Sand which is not washed and which contains a deal of clay in it feels
soft and smooth when held in the hand, hence the term soft sand. Sand which is clean feels coarse in the hand, hence the term sharp. These are terms used by craftsmen. When soft sand is used, the mortar is very smooth and plastic and it is much easier to spread and to bed the bricks in than a mortar made of sharp or clean sand.
Naturally the bricklayer prefers to use a mortar made with soft or unwashed sand, often called ‘builders’ sand’. A good washed sand for mortar should, if clenched in the hand, leave no trace of yellow clay stains on the palm.