July 10, lillfl 



HARDWOOD RECORD 



19 



Schedules for Drying Hardwoods 



Editor's Note 



The Forest Products Laboratory of Mailisuii, Wis,, has prepared a series of articles on various phases of kiln- 

 drying hardwoods, and they will be published siMially in IIaiidwood Record. The opening article of the series is 

 by D. R. Brewster, forest examiner, "Drying Si.-hi-dules Recommended for Various Hardwoods," and will be com- 

 menced in this issue and concluded in the issue of July 23. Accurate drying schedules constitute the only means 

 of controlling the human factor iu kiln-drying, so it is anticipated that these correctly determined schedules will 

 be invaluable to woodworkers as a basic plan. Articles by other engineers will follow later. 



The aeeompanying series of drying schedules has been prepared 

 for the use of those who are interested in the kiln-drying of hard- 

 wood stock for furniture and general interior use. They have been 

 devised primarily to meet the most exacting requirements of furni- 

 ture manufacture, to the end that the kiln-dried wood will be at 

 its best in stability, beauty, and freedom from tendencies which 

 might result in ultimate dissatisfaction when it is put into 

 service. 



Furniture stock is usually valuable, high-grade material. Care 

 in drj'ing such stock so as to leave it in the best possible condition 

 with a minimum loss is distinctly worth while, even if the time of 

 drying must be extended somewhat beyond the period needed for 

 less exacting demands. This material, particularly where joints, 

 mortises, laminations, glued construction, resawiug, and shaping are 

 involved, must not only be free from such obvious defects as checks, 

 splits, warp, etc., but must also be free from brittleness, marked 

 easehardening, and internal stresses, and should be dried to a uni- 

 form moisture content. The schedules and the figures given on 

 time of drying are applicable especially to careful drying iu which 

 high standards of this sort are the aim to be constantly kept in 

 view. 



For uses in which the highest quality of material is not essential 

 or desired and where a certain reduction in quality is allowable, 

 somewhat higher temperatures and lower humidities may be used 

 to produce more rapid drying, according to the particular require- 

 ments and the judgment of the operator. 



The schedules given should \t.- Mii;:il>le for use in any type of 

 kiln in which the temperature aud humidity can be correctly 

 measured and accurately controlled by the operator, and in which 

 the circulation is positive, ample, and uniform in all parts of the 

 kiln. Care should be taken to see that thermometers and samjjles 

 are properly placed so that the drying conditions and rate of dry- 

 ing at the warmest point in the kiln may be known at all times. 

 The conditions specified in the schedules are intended to apply at 

 this point, regulation being in accordance with the moisture con- 

 tent of the samples in this part of the kiln. 



These schedules are recommended for drying one-incli stock 

 for furniture and interior use; for drying lumber thicker than one 

 inch see supplementary instructions attached. 



Satisfactory results cannot be expected unless the progress of 

 drying is closely watched by frequent determinations of moisture 

 content and distribution and unless stresses which may cause dam- 

 age to the lumber are sufficiently relieved by steaming at the 

 proper time. 



Adaptation of Schedules 



Safe limits of temperature and humidity differ widely for differ- 

 ent kinds and thicknesses of wood, depending upon (1) the rate 

 of transfusion of moisture from the interior to the exterior, (2) 

 the amount of shrinkage, (3) the degree of the plasticity of the 

 wood at different temperatures, (4) the density and structure of 

 the wood, aud other factors. The eight kinds of hardwoods for 

 which this series of schedules has been prepared have been classi- 

 fied, in accordance with their drying requirements, in the relative 

 order shown in the following table. Thus, the first wood, birch, 

 which can be dried satisfactorily with high temperatures and low 

 humidities has been given the most severe schedule, while the 

 last wood, southern lowland oak, the most diflieult to dry, has been 

 given the least severe schedule, as shown in the following table: 



Conditions of the Air Entering the Pile 

 Temperature (Deg. F.) Relative Humidity — Per Cent 



The eight schedules cover, as a group, approximately tlic range 

 of temperatures and humidities ordinarily used and considered safe 

 in kiln-drying hardwoods. Final temperatures up to 180 degrees 

 and final humidities down to 10 per cent are sometimes used with 

 some of these woods after they reach a moisture content of 10 per 

 cent or less, although this practice decreases the margin of safety. 

 Drying op Other Hakdwoods 

 Some one of these schedules will be found adequate for drying 

 any of the other commercial hardwoods which have not been 

 specifically mentioned. In general, the lighter hardwoods, such as 

 chestnut, yellow poplar, basswood, butternut, and cottonwood, can 

 be dried with a fairly rapid schedule such as No. 1 or No. 2. 

 Denser, heavier or more cross grained woods, such as ash, elm, 

 beech, tupelo", sycamore, hickory, and cherry, will require a milder 

 schedule of about the range of No. 5 or No. 6. 



The right schedule to use with each kind of wood can be de- 

 termined by any intelligent operator by starting with schedule No. 

 8, which is safe for any of these woods, aud gradually increasing 

 the severity of conditions up to the point where a close watch of 

 the wood indicates that a further increase may cause checking, 

 warping, or other injury. 



Reading op Graphs and Tables 

 The schedules are arranged in both graphic and tabular form. 

 The graphs give the operator a mental picture of the changes in 

 drying conditions and also the stage in the run when such changes 

 are made. The table is convenient for reference. In both cases 

 the moisture content of the wood is the basis of regulation of kiln 

 conditions. The moisture content is expressed in per cent of the 

 dry weight. 



To determine from the graph the conditions to be used at any 

 stage of the run; first, find on the horizontal line at the bottom 

 the figure representing the moisture content of the wood to be 

 dried; next, from tliis point follow up the vertical line to its in- 

 tersections with the humidity and temperature graphs; then from 

 these points of intersection, follow the horizontal lines to the left 

 hand side, where the figures will indicate the degree of tempera- 

 ture and per cent of humidity called for. 



In the same way, kiln conditions to be used can be determined 

 from the drying table by first finding in the fifth column the figure 

 representing the moisture content of the wood to be dried, and then 

 taking the temperature and humidity given in the same horizontal 

 line in the first and second columns. In the third aud fourth 

 columns are given the wet bulb and dew-point temperatures which 

 represent the humidity shown in the second column, at the tem- 



