A really first-class rubber (see p. 104) will not be easily scored by a knife even in the centre, and the finger will make no impression upon it.
Method of distinguishing Clamp-burnt, Kiln-burnt, and Machine-made Brides. - In clamp-burnt bricks the traces of the breeze mixed with the clay can generally be seen.
Kiln-burnt bricks very often have light and dark stripes upon their aides, caused by their being arranged while burning with intervals between them. Where the brick is exposed it is burnt to a light colour; where it rests upon or against other bricks it is dark.
In some cases care is taken to prevent this, and the best kiln-burnt bricks are of an uniform colour.
Machine-made bricks may generally easily be distinguished, if wire-cut, by the marks of the wires; if moulded, by the peculiar form of the mould, letters on the surface, etc., or sometimes by having a frog on both sides.
In many cases the marks made by pronged forks, used for hacking the bricks, may be seen on their sides.