Good building bricks should be sound, free from cracks and flaws, also from stones, or lumps of any kind.

Lumps of lime, however small, are specially dangerous; they slake when the brick is exposed to moisture, and split it to pieces.

A small proportion of lime finely divided and disseminated throughout the mass is an advantage, as it affords the flux necessary for the proper vitrification of the brick.

In examining a brick, lumps of any kind should be regarded with suspicion and tested.

Shape And Surface

In order to ensure good brickwork the bricks must be regular in shape and uniform in size.

Their arrises (or edges) should be square, straight, and sharply defined.

Their surfaces should be even, not hollow; not too smooth, or the mortar will not adhere to them.

Absorption

The proportion of water that a brick will absorb is a very good indication of its quality.

Insufficiently burnt bricks absorb a large proportion and are sure to decay in a short time.

It is generally stated in books that a good brick should not absorb more than 1/15 of its weight of water.

The absorption of average bricks is, however, generally about 1/6 of their weights, and it is only very highly vitrified bricks that take up so little as 1/13 or 1/15. (See p. 114.)

1 Spons' Illustrated Price Book.

Texture

Good bricks should be hard, and burnt so thoroughly that there is incipient vitrification all through the brick.

This may be seen by examining a fractured surface, or the surface may be tested with a knife, which will make hardly any impression upon it unless the brick is underburnt.

A brick thoroughly burnt and sound will give out a ringing sound when struck against another. A dull sound indicates a soft or shaky brick.

A well-burnt brick will be very hard, and possesses great power of resistance to compression. (See p. 115.)