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CIRCULAR 123

APRIL

ARL15
1958

HIGH TEMPERATURE DRYING e6
Saca6e~~cv4~ztad

c~gyncuifturat 'xperinent
E. V. Smith, Director

Statbonl

of th4e

ALABAMA POLYTECHNIC INSTITUTE
Auburn, Alabama

CONTENTS
Page
PROCEDURE
DISCUSSION OF RESULTS

3
4

Drying Rate................ Drying Defects Hygroscopicity and Swelling
SUMMARY AND CONCLUSIONS

4 6 9 _11

FIRST PRINTING

3M,

APRIL 1958

IIGH TEMPERATURE DRYING
o Soutert Hadwood
D. B. RICH ARDS, Forester

THE

IDEA Of

drying lumber at temperatures above the boiling

point of water has intrigued lumbermen for years. Since World War II there has been a renewed interest in this subject, stimulated largely by successful applications in Europe and encouraging research results with Canadian woods. It is well known that different species and different thicknesses of lumber differ considerably in drying rate and in tolerance to accelerated drying schedules. In view of these facts, the present exploratory study was designed to learn if useful lumber thicknesses of representative southern hardwoods could be dried at temperatures above the boiling point of water without excessive degrade. The effectiveness of high temperature drying in reducing the hygroscopicity of wood was also to be evaluated. PROCEDURE Red oak, white oak, hickory, beech, blackgum, sweetgum, and yellow poplar were the hardwoods selected for this study. Southern pine sapwood, an easily dried softwood, was used for comparison.

1/2, A freshly cut bolt of each wood was sawed into boards 1/4,
3/4, and 1 inch thick and about 6 inches wide. In addition to flat

sawed sapwood boards, both flat sawed and quarter sawed heartwood boards were cut whenever it was practical to do so. From the bolts of blackgum and hickory, several 6/4 boards were also obtained. The boards were cut to a length of 18 inches, end
1 Based on a cooperative study supported by the Birmingham Research Center, Southern Forest Experiment Station, U.S. Forest Service.

coated with Moore's Endtite, and weighed. They were then stacked with adequate spacing in a forced circulation drying oven and dried to constant weight at a temperature of 110 ° C. (230 ° F.). During the drying process, the boards were removed at intervals and weighed. From these data drying curves were plotted. After the boards were dried, three thin cross sections were cut from the central region of each. All sections were examined for internal checking and collapse. One section from each board was used to test for case hardening; one was impregnated with water under pressure, then oven dried, and the shrinkage calculated; and one was weighed and measured at equilibrium with successively higher relative humidities up to 100 per cent and then at equilibrium with successively lower humidities down to 12 per cent.
DISCUSSION of RESULTS Drying Rate

In general, the rate of drying was rapid until 30 per cent moisture content 2 was reached, then it slowed down conspicuously. The drying curves for quarter sawed beech heartwood are presented as an example, Figure 1, since they are fairly representative of the hardwoods studied. As expected, the thinnest boards dried most rapidly. With the exception of white oak heartwood, all of the 1/ -inch boards 4 dried from green condition to 4 per cent moisture content in less than 6 hours. The 1/ -inch white oak heartwood required 8 to 11 4 hours. In order that all species could be compared on the same basis, the time required to dry from 60 per cent to 4 per cent moisture content was calculated, Table 1. Some of the species had a high initial moisture content (above 90 per cent) and required 1 to 3 hours to dry to 60 per cent. Hence, the time required to dry from green to 4 per cent would be 1 to 3 hours longer than the values of Table 1. In spite of this fact, a 24-hour drying schedule seems possible for many of these woods. Although the relationship was not at all consistent, on the average the -inch boards dried in about half the time of the 1-inch boards, Table 1. Since the 3/ 4 -inch boards displayed a drying rate intermediate between that of the 1-inch and the 1 /2-inch boards, they are not shown in Table 1 or Figure 1.
2All moisture contents reported are expressed as a per cent of the oven-dry weight.
[4]

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HOURS OF DRYING AT I10' C Figure 1. Drying curves for quarter sawed beech heartwood dried at 1100 C. These are similar to curves for other hardwoods dried at this temperature.

TABLE 1.

TIME REQUIRED TO DRY LUMBER OF VARIOUS SPECIES FROM 60 PER ° CENT TO 4 PER CENT MOISTURE CONTENT AT 110 C. 1

Species

Position

Sawing method 1-inch lumber Hours 19.1 16.8 14.2 24.4
19.2

/2-inch lumber Hours 15.0 8.3 5.0 18.0 8.7
3.2

Beech Beech Blackgum Sweetgum Sweetgum Sweetgum Southern pine Yellow poplar Red oak Red oak White oak White oak White oak Hickory Hickory

Heartwood Sapwood Sapwood Heartwood Heartwood Sapwood Sapwood Sapwood Heartwood Sapwood Heartwood Heartwood Sapwood Sapwood Heartwood

Quarter Flat Flat Flat Quarter Flat Flat Flat Quarter Flat Flat Quarter Flat Flat Quarter

17.5 7.6 18.0 20.0 17.7 18.9 28.6 17.3 21.7 24.7

8.4 8.2 5.8 15.0 16.4 8.3 9.3 12.7

[5]

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SOUTHERN

PINE

Figure 3. 110 C.

Cross sections of 1 inch boards of various species that were

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Figure 4. Cross sections of thick boards after high temperature drying. Both of the redgum sections and the smallest blackgum section are from 5 4 boards. The

other two blackgum and the two hickory sections are from 6 4 boards.

Figure 5. Cross sections bond sawed to reveal case hardening. The kiln-dried specimens were seasoned on a mild schedule, with special attention, to assure stress-free stock. The air-dried material was seasoned inside a building.

I 5]

Figure 6. Case hardening specimens similar to those of Figure 5, except that intervening prongs were cut out to reveal the full tendency of the outer prongs to bow inward.

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RELATIVE HUMIDITY (PER CENT)

Figure 7.

Moisture content of high-temperature-dried and air-dried wood at equiEach point is the

librium with various relative humidities at room temperature. average of all species and thicknesses used in this study.

swelling than the controls at both 90 and 100 per cent humidity. This abnormal swelling of beech and blackgum accounts for the three negative values in Table 2. The high-temperature-dried hardwood cross sections that were saturated with water by pressure impregnation and then oven dried yielded some additional information of interest. Allowed to reach equilibrium with room conditions after initial high temperature drying, they had a moisture content of about 6 per cent. After water saturation and redrying at 105' C. for 24 hours, their
TABLE 2. THE REDUCTION OF HYGROSCOPICITY AND DIMENSIONAL CHANGE ° C., AS A PERCENTAGE OF THE WOOD DUE TO DRYING AT 110
SAME PROPERTY OF AIR-DRIED CONTROLS

OF

Species of
wood

Poplar Hickory Beech Blackgum Southern pine

Per cent reduction in moisture content of wood at equilibrium with the following relative humidities 100 per cent 90 per cent 15.3 26.7 13.0 26.6 6.83 21.9 16.8 21.1 9.1 21.5 [10]

Per cent reduction in dimensional change of wood between oven dry and equilibrium with the following relative humidities 90 per cent 32.1 14.5 9.4 -9.6 27.4 100 per cent 28.8 7.3 -24.2 -7.1 16.5

oven dry dimensions were about 2 per cent greater than their previous dimensions at 6 per cent moisture. This seems to indicate that some type of abnormal shrinkage, probably involving collapse, had been released during the period of water saturation. Since at that time the specimens were in the form of relatively thin cross sections, this abnormal shrinkage did not occur again (at least not to the same extent) during the subsequent oven drying. This indication of collapse may help to explain the seemingly conflicting data concerning dimensional stabilization. When the specimens were brought to equilibrium at 90 per cent relative humidity or higher, some collapse may have been released, causing abnormal swelling that partially or wholly counteracted any stabilizing influence of high temperature drying. In addition to releasing abnormal shrinkage, the period of water saturation seemed also to reduce the acquired dimensional stability since these specimens shrank nearly normally upon subsequent oven drying. It is possible that this normal amount of shrinkage could be the result of a balancing out of any residual stabilization by a mild amount of collapse during the second drying. Either of these explanations still serves as a warning that any severe cyclic exposure that includes water impregnation or prolonged soaking may remove part or all of the dimensional stability imparted by high temperature drying. The presence of collapse not only confuses the question of dimensional stabilization but looms as the principal problem to be encountered in the high temperature drying of green hardwoods. In this connection it is noted that the internal checking in redgum and the oaks was probably due in large measure to a severe condition of collapse. The recently reported work of Ladell3 emphasizes the importance of the collapse problem in high temperature drying of Canadian yellow birch. At present collapse can best be controlled by careful air seasoning to about 30 per cent moisture content prior to high temperature drying. SUMMARY and CONCLUSIONS A temperature of 1100 C. was used to dry various southern hardwoods from the green condition. It was found that 1 inch lumber of yellow poplar, beech, blackgum, hickory, and sapgum could be dried in a short time without excessive visible defect.
SLadell, J. L. High-temperature drying of yellow birch. Forest Products Jour. VI (11): 469-475. 1956.

[11]

Hygroscopicity of the wood was noticeably decreased by this high temperature treatment. Some improvement in dimensional stability was observed. However, display of this property was erratic, probably due to abnormal swelling associated with release of mild collapse that had occurred during drying. Case hardening was in evidence, but it is felt that conventional techniques can control this condition when necessary. Until better techniques are found for solving the problem of mild collapse, it is recommended that controlled air seasoning be used prior to high temperature drying of hardwoods. Redgum, red oak, and white oak displayed such severe internal checking that it seems impractical to dry normal lumber thicknesses of these species at high temperatures.