Furnaces are fire-places constructed to serve particular purposes,
and are chiefly of two kinds, “Wind furnaces” and “Blast furnaces.”
Of the first kind the common house grate is an instance, of the
second the blacksmith’s forge. The fire in a wind furnace is more
or less shut up, so that the draught of air entering it shall pass
from the ash-pit right up into the fire, and through it into the
flue or chimney--the latter being tall, and of certain proportions,
so as to ensure the requisite draught. These furnaces are used where
heat of the very highest degree is not required, as in glass-houses,
pottery-kilns, &c. The “Reverberatory furnace” is a modification of
the wind furnace, and is used to throw heat on to the surface of
substances, as in roasting ores of metals, to drive off the sulphur,
arsenic, &c., or in the making of soda, litharge, and other processes
where the admission of hot air with the flame is either beneficial,
or at least not detrimental; fig. 1 shows the construction of this
kind of furnace. Blast furnaces are for the production of the very
highest degrees of temperature, and in these the air is forced
into the fire by blowing machines or bellows, often worked by
steam-engines; such furnaces are used for the smelting and casting
of iron, &c. (fig. 2). A good blast furnace for small purposes may
be made by two crucibles--those made of coarse blacklead and clay,
and called “Blue pots,” are the best--one placed inside the other,
the outer one having a hole at the lower part for the nose of the
bellows, the inner one having the bottom cut off and a grating of
iron put in to lodge just above the lowest part; the space between
the two should be filled with powdered fire-brick or broken-up
crucibles (fig. 3).