al to be used in its construction must be of the best
procurable.

Boiler settings to-day consist almost universally of brickwork--two
kinds being used, namely, red brick and fire brick.

The red brick should only be used in such portions of the setting as are
well protected from the heat. In such location, their service is not so
severe as that of fire brick and ordinarily, if such red brick are
sound, hard, well burned and uniform, they will serve their purpose.

The fire brick should be selected with the greatest care, as it is this
portion of the setting that has to endure the high temperatures now
developed in boiler practice. To a great extent, the life of a boiler
setting is dependent upon the quality of the fire brick used and the
care exercised in its laying.

The best fire brick are manufactured from the fire clays of
Pennsylvania. South and west from this locality the quality of fire clay
becomes poorer as the distance increases, some of the southern fire
clays containing a considerable percentage of iron oxide.

Until very recently, the important characteristic on which to base a
judgment of the suitability of fire brick for use in connection with
boiler settings has been considered the melting point, or the
temperature at which the brick will liquify and run. Experience has
shown, however, that this point is only important within certain limits
and that the real basis on which to judge material of this description
is, from the boiler man's standpoint, the quality of plasticity under a
given load. This tendency of a brick to become plastic occurs at a
temperature much below the melting point and to a degree that may cause
the brick to become deformed under the stress to which it is subjected.
The allowable plastic or softening temperature will naturally be
relative and dependent upon the stress to be endured.

With the plasticity the determining factor, the perfect fire brick is
one whose critical point of plasticity lies well above the working
temperature of the fire. It is probable that there are but few brick on
the market which would not show, if tested, this critical temperature at
the stress met with in arch construction at a point less than 2400
degrees. The fact that an arch will stand for a long period under
furnace temperatures considerably above this point is due entirely to
the fact that its temperature as a whole is far below the furnace
temperature and only about 10 per cent of its cross