3. It has likewise a great reducing power.

The best kind of charcoal is that of pinewood, linden, willow, or
alderwood, or any other soft wood. Coal from the firwood sparkles too
freely, while that of the hard woods contains too much iron in its
ashes. Smooth pieces, free from bark and knots, should be selected. It
should be thoroughly burnt, and the annual rings or growths should be
as close together as possible.

If the charcoal is in masses, it should be sawed into pieces about six
inches in length by about two inches broad, but so that the
year-growths run perpendicular to the broadest side, as the other
sides, by their unequal structure, burn unevenly.

That the substance under examination may not be carried off by the
blast, small conical concavities should be cut in the broad side of
the charcoal, between the year-growths, with a conical tube of tin
plate about two or three inches long, and one quarter of an inch at
one end, and half an inch at the other. These edges are made sharp
with a file. The widest end of this charcoal borer is used for the
purpose of making cavities for cupellation.

In places where the proper kind of charcoal is difficult to procure,
it is economical to cut common charcoal into pieces about an inch
broad, and the third of an inch thick. In each of these little pieces
small cavities should be cut with the small end of the borer. When
these pieces of charcoal are required for use, they must be fastened
to a narrow slip of tin plate, one end of which is bent into the form
of a hook, under which the plate of charcoal is pushed.