20 



HARDWOOD RECORD 



August 10, 1919 



Schedules for Drying Hardwoods 



„ /r .s f.r SfS-wS! SSE-T.1 frsis,!:'. 



Editor's Note ^ , 



n R Brewster of the Madison, Wis., laboratory, begun In 

 re so inten-elltea that both should be considered together. 



The approximate minimum and average number of days requ^ed to 

 dry one inch stock (plain-sawed in the case of oak) from different 

 „.oisture contents to 5 per cent is given in the sixth and seventh column 

 of the table. This time applies only to that portion of the charge 

 which is subjected to the actual conditions specified and not necessarily 

 to the drying time of the charge as a whole. If the run is to be con- 

 tinned until the entire kiln charge has dried to the required moisture 

 per cent, the time of the run will depend upon the rate of drying at 

 the coolest point in the kiln. ^ ., * „ 



In the runs which the figures in the tables have been based, the tern- 

 perature and humidity in all parts of the kiln have been both uniform 

 and accurately measured, the circulation has been positive and ample 

 at all points, the width of the pile has not been more than 5 feet, 

 and the lumber has been piled between one-inch stickers placed so 

 as not to obstruct the circulation of air through the pile. If conditions 

 other than these are present, more or less variation from the figures 

 • may be expected. 



It is probable that no two runs of any wood, even when of identical 

 material and in the same kiln or type of kiln, were ever dried to the 

 same moisture content with exactly the same rate of drying, lor 

 this reason it is safer to use moisture content rather than "the num- 

 ber of days in the kiln" as a basis of regulating kiln conditions, even 

 though previous experience may indicate the probable length of the 

 run Just because one run has dried in a certain number of days the 

 operator is not justified in assuming that the next run will dry to the 

 same moisture content in the same number of days. 



The attempt to dry successive runs in the same length of time with- 

 out regard to moisture content, particularly where the stock is not 

 similar, is a serious cause of loss and failure in commercial kUn- 

 drying practice. Uniformly good results can be secured with indi- 

 vidual runs only by using moisture content as a guide at all times 

 and disregarding number of days in the kiln, except as a matter of 

 record which can be used to determine the average time required for 

 different kinds of stock in estimating kiln capacity, allotment budgets, 

 ■ and production schedules. 



Injuries Before Kiln Drying 

 Injuries received in previous air drying should always be taken into 

 account in examining lumber in the kihi or in judging the material 

 after it has been dried. Frequently injuries, such as cheeks and honey- 

 combing, are ascribed to the kiln drying operation, when in reality 

 they were due primarUy to the condition of the material before it was 

 placed in the kiln, and should be chargeable to the previous treat- 

 ment. 



A check or honeycomb when once started in a piece of wood, par- 

 ticularly in the thicker sizes, is very apt to increase in size in spite 

 of anything that can be done. The extension is similar to that of a 

 crack in a pane of glass. These checks, particularly in fully air- 

 seasoned stock, may have closed up again so as to become almost 

 invisible by the time the lumber is loaded into the kiln, but will 

 usually open up again in further drying. 



At the time of loading into the kiln, a careful examination, includ- 

 ing the cutting of sample boards, should be made to determine the 

 amount of checking, warping, honey-combing and other injuries and 

 defects present, and a permanent record made. A similar inspection 

 after drying wUl then show correctly the effect on the lumber of the 

 drying conditions in the kiln. The best drying schedule to use for a 

 particular kind of stock can be worked out only when these injuries 

 due to air-seasoning are taken into account. 



Treatment Preliminary to Drying 

 Before drying is started the lumber should be "steamed" with 

 saturated air, at or slightly above the same temperature at which 

 drying is to begin. This treatment should be continued untU the 



interior of the stock, as well as all parts of the load, have been 

 heated up to the initial drying temperature. The time necessary wdl 

 vary with the thickness of the stock, the piling of the lumber, the 

 construction of the kiln, and the rate of circulation of the saturated 

 air through the pile. With a positive circulation three hours for each 

 inch in thickness should be ample. If the air is stagnant three days 

 may be needed to heat up all parts of the charge. Such a long treat- 

 ment is apt to darken light colored woods and, when used with air- 

 seasoned lumber, may set up strong internal stresses m the lumber 

 After the preliminary steaming, the humidity should be reduced 

 very gradually from saturation to the point specified for beginning of 

 drying. Too rapid lowering of the humidity is liable to cause end and 

 face checks The drying conditions corresponding to the moisture 

 content of the stock may be established by the end of the first twenty- 

 four hours after the preliminary steaming. 



Drying of Air-Seasoned Lumber 

 These schedules may be used to dry either green or air-seasoned 

 material. The drying of partially or fully air-seasoned lumber may be 

 started with whatever conditions are specified for the moisture content 

 of the stock concerned. The preliminary steaming treatment, m the 

 case of air-seasoned material, serves both to heat the lumber through 

 and to soften up the outside enough to relieve any "set" condition 

 that may be present. In applying the figures on time of drying to 

 air-seasoned stock, from one to three days should be added for the 

 time required for preliminary steaming and the establishment of dry- 



ing conditions. 



Drying op Stock Thicker Than One Inch 

 The schedules are suitable as they stand for all stock 1 inch or less 

 and up to IV. inches in thickness. Above this point it becomes more 

 and more difficult to obtain good results in drying as the thickness of 

 the material increases. The rate of transfusion of moisture from the 

 inside to the outside is very much slower in the thicker stock than 

 in 1-iiich boards. Rapid drying at high temperatures under such con- 

 ditions is very apt to cause checking early in the run and later on a 

 casehardening or "setting" of the outer portion which prevents the 

 center from shrinking and results in internal checks known as honey- 

 combing High temperatures cause the center to shrink more than low 

 temperatures, and consequently increase the liability to honey-comb- 

 ing. Stresses, due to unequal shrinkage, which cause cuppmg, warp- 

 ing, twisting, and edge springing, are also increased by the use of 

 too high temperatures and low humidities, particularly with thick 



^ °Some stock two inches thick can be dried satisfactorily with the 

 same schedule as recommended for 1 inch. It is doubtful, however, 

 whether these schedules are safe for all two-inch stock. Temperatures 

 5° F lower than those given in each schedule are therefore recom- 

 mended for thicknesses ranging from 1% to 21/2 inches. For each 

 additional inch in thickness a further decrease of 5 degrees should 

 be made, except that it is unnecessary to go below a minimum initial 

 temperature of 100 degrees, provided the humidity in the kiln is cor- 

 rectly measured and under proper control and the circulation 13 uni- 

 form and ample. 



Watching Condition of Stock 

 The schedules should not be used blindly. KUn conditions vary in 

 spite of the best control. Successive charges differ both in their 

 original condition and in the way they dry. The only safeguard 

 against damage is a close watch at all times of the condition of 

 the stock, particularly of stock thicker than 1 inch. 



If surface checks appear a humidity higher than that given m the 

 schedule should be used until checking stops. A very wide difference 

 between the moisture content of the center and outside is a danger 

 signal and usually means that a higher humidity should be used to 



