MIXING THE MORTAR. After the lime is slaked, the sand is spread evenly over the paste, and the ingredients are mixed with a shovel or hoe, a little water being added occasionally if the mortar is too stiff. The mixing should be thorough, i.e., should be continued until the mortar is of a uniform color.
To determine .whether the proportion of sand is right, hold the hoe-handle nearly horizontal and lift up a hoeful of mortar. If the mortar will not of itself slide from the hoe, it does not contain enough sand; and if a hoeful of mortar can not be thus lifted up, it contains too much sand. The brick-mason on the wall by a somewhat similar process checks the proportions of the mortar by the way in which it slips, from the trowel. If there is an excess of sand, the mortar will be "brash" or "short," and will drop from the trowel so abruptly as to make it impossible to "string out the mortar," i.e.,
to spread the mortar over several bricks by simply allowing it to flow from the trowel as the latter is drawn along. On the other hand, if there is an excess of paste, the mortar will not flow from the trowel, at least in sufficient quantity to make the joint. This method of proportioning gives a mortar that works well under the trowel, and with reasonably clean sand also a mortar of practically maximum strength.
If the sand is very fine and contains a good deal of finely pulver ized clay, the above test may be satisfied when the mortar contains too little lime; but lime paste is so cheap, and lime mortar is so weak, that a sand with any considerable amount of clay should not be used in lime mortar, since the clay is a source of weakness.