This interior checking is called honeycombing.

Hardwood lumber is commonly air-dried from two to six months, before being kiln-dried.

For the sake of economy in time, the tendency is to eliminate yard-drying, and substitute kiln-drying.
Kiln-drying of one inch oak, takes one or two weeks, quarter-sawn boards taking one and a half times as long as plain-sawn.
The best method of drying is that which gradually raises the temperature of both the wood and of the water which it contains to the point at which the drying is to take place.

Care is therefore taken not to let the surface become entirely dry before the internal moisture is heated.

This is done by retaining the moisture first vaporized about the wood, by means of wet steam.