open, clean,
white and flinty, the brick in all probability is of a good quality. If
this fracture has the fine uniform texture of bread, the brick is
probably poor.

In considering the heavy duty of brick in boiler furnaces, experience
shows that arches are the only part that ordinarily give trouble. These
fail from the following causes:

Bad workmanship in laying up of brick. This feature is treated below.

The tendency of a brick to become plastic at a temperature below the
fusing point. The limits of allowable plastic temperature have already
been pointed out.

Spalling. This action occurs on the inner ends of combustion arches
where they are swept by gases at a high velocity at the full furnace
temperature. The most troublesome spalling arises through cold air
striking the heated brickwork. Failure from this cause is becoming rare,
due to the large increase in number of stoker installations in which
rapid temperature changes are to a great degree eliminated. Furthermore,
there are a number of brick on the market practically free from such
defects and where a new brick is considered, it can be tried out and if
the defect exists, can be readily detected and the brick discarded.

Failures of arches from the expansive power of brick are also rare, due
to the fact that there are a number of brick in which the expansion is
well within the allowable limits and the ease with which such defects
may be determined before a brick is used.

Failures through chemical disintegration. Failure through this cause is
found only occasionally in brick containing a high percentage of iron
oxide.

With the grade of brick selected best suited to the service of the
boiler to be set, the other factor affecting the life of the setting is
the laying. It is probable that more setting difficulties arise from the
improper workmanship in the laying up of brick than from poor material,
and to insure a setting which will remain tight it is necessary that the
masonry work be done most carefully. This is particularly true where the
boiler is of such a type as to require combustion arches in the furnace.

Red brick should be laid in a thoroughly mixed mortar composed of one
volume of Portland cement, 3 volumes of unslacked lime and 16 volumes of
clear sharp sand. Not less than 2-1/2 bushels of lime should be used in the
laying up of 1000 brick. Each brick should be thoroughly embedded and
all joints filled. Where red brick and fire brick are