section nearest the
fire approaches the furnace temperature. This is borne out by the fact
that arches which are heated on both sides to the full temperature of an
ordinary furnace will first bow down in the middle and eventually fall.

A method of testing brick for this characteristic is given in the
Technologic Paper No. 7 of the Bureau of Standards dealing with "The
testing of clay refractories with special reference to their load
carrying capacity at furnace temperatures." Referring to the test for
this specific characteristic, this publication recommends the following:
"When subjected to the load test in a manner substantially as described
in this bulletin, at 1350 degrees centigrade (2462 degrees Fahrenheit),
and under a load of 50 pounds per square inch, a standard fire brick
tested on end should show no serious deformation and should not be
compressed more than one inch, referred to the standard length of nine
inches."

In the Bureau of Standards test for softening temperature, or critical
temperature of plasticity under the specified load, the brick are tested
on end. In testing fire brick for boiler purposes such a method might be
criticised, because such a test is a compression test and subject to
errors from unequal bearing surfaces causing shear. Furthermore, a
series of samples, presumably duplicates, will not fail in the same way,
due to the mechanical variation in the manufacture of the brick. Arches
that fail through plasticity show that the tensile strength of the brick
is important, this being evidenced by the fact that the bottom of a
wedge brick in an arch that has failed is usually found to be wider than
the top and the adjacent bricks are firmly cemented together.

A better method of testing is that of testing the brick as a beam
subjected to its own weight and not on end. This method has been used
for years in Germany and is recommended by the highest authorities in
ceramics. It takes into account the failure by tension in the brick as
well as by compression and thus covers the tension element which is
important in arch construction.

The plastic point under a unit stress of 100 pounds per square inch,
which may be taken as the average maximum arch stress, should be above
2800 degrees to give perfect results and should be above 2400 degrees to
enable the brick to be used with any degree of satisfaction.

The other characteristics by which the quality of a fire brick is to be
judged are:

F