by the
heat, and ventilation also serves the purpose of bringing the heat in
contact with the wood. If, however, plain, dry heat is applied to the
wood, the surface will become entirely dry before the interior
moisture is even heated, let alone removed. This condition causes
"case-hardening" or "hollow-horning." So it is very essential that
sufficient humidity be maintained to prevent the surface from drying
too rapidly, while the heat is allowed to penetrate to the interior.

This humidity or moisture is originated by the evaporation from the
drying wood, or by the admission of steam into the dry kiln by the use
of steam spray pipes, and is absolutely necessary in the process of
hastening the drying of wood. With green lumber it keeps the sap near
the surface of the piece in a condition that allows the escape of the
moisture from its interior; or, in other words, it prevents the
outside from drying first, which would close the pores and cause
case-hardening.

The great amount of latent heat necessary to evaporate the water after
it has reached the surface is shown by the fact that the evaporation
of only one pound of water will extract approximately 66 degrees from
1,000 cubic feet of air, allowing the air to drop in temperature from
154 to 84 degrees Fahrenheit. In addition to this amount of heat, the
wood and the water must also be raised to the temperature at which the
drying is to be accomplished.

It matters not what type of dry kiln is used, source or application of
heating medium, these underlying principles remain the same, and must
be the first things considered in the design or selection of the
equipment necessary for producing the three essentials of drying:
Heat, humidity, and circulation.

Although these principles constitute the basis of all drying problems
and must, therefore, be continually carried in mind in the
consideration of them, it is equally necessary to have a comprehensive
understanding of the characteristics of the materials to be dried, and
its action during the drying process. All failures in the past, in the
drying of timber products, can be directly attributed to either the
kiln designer's neglect of these things, or his failure to carry them
fully in mind in the consideration of his problems.

Wood has characteristics very much different from those of other
materials, and what little knowledge we have of it and its properties
has been taken from the accumulated records of experienc