Besides the colour, the importance of which for facing bricks we have already emphasised, a good brick should be homogeneous, without fissures or flaws, not frost-cracked, easily cut and very resisting.
Homogeneousness is tested by superficial appearance and also by fracture, which should be clean-cut and show a fine close grain without sign of crumbling away.
The non-liability to frost-cracking is an important quality, for without it a building would suffer from the weather. This quality is approximately tested by measuring the quantity of water absorbed by the dried bricks; this should vary from I 2 to 16 per cent, of their weight; but this measurement is only an indication, and so is the method of impregnating the brick with a saturated solution of sulphate of soda in order to see if it loses small fragments, which would show its liability to crack.
Direct experiment is the most conclusive; it consists in plunging several bricks into water, and afterwards subjecting them to a temperature of several degrees below zero (Essai officiel des terres cuites, p. 381).
Ease of cutting is useful in order that the mason may be able to cut the brick without difficulty to fill up gaps or to complete a layer, etc.
The resistance is proportional to the hardness of the bricks. It is recognised by the clear sound which should be given when they are struck together. This resistance is measured directly by making the bricks support greater and greater weights until they are crushed or break, according as we attempt their crushing or fracture. In buildings bricks tend rather to become crushed.
According to experiments which have been made since fairly remote times, a well-fired brick will be crushed under a weight varying from 110 to 150 kilogrammes per square centimetre. This weight is reduced to 90 or even to 60 kilogrammes if the brick is insufficiently fired. These numbers are mere indications, and vary considerably with the manner in which the bricks have been made and the degree to which they have been baked. In practice, bricks are only subjected to the tenth part of the pressure which would crush them.
The effort required to produce rupture is very much less; subject to the remarks made above, the pressure has been found to be on an average 30 kilogrammes per square centimetre.
The density of bricks is also a sign of good quality, because well-blended clays give bricks of very close grain, very homogeneous, and consequently of great density. But here again we must have recourse to direct experiment, for there are certainly bricks which differ in weight and are yet as good in quality. The weight of a well-fired and dry brick of dimensions .22 x.11 x .06 to .07 varies from 2 1/2 to 3 kilogrammes. For the official tests of bricks see the end of Part I., p. 377.