When the surface is made permeable to moisture, drying may take place rapidly.

Curtains of canvas are hung all around the lumber on the same principle that windows in newly plastered buildings are hung with muslin.
The moisture is absorbed on the inner surface of the curtain and evaporates from the outer surface.

Improvements in kiln-drying are along the line of moist air operation.

In common practice, however, the moist air principle is often neglected.
There are two methods in operation, the progressive method and the charge method.

In the progressive, the process is continuous, the loads going in at one end of the kiln, and out at the other, the temperature and the moisture being so distributed in the kiln, that in passing from the green to the dry end, a load of lumber is first moistened, then heated, and finally dried out.