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UNITED STATES DEPARTMENT OF AGRICULTURE 
BULLETIN No. 1136 fjj||| 
Washington, D. C. Y May 12, 1923 
KILN DRYING HANDBOOK. 
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 5G 
Air seasoning 63 
PURPOSE. 
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 and present, who are largely responsible for the development 
of the practical technique of kiln drying described in this bulletin. 
23241°— 23 1 1 
