ture used should depend upon the species and condition of
the material when entering the kiln. In general, it is advantageous to
have as high a temperature as possible, both for economy of operation
and speed of drying, but the physical properties of the wood will
govern this.

Many species cannot be dried satisfactorily at high temperatures on
account of their peculiar behavior. This is particularly so with green
lumber.

Air-dried wood will stand a relatively higher temperature, as a rule,
than wet or green wood. In drying green wood direct from the saw, it
is usually best to start with a comparatively low temperature, and not
raise the temperature until the wood is nearly dry. For example, green
maple containing about 60 per cent of its dry weight in water should
be started at about 120 degrees Fahrenheit and when it reaches a
dryness of 25 per cent, the temperature may be raised gradually up to
190 degrees.

It is exceedingly important that the material be practically at the
same temperature throughout if perfect drying is to be secured. It
should be the same temperature in the center of a pile or car as on
the outside, and the same in the center of each individual piece of
wood as on its surface. This is the effect obtained by natural
air-drying. The outside atmosphere and breezes (natural air
circulation) are so ample that the heat extracted for drying does not
appreciably change the temperature.

When once the wood has been raised to a high temperature through and
through and especially when the surface has been rendered most
permeable to moisture, drying may proceed as rapidly as it can be
forced by artificial circulation, provided the heat lost from the wood
through vaporization is constantly replaced by the heat of the kiln.

It is evident that to secure an even temperature, a free circulation
of air must be brought in contact with the wood. It is also evident
that in addition to heat and a circulation of air, the air must be
charged with a certain amount of moisture to prevent surface drying or
case-hardening.

There are some twenty-five different makes of dry kilns on the market,
which fulfill to a varying degree the fundamental requirements.
Probably none of them succeed perfectly in fulfilling all.

It is well to have the temperature of a dry kiln controlled by a
thermostat which actuates the valve on the main steam supply pipe. It
is doubly important to maintain a uniform temperature and avoid
f