Mortar, unless very thin, will not adhere to a dry, porous brick, because the brick robs the mortar of its moisture, which prevents its proper setting. On this account brick should never be laid dry, except in freezing weather, and in hot, dry weather it is impossible to get the bricks too wet. When using very porous brick the wetting of the brick is of more consequence in obtaining a strong wall than any other condition. As wetting the bricks greatly increases their weight and consequently the labor of handling them, besides making it harder on the hands, masons do not like to wet them unless they are obliged to, and it should always be specified and insisted upon by the superintendent, except in freezing weather.
Pressed brick cannot very well be laid dry, and the masons generally wet them for their own convenience, but they will often tell all sorts of stories to escape wetting the common brick.
Brickwork should never be laid when the temperature is below 320, and if it is below 400 and liable to fall below 320 at night, salt should be mixed with the mortar and the bricks heated before laying, and the top of the wall covered with boards and straw at night. It is much better not to lay brick in freezing weather unless the delay occasioned involves a great loss. In building large buildings in the winter time one-third Portland cement should be added to the mortar, then it will not be damaged by freezing. It is necessary that the surface of the bricks be clean and free from frost, snow or ice, when they are laid, otherwise the mortar will not adhere to them.
If the mortar in the upper courses becomes frozen over night, those courses should be taken down and the bricks thoroughly cleaned before being used again. For the effect of freezing on mortar see Section 139.
The excessive wetting of a wall is also injurious, as it takes a long time for the wall to dry out, and it is likely never to dry to a uniform color. For this reason the top of the wall should always be protected at night, or when leaving off work, by boards placed so as to shed the water.