I have read that masons should not "slush" head joints. What is slushing and why should it not be done?
Slushing is the process of shaking or throwing mortar off a trowel into the top of a head joint in an attempt to fill it. Slushing will not create a good head joint. First of all, the mortar is not placed under compression and thus is unlikely to develop adequate contact for bond. Also, in shaking or throwing the mortar off the trowel, the trowel may be struck against the top of the brick. The impact of the trowel on the brick can break the mortar bond, resulting in increased water penetration and reduced wall strength. Form head joints by placing mortar on the end of the brick and shoving the unit into position in the wall.