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HARDWOOD RECORD 



fore prepared to investigate complaints on short 

 notice throughout the whoie territory in which 

 cypress lumber is sold. Very truly yours, 

 George E. Watson. 



Wants Bark of Shellbark Hickory. 



El Centro, Cal., Feb. 19.— Editor Hardwood 



Record: Will you kindly send me a copy of 



your paper? I want to get into communication 



with someone who can furnish the bark of shell- 



bark hickory. I want to get it in large quanti- 

 ties for medicinal use. Any assistance you can 

 render me will greatly oblige. — D. H. Chaplin. 



Manufacturers of shellbark hickory who 

 may be interested in harvesting a crop of the 

 bark for Mr. Chaplin's use are invited to 

 communicate with him on this subject. — 

 Editor. 



Kiln Drying Hardtoood Lumber. 



The Forest Service of the United States De- 

 part of Agriculture recently issued circular No. 

 48 on kiln drying hardwoods, which con- 

 tains so much valuable information on this sub- 

 ject that the Hardwood Record reprints it here- 

 with : 



Difficulty of Drying Wood. 



In drying wood, whether in the form of stand- 

 ard stock or finished product, the application of 

 the requisite heat and circulation must be care- 

 fully regulated throughout the entire process, 

 or warping and checking are almost certain to 

 result. Moreover, wood of different shapes and 

 thicknesses is very differently affected by the 

 same treatment. Finally, the tissues composing 

 the wood, which vary in form and physical prop- 

 erties, and which cross each other in regular 

 directions, exert their own peculiar influence 

 upon its behavior during drying. With our na- 

 tive woods, for instance, summer wood and 

 spring wood show distinct tendencies in drying, 

 and the same is true in less degree of heartwood 

 as contrasted with sapw r ood. Or, again, pro- 

 nounced medullary rays further complicate the 

 drying problem. Flain "oak and quartered oak 

 require different treatment. Even in mahogany 

 and similar tropical woods which are outwardly 

 more homogeneous, various kinds of tissue are 

 differenciated. 



The Water in Wood. 



In the living tree and in green wood there is 

 a large amount of water. Part of this is closely 

 held in the material of the cell walls, and can- 

 not be removed without affecting the physical 

 condition of the wood : the rest, which fills the 

 pores of the wood, is free water. In drying, the 

 free water within the cells passes through the 

 'cell walls until the cells are empty, while the 

 cell walls remain saturated. When all the free 

 water has been removed, the cell walls begin to 

 yield up their moisture. Heat raises the absorp- 

 tive power of the fibers, and so aids the passage 

 of water from the interior of the cells. 



A confusion in the use of the word "sap" is to 

 be found in many discussions of kiln drying ; in 

 some instances it means water, in other cases it 

 is applied to the organic substances held in a 

 water solution in the cell cavities. The term is 

 best confined to the organic substances from the 

 living cell. These substances, for the most part 

 of ( lie nature of sugar, have a strong attraction 

 for water and water vapor and so retard drying 

 and absorb moisture into dried wood. High tem- 

 peratures, especially those produced by live 

 steam, appear to destroy these organic com- 

 pounds, and therefore both to retard and to limit 

 the reabsorption of moisture when the wood is 

 subsequently exposed to the atmosphere. 



Air dried wood, under ordinary atmospheric 

 temperatures, retains from 10 to 20 per cent of 

 moisture, whereas kiln dried wood may have 

 no more than 5 per cent as it comes from 

 the kiln. The exact figures for a given species 

 depend in the first case upon the weather condi- 

 tions, and in the second case upon the tempera- 

 ture of the kiln and the time during which the 

 wood is exposed to it. When wood that has 

 been kiln dried is allowed to stand in the open 

 it apparently ceases to reabsorb moisture from 

 the, air before its moisture content equals that 

 of wood which has merely been air dried in the 

 same place and under the same conditions. 



Kilns and Methods Studied. 



WOODS. 



The studies upon which this report is based 



covered the following woods : White oak, red 



oak, maple, birch, basswood, chestnut, ash, red 

 gum, mahogany, cherry and walnut. 



TIME REQUIRED. 



The time consumed in drying, one of the most 

 important items in the expense account, varies 

 very widely among operators. Take, for exam- 

 ple, 1-inch plain white oak, which is a standard 

 material dried throughout the region studied. 

 As a rule, this is dried from one to two weeks, 

 yet many operators, even when crowded for kiln 

 space, double this period, whereas at the larger 

 and more progressive plants, especially those dry- 

 ing hardwood flooring, it is reduced to five, four 

 or even three days. When the kiln is larger 

 than necessary it is a not uncommon practice to 

 use it as a storage room for surplus stock. 



The time of drying differs widely also with the 

 species, as well as with the intended use. Quar- 

 ter-sawed oak usually requires half again as 

 long as plain oak. Mahogany requires about the 

 same time as plain oak ; ash dries in a little less 

 time, and maple, according to the purpose for 

 which it is intended, may be dried in one-fifth 

 the time needed for oak or may need a slightly 

 longer treatment. For birch the time required 

 is from one-half to two-thirds, and for poplar 

 and basswood, from one-fifth to one-third that 

 required for oak. 



COST. 



The information secured upon cost indicates — 

 especially among smaller operators, where econ- 

 omies-are less carefully studied — the widest di- 

 vergence. The extreme figures, for products not 

 widely different, are 75 cents and $5 per thou- 

 sand feet. 



USE OP KILN DRIED WOODS. 



With the exception of structural timber, nearly 

 all hardwoods are kiln dried before they are 

 made into the finished product. A surface fin- 

 ish such as that demanded in furniture and in- 

 terior work and the high degree of strength and 

 stiffness demanded in vehicle and implement 

 stock are impossible without thorough drying, 

 and this drying is most quickly accomplished in 

 a kiln. For the very exacting requirements of 

 wheel work and of shoe last and printing type 

 wood rapid kiln drying has not yet proved en- 

 tirely satisfactory, but new and improved meth- 

 ods appear to promise success. 



Defects of Present Methods. 

 Dry kilns are at present constructed and op- 

 erated largely without thoroughgoing system. 

 Forms of kiln and modes of operation have com- 

 monly been copied by one woodworking plant 

 after the example of some neighboring establish- 

 ment. In this way it has been brought about 

 that the present practices have many shortcom- 

 ings. The most progressive operators, however, 

 have experimented' freely in the effort to secure 

 special results desirable for their peculiar prod- 

 ucts. 



RESULTS OF MISTREATMENT. 



Mistreatment of the material results in numer- 

 ous defects, chief among which are warping and 

 twisting, checking, casehardening and honey- 

 combing. 



Many woods, as, for example, tupelo and red 

 gum, will warp and twist in drying unless spe- 

 cial care is taken. This difficulty is not con- 

 fined to kiln-drying, but is quite as great in air- 

 drying. Indeed, drying in the open with 

 exposure to the sun often develops the worst 

 examples. In both cases the remedy lies in 

 proper piling. In piling lumber for the kiln the 

 cross sticks should always be placed directly 

 over the trucks, following a perpendicular line. 

 Where the intervals between trucks are so great 

 that intermediate sticks are required, heavy tim- 

 bers should first be laid upon the trucks, to 

 form a foundation for the pile. It is a good 

 practice to place sticks at the very end of the 

 pile. 



If the kiln-drying is too rapid the lumber may 

 open up at the ends in deep checks. This defect 

 is common to all grades of veneer stock and is 

 most conspicuous in thin hardwood veneers. 

 Frequently checks which appear after kiln-drying 

 were originally formed during previous air-dry- 

 ing and are merely reopened in the kiln. These 

 may readily be distinguished from fresh checks 

 formed in the kiln, since their inner surfaces 

 have been filled with dust and darkened by the 

 weather. It appears to be almost impossible to 

 prevent their reopening. 



Casehardening occurs when the kiln-drying is 

 pushed too rapidly without proper precautions. 

 The surface of the wood becomes dry and im- 

 pervious, while the interior remains almost as 

 moist as before. Thorough drying is thus quite 

 prevented, and effort to secure it usually pro- 

 duces honeycombing. 



Honeycombing can occur only together with 

 casehardening. It is, in effect, internal checking 

 in which the checks, following the medullary 

 rays, may run nearly from end to end of the 

 piece but do not, except in extreme cases, show 

 upon the surface. 



Theory of Drying. 



Despite the diversity of practice, it is possible 

 to find among the larger and more enterprising 

 operators a measure of agreement, as to both 

 methods and results, and from this to outline 

 the essentials of a correct theory. 



Before any drying occurs both the wood and 

 the water it contains must be raised to the tem- 

 perature at which the drying is to take place. 

 If the wood is slowly heated and circulation Is 

 meantime suffered to carry off the surface mois- 

 ture as fast as it is vaporized, the surface be- 

 comes entirely dry before the internal moisture 

 is even moderately heated or has begun to move 

 in quantity to the surface. Moreover, if pre- 

 liminary air-drying has taken place, it should 

 be remembered that more moisture has been lost 

 from the surface than from the interior and 

 that it is important that this condition should 

 not be accentuated in the kiln. It is necessary, 

 therefore, that surface drying be delayed in the 

 kiln by retaining the moisture first vaporized 

 about the wood while the continued heat pene- 

 trates to the internal moisture. Thus far this 

 is essentially the moist-air system of drying. 

 Steam may be used to advantage to wet the 

 wood, and though this increases the water con- 

 tent it shortens the time required for drying by 

 quickly raising the wood to the drying tempera- 

 ture. 



When once the wood has been raised to a high 

 temperature through and through, and especially 

 when the surface has been rendered most per- 

 meable to moisture, drying may proceed as rap- 

 idly as it can be forced by artificial circulation, 

 provided the heat lost from the wood through 

 vaporization is constantly replaced by the heat 

 of the kiln. 



The drying is actually carried out by admit- 

 ting hot, unsaturated air to contact with tie 

 heated wood and replacing it constantly as it 

 becomes saturated. The best rate of circulation 

 for the air, as well as the best degree. of humid- 

 ity, is still unsettled. Provided the internal 

 moisture is kept moving toward the surface as 

 rapidly as the surface moisture is removed, the 

 problem of hastening the drying by increasing 

 the circulation and regulating the humidity is 

 comparatively simple. It should be quite pos- 

 sible to discover by simple experiments what 

 means of fulfilling the conditions are suited to 

 the drying of lumber on a commercial scale. 



Successful kiln-drying depends chiefly upon 

 these two principles and upon keeping separate 

 the two distinct stages of the process which they 

 govern. 



To carry out right drying principles a rightly 

 constructed kiln is necessary. The essentials of 

 kiln construction, deferring details for the pres- 

 ent, include the following points : 



The system of heaters and radiators should be 

 capable of maintaining the desired temperature 

 at all times. 



The temperature and humidity of the drying 

 chamber should be under perfect control and pro- 

 tected from outside influences. 



Simple devices should control the circulation 

 as desired, both between the kiln and the outer 

 atmosphere and between the two ends of the 

 kiln. 



