which is covered with a layer of powdered charcoal or other form of
carbon, then another layer of carbon dust is sprinkled over them, and
the box is covered with a close fitting lid. The box is next heated
gradually in an oven, to drive off moisture, and the temperature slowly
raised until the escape of blue smoke from under the lid ceases. The
heat is then increased until the box becomes white hot. It is kept in
this glowing condition for at least two hours. It is then removed from
the fire, allowed to cool, and the contents are tested in a gas flame.
If they have been thoroughly carbonized, they will not glow when removed
from the flame, and the fibers may even be heated white hot before
consuming.

Of course, the method employed to carbonize the materials is suspectible
of variation, but the scientific principles involved are unchangeable,
viz.:

(1) Partial exclusion of air and substitution therefor of a carbon
atmosphere.

(2) Slow heating to drive off moisture and volatile elements.

(3) Intense and prolonged heating of the partly charred objects to
eliminate remaining foreign elements, and to change the carbon from the
combustible form of ordinary charcoal to a highly refractory condition.

[Illustration]

NOTE.--Fig. 1 is photographed from a white iron casting made upon
carbonized coarse lace; the lower portion of the plate shows the lace
embedded in the iron. Fig. 2 is a casting in gray iron upon lace laid on