﻿UNITED STATES DEPARTMENT OF AGRICULTURE 



Washington, D. C. 



May 12, 1923 



By Rolf Thelen, In Charge, Section of Timber Physics, Forest Products Labo- 

 ratory, Forest Service. 



CONTENTS. 



Page. 



Purpose 1 



Moisture in wood 1 



General principles of drying wood 5 



Heat in the kiln 6 



Humidity in the kiln 14 



Air circulation in the kiln 20 



Page. 



Drying and drying stresses 23 



Drying schedules 31 



Kiln types 45 



Piling lumber for kiln drying 53 



Details of kiln operation 56 



Air seasoning 63 



The principal purpose of this bulletin is to present to the dry-kiln 

 operator, in condensed and convenient form, the fundamental facts 

 about the drying of wood which he must know in order to get the 

 most satisfactory results with his kiln. The major portion of the 

 bulletin deals with the kiln drying of lumber, but there are also in- 

 cluded specific suggestions concerning the drying of other forms 

 of wood. 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 are for the most 

 part based on extensive investigations and experiments by the For- 

 est Products Laboratory of the Forest Service, Department of Agri- 

 culture, Madison, Wis., tested out in commercial practice. 



MOISTURE IN WOOD. 



The purpose of drying or seasoning wood is to remove a certain 

 amount of the moisture which is naturally present in it, and which 

 if allowed to remain would interfere with its use for most construc- 

 tion purposes. The exact amount of moisture to be removed de- 



Note. — Acknowledgement is made by the author to the members of the Section of 

 Timber Physics, both past axid present, who are largely responsible for the development 

 of the practical technique of kiln drying described in this bulletin. 



23241°— 23 1 1 



