usion point. In view of the fact that the critical temperature of
plasticity is below the fusion point, this is only important as an
indication from high fusion point of a high temperature of plasticity.

Hardness. This is a relative quality based on an arbitrary scale of 10
and is an indication of probable cracking and spalling.

Expansion. The lineal expansion per brick in inches. This characteristic
in conjunction with hardness is a measure of the physical movement of
the brick as affecting a mass of brickwork, such movement resulting in
cracked walls, etc. The expansion will vary between wide limits in
different brick and provided such expansion is not in excess of, say,
.05 inch in a 9-inch brick, when measured at 2600 degrees, it is not
particularly important in a properly designed furnace, though in general
the smaller the expansion the better.

Compression. The strength necessary to cause crushing of the brick at
the center of the 4-1/2 inch face by a steel block one inch square. The
compression should ordinarily be low, a suggested standard being that a
brick show signs of crushing at 7500 pounds.

Size of Nodules. The average size of flint grains when the brick is
carefully crushed. The scale of these sizes may be considered: Small,
size of anthracite rice; large, size of anthracite pea.

Ratio of Nodules. The percentage of a given volume occupied by the flint
grains. This scale may be considered: High, 90 to 100 per cent; medium,
50 to 90 per cent; low, 10 to 50 per cent.

The statement of characteristics suggested as desirable, are for arch
purposes where the hardest service is met. For side wall purposes the
compression and hardness limit may be raised considerably and the
plastic point lowered.

Aside from the physical properties by which a fire brick is judged, it
is sometimes customary to require a chemical analysis of the brick. Such
an analysis is only necessary as determining the amount of total basic
fluxes (K_{2}O, Na_{2}O, CaO, MgO and FeO). These fluxes are ordinarily
combined into one expression, indicated by the symbol RO. This total
becomes important only above 0.2 molecular equivalent as expressed in
ceramic empirical formulae, and this limit should not be exceeded.[75]

From the nature of fire brick, their value can only be considered from a
relative standpoint. Generally speaking, what are known as first-grade
fire brick may be divided into three classes, suitable for various
conditi