salt is mixed with the mortar to prevent freezing, but it is undesirable, as it usually causes efflorescence, or the white deposit often seen spread over a wall.

Portland cement mortar and Portland cement and lime mortars are not affected by changes in temperature.