2 BULLETIN 1136, U. S. DEPARTMENT OF AGRICULTURE 
the most satisfactory results with his kiln. Naturally the major 
portion of the publication deals with the kiln drying of lumber, but 
specific suggestions for the drying of other partly manufactured 
wood specialities are also included. .The general information is 
applicable to all kinds of drying. | 
No attempt has been made to present detailed data in substantia- 
tion of the information given. The conclusions, which have been 
tested out in commercial practice, are for the most part based on 
extensive investigations and experiments by the Forest Products 
Laboratory of the Forest Service, Department of Agriculture, Madi- 
son, Wis. 
MOISTURE IN WOOD 
The chief aim in seasoning wood, whether such drying takes place 
in the kiln or elsewhere, is to remove part of the moisture naturally 
present in it, which if allowed to remain would ordinarily interfere 
seriously with its use. The amount of moisture to be removed de- 
pends upon both the quantity present and the service for which the 
wood is intended. It is rarely necessary, except in test pieces, or 
even desirable to remove all the moisture, that is, to bring about an 
oven-dry condition. 
Moisture in wood is commonly called sap, a word that has severa 
meanings. Since lack of a precise definition causes much confusion, 
this bulletin will avoid the use of the term “sap.” 
The moisture in both sapwood and heartwood consists almost en- 
tirely of water, although it does contain small percentages of organic 
and mineral matter. In the sapwood these substances are principally 
sugars of various kinds, and in the heartwood they include tannins, 
‘coloring matter, and various other chemicals. For present purposes, 
sap will be considered as water only. 
Water occurs in wood in two distinct forms, usually called “ free * 
water and “imbibed ” water. The free water is in the cell cavities, 
and the imbibed is in the cell walls. Imagine each cell of a piece 
of wood to be a small bucket made of some absorbent material. 
When such a bucket is filled with water, a certain quantity of the 
water is absorbed by the sides and bottom, in addition to the pailful 
which they inclose. The pailful is free water, that absorbed by the 
walls is imbibed water, and the sum of the two represents all the 
water the bucket, or the cell, can hold. A portion or even all of 
the free water can be removed from the cell without changing the 
amount of imbibed water in its walls, but when it has become empty 
further draining removes water from the walls themselves and they 
begin to dry out. The point at which the cell becomes empty while 
the walls are still full is called the “ fiber-saturation point”; it has 
a very important bearing upon the process of drying and for that 
reason will be discussed more fully later. 
In most living trees both heartwood and sapwood contain some 
free water. The amount, which depends on a number of factors, 
varies considerably, although sapwood almost always has more mois- 
ture than heartwood. Similarly the butt may contain much more 
than the top, as is evidenced by the sinker stock of redwood and of 
sugar pine. The season of the year in which trees are felled may 
have some influence upon the moisture in their sapwood, although 
this influence is never important. 
