This is a somewhat vague term commonly used in the description of
bricks. By general agreement it is recognised that a brick which is to have a moderately good compressive strength, reasonable resistance to saturation by rainwater and sufficient resistance to the disruptive action of frost should be hard burned. Without some experience in the handling, and of the behaviour, of bricks in general it is very difficult to determine whether or not a particular brick is hard burned.
A method of testing for hardness is to hold the brick in one hand and give it a light tap with a hammer. The sound caused by the blow should be a dull ringing tone and not a dull thud. Obviously different types of brick will, when tapped, give off different sorts of sound and a brick that gives off a dull sound when struck may possibly be hard burned.