﻿KILN DRYING HANDBOOK. 31 



STEAMING TO KILL MOLDS AND WOOD-BORERS. 



The kiln operator is frequently confronted with the necessity of 

 handling stock showing evidences of decay, mold, stain, or the action 

 of borers. Under ordinary drying conditions in the kiln, borers will 

 be killed and the growth of decay, molds, and stains will be arrested, 

 except possibly in the case of stains similar to the brown stain of 

 western yellow pine. When drying is carried on at low temperatures 

 and high humidities, however, conditions are favorable to the growth 

 of many of these parasites, and sometimes they may cause trouble 

 in the kiln. The growth of mold on heavy oak wagon stock during 

 the early stages of the drying is not uncommon, and borers are oc- 

 casionally found working in hickory wagon-axle stock in the kiln. 

 The remedy usually applied is steaming for a period of about two 

 hours at a temperature of about 180° F. This treatment may have 

 to be repeated periodically in the case of molds, until the surface 

 of the stock becomes dry enough to inhibit further growth. 



DRYING SCHEDULES. 



A drying schedule is a prescription or rule for the operation of 

 the kiln during the drying period. Drying schedules are usually 

 presented in the form of curves or tables showing the temperatures 

 and humidities to be used at various stages of the drying, it being 

 taken for granted that a kiln of suitable type, with ample and uni- 

 form circulation, etc., is available. Obviously, successful drying can 

 not be accomplished if the kiln is incapable of doing the work re- 

 quired of it. The temperatures and humidities in drying schedules 

 are based upon either the length of time the stock has been in the 

 kiln or the current moisture content of the stock. The latter basis 

 is used exclusively by the Forest Products Laboratory, since it is 

 logical and of universal application. 



KILN SAMPLES. 



To use a drying schedule based upon the current moisture content 

 of the stock requires a system by which this current moisture can be 

 determined with ease and certainty. The best system so far de- 

 veloped depends upon the use of kiln samples. Kiln samples are 

 short pieces of typical stock of known original moisture content, 

 which are placed in different parts of the kiln and are periodically 

 weighed to determine the loss of moisture. The current moisture 

 content is computed from the original moisture content and the loss 

 in weight, and is assumed to be the average moisture content of the 

 stock represented by the samples. 



Kiln samples are prepared as follows: Several boards, represent- 

 ing both fast drying and slow drying stock, are selected from the 

 stock to be dried, and from each one or more samples about 2 feet 

 long are cut. The sample should be cut not less than 2 feet from the 

 end of the board, if possible, and the end 2 feet discarded. Each 

 sample should be cut 2 inches longer than desired, a moisture section 

 cut immediately from each end, and the moisture determination 

 made. The average of these two moistures is assumed to be the 

 average moisture content of the sample. 



