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