﻿2 BUI.I.ITIX 1136, U. S. DEPARTMENT OF AGRICULTURE. 



pends upon the quantity present and the purpose for which the 

 wood is to he osed. Rarefy, except far test pieces, is it necessary 

 or desirahle to remove all the moisture, producing an oven-dry or 

 bone-dry condition. 



Moisture in wood is commonly called sap. There is no uni- 

 versally accepted definition of this word "sap," and its use causes 

 much confusion. The moisture, or sap, in both sapwood and 

 heartwood consists almost entirely of water. It does contain, 

 however, 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. I^or the purposes of this bulletin, 

 sap will be considered to be water only. 



Water occurs in wood in two distinct forms, spoken of as "free" 

 water and "imbibed" water. The free water exists in the cell 

 cavities and the imbibed water in the cell walls. Imagine each 

 cell of the wood to be a small bucket of some porous or absorbent 

 material. If this bucket is filled with water, a certain amount 

 will be absorbed by the sides and bottom, in addition to the " pail- 

 ful" inside the bucket. This 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 all 

 of the free water can be removed from the cell without changing 

 the amount of imbibed water in the walls; but when the bucket 

 is empty further drying removes water from the walls themselves 

 and they begin to dry out. This point at which the bucket becomes 

 empty is called the "fiber-saturation point." It has a very im- 

 portant bearing upon the process of drying and will be discussed 

 more fully later. 



In most living trees there is some free water in both heart wood 

 and sapwood. The amount varies considerably depending on a num- 

 ber of factors. Thus, sapwood almost always contains more mois- 

 ture than heartwood. The butt may contain much more than the top, 

 as is evidenced by the sinker stock of redwood and sugar pine. The 

 season of year in which the trees are felled may have some influ- 

 ence upon the moisture in the sapwood, but this influence is not very 

 important. There are a number of instances on record in which 

 there was more moTsture present in the sapwood in winter than in 

 summer. The common conception is that the reverse is true. 



Species and locality of growth have an important bearing upon 

 the amount of moisture in the living tree. Species growing in 

 Mvampy regions are apt to contain much more moisture and to be 

 harder to dry than similar upland species. The oaks are an excellent 

 illustration of this fact. On the other hand, certain species contain 

 comparatively large amounts of water, even though growing under 

 reasonably dry conditions. All of these variations must be taken 

 into consideration in the drafting of drying schedules and in the 

 actual drying operation. 



MOISTURE DETERMINATION. 



To dry stock successfully and to know when it has reached the 

 proper dryness, it is essential that the operator be able to determine 

 the amount of moisture in wood at an}' time. There are several 



