US3535795A

US3535795A - Process of drying lithocarpus densiflora rehd. (tanoak) wood with radio wave energy

Info

Publication number: US3535795A
Application number: US758097A
Authority: US
Inventors: Carl M Olsen
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1968-09-06
Filing date: 1968-09-06
Publication date: 1970-10-27
Anticipated expiration: 1987-10-27

Description

United States Patent O1 ice U.S. Cl. 34-1 Claims ABSTRACT OF THE DISCLOSURE After harvesting and before seasoning degrade has occurred due to nature or forced evaporation drying, tanoak wood is subjected to radio wave energy to effect a temperature of about 220 F throughout the tanoak wood. The subjection of the tanoak wood to radio wave energy is continued until about one third or more of the desired weight loss in the wood is attained. While the moisture is being removed, the radio wave power is maintained at a level which prevents the temperature of the moisture contained in the tanoak wood from exceeding about 220 F. After about one third of the desired weight loss in the tanoak wood is attained by radio Wave energy, it may be dried further either by standard non-radio wave evaporative drying techniques or by continuing to subject the tanoak wood to radio wave energy.

BACKGROUND OF THE INVENTION Lithocarpus densiflora Rehd. (Tanoak) Wood, hereinafter referred to as tanoak, is biologically classified as a hardwood. It has a finer grain structure than most other hardwoods, and thus does not split, flake and peel as much in use. Because of its excellent resilience characteristics, it has been found to be particularly suited for making baseball bats. The hardwood characteristics of tanoak wood also make it attractive for other hardwood applications, such as, constructing hardwood floors, tool handles, riot sticks, and furniture.

Unfortunately, tanoak wood cannot be economically dried by standard non-radio wave evaporative drying techniques, such as, air drying, kiln drying, infrared drying and the like. Even though the great demand for hardwoods is depleting the stands of other hardwoods, tanoak wood still has been considered economically useless because it has been so difficult and expensive to dry.

The principal problem encountered in drying tanoak wood by standard non-radio wave evaporative drying techniques is tanoak woods tendency to suffer irreversible seasoning degrade as a result of unequal shrinkage produced in the wood as it is dried, Seasoning degrades common to tanoak wood are warping, checking, splitting, grain separation and cell collapse. This seasoning degrade is detrimental to the quality of the tanoak wood produced and results in a very poor drying process yield of useful tanoak wood. This occurs even when tanoak wood is dried under conditions of carefully controlled humidity and temperature. For example, when tanoak wood is dried under such carefully controlled conditions, three to six months are required to complete the drying process and the maximum process yield will be about fifty percent of useful tanoak wood with an average process yield of less than about thirty percent. The long time required to complete the drying process and the careful control of the humidity and temperature conditions. require a substantial investment in equipment and storage facilities. Even with such an investment, the average yield is so poor than tanoak wood has been too expensive for most uses of hardwoods. If tanoak wood is not 3,535,795 Patented Oct. 27, 1970 dried under such carefully controlled humidity and temperature conditions, excessive seasoning degrade occurs. For the purpose of this application and in view of the above discussions, seasoning degrade is considered excessive when the average process yields is less than about thirty percent of useful tanoak wood. For the foregoing reasons, both the lumber industry and US. Forest Service have classified tanoak wood as a weed.

Since seasoning degrade is irreversible, higher process yields can be obtained only by preventing damaging non-radio wave evaporative dying after the tanoak is harvested.

Drying wood with electromagnetic energy having a frequency in the radio wave range, i.e., about 10 kilohertz (kHZ.) to about 1000 gigahertz (gHz.) has been investigated for a number of years. Some of these investigations have involved drying green soft and hard woods exclusively with radio wave energy. Green wood is wood that has not suifered seasoning degrade due to loss of moisture after being harvested and before being subjected to a forced evaporative drying process. However, when compared to the costs of standard non-radio wave evaporative wood drying techniques, drying wood exclusively with radio wave energy has been economically impractical for large scale industrial applications. Generally, drying wood exclusively with radio wave energy has been economically impractical for two reasons. Firstly, radio wave energy has been more expensive than that provided by other available energy sources for drying woods. To favorably compete with the other available energy sources, drying with radio wave energy had to result in an enhanced quality product or yield produced in a much shorter time. Although in some cases the product quality, yield or production time have been enhanced slightly when drying wood exclusively with radio wave energy, it has not been enough to justify drying wood exclusively with radio wave energy in large scale industrial applications.

Another reason why it has been economically impractical to dry wood exclusively with radio wave energy is associated with the manner in which investigators have subjected the wood to radio wave energy. In an attempt to favorably compete with the other available energy sources, it has been the practice to dry wood with radio wave energy as rapidly as possible. This requires very high power levels which generally create extreme temperatures internally of the Wood causing the wood to be damaged. Commonly this damage is in the form of rupture, shrinkage, internal cell collapse, and internal scorching.

In certain cases, it has been found economically practical to dry Wood with radio wave energy techniques when combined with standard evaporative air drying techniques. In most of these cases, the wood is first subjected to air, generally, hot, to reduce its moisture content from that of its green state to a level of about twenty percent by weight, dry basis and then subjecting it to radio wave energy to dry it to the desired final weight. In other cases, wood is subjected simultaneously to radio wave energy and hot air until the desired final weight is achieved. When expressing moisture content percentages herein, it will be on the dry basis standard, determined by dividing the weight loss of the wood by the desired final weight of the wood and multiplying by The desired final Weight of tanoak wood for optimum physical characteristic has been determined experimentally and includes some residual moisture content usually 6 to 10 percent by weight determined by dividing the difference between the final desired weight and the fully dried weight of the wood by such fully dried weight and multiplying However, in the case of green tanoak wood, it was found that excessive seasoning degrade will occur when drying it with air whether tanoak Wood is subjected to air drying in the presence or in the absence of radio wave energy, and whether the air is cool or hot.

Considerable advantage is therefore to be gained by providing a process of drying tanoak wood which results in tanoak wood being an economically useful hardwood. Additional advantages are to be gained by providing a process of drying tanoak wood which is economically practical for large scale industrial applications.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to render tanoak wood economically useful.

More specifically, it is an object of this invention to minimize the tendency of tanoak wood to suffer seasoning degrade caused by unequal shrinkage as it is dried.

Another object of this invention is to provide a high yield tanoak wood drying process.

It is a further object of this invention to provide a process of drying tanoak wood which is suitable for large scale industrial processing.

It is still another object of this invention to reduce the time required to dry tanoak wood without excessive seasoning degrade caused by unequal shrinkage.

It is yet a further object of this invention to dry tanoak wood economically with radio wave energy.

Yet another object of this invention is to dry tanoak wood with radio wave energy without forming destructive high temperature internally of the tanoak wood.

Still a further object of this invention is a. process for drying tanoak Wood in which the tendency of the tanoak wood to check, split, suffer grain separation or warpage during drying is reduced.

The present invention is a process of drying tanoak wood with radio wave energy, i.e., electromagnetic energy having a frequency in the range of about 1 megahertz to about 30 gigahertz. More particularly, it is a process of drying tanoak wood by subjecting it to radio wave energy before non-radio wave evaporative drying effects excessive seasoning degrade and thereafter continuing to subject the tanoak wood to radio wave energy at least until non-radio wave evaporative drying does not effect excessive seasoning degrade.

In accordance with the process of the present invention, tanoak wood is subjected to radio wave energy at a power level suflicient to produce a temperature in the range of about 180 F. to 230 F. throughout the tanoak wood before non-radio wave evaporative drying effects excessive seasoning degrade. If the temperature of the tanoak wood is less than 180 F. at the commencement of subjecting it to radio wave energy some moisture will be lost in increasing its temperature to the above range. The subjection of tanoak wood to'the radio wave energy is preferably continued until its moisture content is reduced by an amount at which subsequent non-radio wave evaporative drying does not effect excessive seasoning degrade. As tanoak wood is dried, its moisture content is not maintained uniform throughout its volume. Hence, the language moisture content refers to the average moisture content of the tanoak wood.

Tanoak wood can be dried exclusively with radio wave energy. However, drying with radio wave energy can be terminated prior to reaching the final desired moisture content level by weight-and non-radio wave evaporation drying employed to reduce the moisture content of the tanoak wood to its final desired level by weight without effecting excessive seasoning degrade. Furthermore, if non-radio wave evaporative forced drying techniques such as kiln drying, are employed to dry tanoak wood to its final moisture content level by weight rather than natural air drying techniques and such drying is commenced before the tanoak wood cools substantially below the temperature it has at the termination of subjecting it to radio wave energy, it is not necessary to remove as much moisture from the tanoak wood with radio wave energy in order to avoid excessive seasoning degrade. Process yields of useful tanoak wood as high is ninety-five percent have been obtained by subjecting green tanoak wood to radio wave energy to effect a temperature in the range of 180 F. to 230 F. throughout the tanoak and thereafter continuing to subject it to radio wave energy until about /3 or more of the desired weight loss in the tanoak wood is attained prior to placing the tanoak wood in kilns. Lesser process yields of useful tanoak wood are obtained when less moisture is removed by radio wave energy prior to drying the tanoak wood in kilns.

If the tanoak wood is allowed to cool substantially after drying with radio wave energy (e.-g., to a temperature of less than 180 F.) before placing it in a forced air dryer or subjecting it to natural air drying techniques to dry tanoak wood to its final moisture content level by weight, more moisture must be removed with radio wave energy to obtain the same process yields of useful tanoak wood as in the case where forced drying is commenced before the tanoak wood cools.

The yield of useful tanoak from the process of the present invention varies according to the amount of moisture removed from the tanoak wood by non-radio wave evaporative drying since seasoning degrade is irreversible, hence, cumulative. Thus, non-radio wave evaporative drying before, simultaneously with and after drying with radio wave energy, must be controlled so that its cumulative effect does not produce excessive seasoning degrade.

When subjecting tanoak wood to radio wave energy, the yield of the process is optimized by selecting the radio wave power level so that the internal temperature of the tanoak wood is maintained below 230 F. at all times. If the internal temperature of the tanoak wood is allowed to exceed this level, damaging cell collapse, shrinkage, rupture and food charring are found to occur. To reduce these effects, the radio wave power level is adjusted during drying to maintain the internal temperature below about 230 F. and, preferably, not higher than about 220 F.

By drying tanoak wood by the process of the present invention, excessive seasoning degrade can be avoided and yields of useful tanoak wood at least as high as ninety-five percent obtained. Although yields between thirty and fifty can be obtained by conventional nonradio wave evaporative drying techniques, greater expense and longer drying times are required than in the cases where radio wave energy is used in drying the tanoak wood, e.g. four to six months as compared to one to seven days.

The objects and advantages of the process of the present invention will become more apparent from the following detailed description of the present invention considered together with the appended claims.

DESCRIPTION OF PREFERRED PROCESS The process of the present invention will be described in detail as practiced to dry tanoak wood used for making baseball bats. To obtain acceptable drying yields, at least above thirty percent and preferably higher when drying tanoak wood to be used in manufacturing baseball bats, prior to drying the harvested tanoak logs or wood are kept under conditions which prevent the loss of moisture by non-radio wave evaporative drying sulficient to cause excessive seasoning degrade. Moisture loss by nonradio wave evaporative drying can be prevented various ways. For example, if the tanoak wood is not dried shortly after harvesting, the tanoak logs could be placed in log ponds until they are to be dried. Also, they could be stored in an environment having a relatively humid atmosphere of, for example, at least sixty percent and, preferably, at a temperature not greater than about R, such as found in storage sheds. At a relative humidity of above eighty percent, the tanoak wood can be kept for several months without suffering excessive seasoning degrade.

However, if the tanoak wood is subjected to radio wave energy shortly after harvesting, very little moisture 'will be lost through non-radio wave evaporative drying and excessive seasoning degrade will not occur. For example, if tanoak wood is allowed to stand in the atmosphere after harvesting for about one week before subjecting it to radio wave energy, free moisture will be lost by gravity fiow and some moisture 'will be lost due to evaporative air drying. The moisture loss that does occur as a result of evaporative air drying is slight and the tanoak :wood is still considered to be in the green state. Yields greater than ninety-five percent have been obtained from tanoak wood which, after harvesting and debarking, has been allowed to stand for one week in an atmosphere having a relative humidity of about sixty-five percent at a temperature of about 65 F.

Other techniques than those specifically described can be employedto prevent moisture loss which results in excessive seasoning degrade. However, the specifically described techniques are convenient and make use of equipment and facilities presently employed in standard wood drying processes.

Tanoak wood in various states of processing, such as logs, cut lumber and preshaped lumber, can be dried with radio Wave energy in accordance with the method of the present invention. In manufacturing baseball bats, it has been found particularly advantageous to debark and cut green tanoak timber into billets having a cross sectional area of about square inches and a length of about 40 inches before subjecting them to radio wave energy, Furthermore, greater yields are obtained if sharp corners are removed by preshaping the tanoak billets to have a cross section in the plane of the growth rings defining a curved outline form such as circular or ovular. An additional advantage attendant to preshaping is that less radio wave energy is required to dry the preshaped tanoak billet because the preshaped tanoak billet contains less total water.

Standard equipment employed in lumber mills can be used to cut and shape the tanoak wood into the desired billets. For example, a tanoak log from which billets are to be made might be passed through a debarker to remove the bark from the tanoak wood. After debarking the tanoak log, the tanoak wood would be passed through a saw, such as a circular saw, band saw, or log gang saw, to cut it into pieces having a cross sectional size that is desired for the billets. If the billets are to be preshaped to have a circular or ovular cross section, the tanoak billets would be passed through a contour cutting device, such as a lathe or shaper. After cutting the tanoak wood into pieces of the desired cross sectional size, they are passed through a trimmer to cut them into billets of the desired I length of 40 inches.

The preshaped billets are then subjected to radio wave energy so that exposure to non-radio wave evaporative drying does not result in excessive seasoning degrade. The billets may be subjected to radio wave energy in a continuous process or in a batch process. In a continuous process, the tanoak billets may be passed through a conveyorized applicator of radio wave energy, for example, at microwave frequencies, i.e., 1 mHz. to 30 gHz., a conveyorized cavity of the type described in the British Pat. 1,043,290, published Sept. 21, 1966 by Morris R. Jeppson or in the U.S. application Ser. No. 675,172, filed Oct. 13, 1967 by Jerome R. White and assigned to the assignee of this application, At lower radio wave frequencies, conveyorized dielectric applicators would be employed, such as described in the U.S. Pat. 2,868,939, issued Jan. 13, 1959 by R. V. Pound. Preferably, the conveyorized radio wave applicator would be positioned so as to receive the preshaped billets directly from the trimmer via a linking conveyor means.

If the tanoak billets are to be exposed to radio wave energy in a batch process, they could be placed in a large closed cavity applicator, for example, at microwave frequencies of the multimode microwave cavity types described in the U.S. Pat. 2,618,735, issued Nov. 18, 1952 to W. M. Hall or in the US application Ser No. 624,503, filed Mar 20, 1967 by Rexford E. Black and assigned to the assignee of this application. At lower frequencies, a dielectric heating applicator, for example, of the type described in U.S. Pat. 2,783,344, issued Feb. 26, 1957 to H. R. Warren could be used. To dry the preshaped billets, they are removed from the trimmer and, to minimize nonradio wave evaporative drying, immediately placed in the radio wave applicator. The particular type of radio wave applicator forms no part of the process of the present invention. Equivalent radio wave applications at those referred to hereinabove may be used with equal facility.

For large capacity operations, a number of billets are subjected at one time to radio wave energy. In drying a large number of billets at one time, for example, in a batch process by radio Wave energy at microwave frequencies, they are placed in the multimode cavity in a configuration providing spacing of at least about one inch between the sides of adjacent billets. By placing the billets in such a configuration, a high degree of uniform drying can be obtained.

The amount of microwave energy required to effect a temperature in the range of 180 .F. and 230 F. throughout the tanoak billets and to remove a certain amount of moisture therefrom depends upon the number of billets being dried, the initial temperature of the billets, the size of the billets, and the initial moisture content of the billets. The energy level is selected so that the billets Will not be damaged by high internal temperatures created by subjecting the billets to too much energy. However, the energy must be sufficient to effect the desired internal temperature and remove the desired amount of moisture.

The level of power at which the tanoak billets are subjected to microwave energy determines the rate at which the temperature of the billets reaches the range of 180 F. and 230 :F. and at which the moisture is removed from the billets. The rate is important because if the rate is too low, non-radio wave evaporative drying can become sufficiently great to cause undesirable seasoning degrade, even to the extent of being excessive. If the power level is too high, the tanoak billets shrink excessively and be come undesirably dense, and often rupture or suffer internal scorching. The power should be maintained at a level at which less than thirty percent shrinkage in diameter occurs. In the case of green tanoak billets having a size and shape as described above, it Was found that efficiency and yields are optimized if a billet is subjected to microwave power in the range of 0.3 kw. to 5 k w.

The above optimum power range is accurate for green tanoak billets of the above-described size and shape. For tanoak billets of different sizes, shapes, and moisture content the optimum power range Would change. However, the optimum power range easily can be determined for tanoak wood of any size and shape at various moisture content levels by experimental methods. Such a method would be to expose various pieces of tanoak wood of a particular size, shape and moisture content to different levels of power and inspect the pieces for excessive shrink age or seasoning degrade after the desired amount of moisture has been removed.

As described hereinbefore, drying tanoak wood in accordance with the process of the present invention can be conducted in various ways including exclusively with radio wave energy, with radio wave energy combined with forced evaporative drying and with radio wave energy combined with natural air drying.

In one case, tanoak wood billets were dried exclusively with radio wave energy at microwave frequencies in a closed multimode microwave cavity. The tanoak bil lets were initially at room temperature, had a size and shape described hereinbefore and had an initial average green state weight of eleven and one-half pounds and an average dry weight of four and one-half pounds. The billets were placed in the microwave cavity in a configuration with a spacing of about one inch between the sides of adjacent billets. Microwave power of 30 kw. at a frequency of 2450 mHz. was coupled into the cavity for about four hours to remove about four and three-quarters pounds of moisture and, thereby, reduce the moisture content of the billets to about fifty percent, by weight. At this moisture content level, the internal temperature of the tanoak wood billets began to increase above 230 F. as additional moisture was removed at the input power level of 30 kw. If the input power level is maintained at 30 kw. as the tanoak billets are dried below a moisture content of fifty percent by weight, the tanoak billets often rupture or suffer internal scorching.

To minimize such damage to the billets, the power was decreased to maintain the internal temperature of the billets below 230 F. as the average moisture content of the billets was reduced below fifty percent by weight. For example, as the average moisture content is reduced from about fifty percent to about forty percent by weight, the microwave power level should be reduced to a maximum of about 1.125 kw. per billet. This is accomplished by gradually reducing the microwave power from 30 'klW. to about 22.5 klW. as the moisture content is reduced from fifty percent to forty percent by weight. As the billets are further dried with microwave power to reduce their average moisture content to about twenty percent by weight, the microwave power level should be reduced to a maximum of about 0.375 kw. per billet by gradually reducing the power level to about 7.5 kw. At moisture content levels below fifty percent, the reduction in the microwave power level is generally linear with the reduction of the moisture content of the tanoak billets. When drying tanoak billets in this manner, a process yield of ninety-five percent was obtained and diameter shrinkage was maintained at about ten percent.

The optimum power level for drying tanoak wood at various moisture content levels can be determined experimentally. One technique that might be employed would be to subject samples of tanoak wood having various beginning moisture content levels of power for a definite period. Immediately upon the termination of the application of the power, the internal temperature of the samples would be measured by inserting a thermometer into the sample and the ending moisture content determined by weight measurements as discussed above. The optimum power levels are those which produce an internal temperature in the range of 212 to 230 F. By using this observed data as a guide in setting the radio wave power level for drying the tanoak wood, the tanoak wood can be dried exclusively with radio wave power without subjecting it to excesive levels of power. Thus process yields of useful tanoak billets as high as ninety five percent may be obtained by drying green tanoak billets exclusively with microwave energy in the manner described above.

Once the tanoak billets are dried to a moisture content of about fifty percent by weight, they can be dried to their final moisture content by any of the non-radio wave evaporative drying techniques commonly employed in the lumber industry without causing excessive seasoning degrade. For example, the tanoak billets could be removed from the microwave applicator and placed in air for drying to the final desired moisture content by normal air drying.

Process yields of useful tanoak billets as high as ninety percent can be obtained by commencing microwave drying before the moisture content of the billets is reduced below about eighty five percent by weight by non-radio [wave evaporative drying and, thereafter, continuing to dry the billets with microwave energy until their moisture content is reduced to about fifty percent by weight.

The process yield of useful tanoak billets depends upon the amount of moisture lost through non-radio wave evaporative drying before commencing, during, and after terminating radio wave drying. For example, if radio wave drying is started before non-radio wave evaporative drying reduces the moisture content of the tanoak billets below about sixty percent and is continued at least until the moisture content is reduced to about fifty percent, excessive seasoning degrade will be avoided and process yields of at least thirty percent will be realized. Process yields of fifty percent will be obtained if radio wave drying is commenced before the moisture content of the tanoak billets is reduced below about seventy percent by weight and continued at least until the moisture content is reduced to about fifty percent. Although process yields between thirty and fifty percent can be realized by conventional non-radio evaporative drying techniques, much longer drying times are required than when radio wave energy is used to dry tanoak wood, e.g., four to six months as compared to one to seven days.

With an exception to be discussed in detail hereinbelow, generally the process yield of useful tanoak billets will be less than optimum if radio wave drying is terminated before the moisture content reaches fifty percent by weight. The amount of the decrease depends upon how much the moisture content of the tanoak wood is above fifty percent by weight when the radio wave drying is terminated.

If the tanoak billets are placed in kilns or other force type non-radio Wave evaporative dryers after being subjected to radio wave energy, but before they cool substantially from the temperature they have at the termination of being subjected to radio wave energy, the tanoak billets can have much higher moisture content levels when subjected to non-radio evaporative drying without suffering seasoning degrade than in those cases when they are allowed to cool before being placed in the kilns and the like. In one case, green tanoak billets each of a size and shape described hereinbefore are placed in a microwave oven and subjected to 30 kw. of microwave power at frequency of 2450 mHz. until about three pounds of moisture are removed. The tanoak billets are then removed from the microwave cavity and placed in a kiln, preferably before the temperature of the tanoak billets falls below F. and in a configuration to permit good circulation of air. The kiln is heated to a temperature in the range of 212 F. to 220 F. To reduce the moisture content of the tanoak billets to the desired final moisture content by weight, they remain in the kiln for about one week, the exact time depending upon the exact final moisture content desired as well as the volume of air flow and the temperature of the air directed through the kiln oven. Process yields of useful tanoak wood billets as high as ninety-five percent have been achieved by drying the billets in this manner.

Excessive seasoning degrade can be avoided, i.e., process yields greater than thirty percent can be obtained by subjecting green tanoak billets to radio wave energy to raise their temperature throughout to the range of 180 F. to 230 F. and, after being brought to temperature, placing them in a force type dryer, such as a kiln, to dry the billets to their final desired moisture content. For example, eighty (80) green tanoak wood billets at room temperature and having a size, shape and moisture content as described hereinbefore were subjected to 30 kw. of microwave power for about twenty minutes to raise their temperature to about 212 F. The billets were then removed from the microwave applicator and placed, for example, in a kiln before their temperature fell below about 180 F. Drying the tanoak billets with radio wave energy and forced type dryers in the foregoing manner has the advantage of requiring less radio wave power than the other ways of conducting the process of the present invention but the average process yield may be lower.

It is possible to combine radio wave and air drying techniques to dry the tanoak billets after the tanoak wood no longer suffers excessive seasoning degrade when exposed to non-radio wave evaporative drying. In such cases, a current of hot air at a temperature in the range of about 210 F. to 220 F. would be established through the radio wave applicator in contact with the tanoak billets. The hot air current serves to aid in drying the billets and exhausting the applicator.

However, when the tanoak billets are in a condition such that seasoning degrade is caused when they are exposed to non-radio wave evaporative drying, it is preferred to avoid establishing a current of hot air through the applicator for two reasons. Firstly, the hot air tends to promote undesirable non-radio wave evaporative drying. Secondly, the hot air current or even a cold air current, exhausts the moisture liberated from the billets. It is desirable to keep the atmosphere in the applicator humid during radio wave drying. The humid atmosphere keeps the surface of the tanoak billets moist, thereby preventing the occurrence of any significant surface evaporation. By confining at least some of the moisture removed from the tanoak billets in the applicator so that a moist atmosphere having a relative humidity of at least about seventy (and preferably about eighty-five) percent surrounds the tanoak billets, surface evaporation can be reduced and undesirable seasoning degrade such as surface checking prevented.

In drying tanoak wood with radio wave energy, the radio wave energy can be in the form of continuous wave power, pulse power or modulated power. The use of pulse or modulated power would allow the use of radio wave energy at higher absolute power levels than possible with continuous wave power. Furthermore, it is possible to conduct radio wave drying at intervals whereby radio wave drying is terminated for extended periods. If the radio wave drying is terminated while the tanoak wood is still susceptible to seasoning degrade when exposed to non-radio wave evaporative drying, steps such as described hereinbefore should be taken to prevent moisture lost through non-radio wave evaporative drying so that the process yield is not reduced by the interruption of the radio Wave drying.

By the process of the present invention, tanoak wood can be processed so that it is an economically useful hardwood which can compete with other hardwoods for the various hardwood uses. Furthermore, it has been found that baseball bats manufactured from tanoak wood may be superior to those manufactured from other woods, commonly ash and hickory. Two of the more significant superior characteristics of tanoak baseball bats are fine grain structure which appears to contribute to longer life and a resilience which appears to provide an ability to propell a baseball farther than baseball bats made from other woods. It is believed that tanoak wood has a lesser tendency to split in use in comparison to the other hardwoods due to its fine grain structure. It is also believed that tanoak baseball bats will propell a baseball a distance about three percent farther than baseball bats of other woods because ofthe superior resilience of tanoak wood.

While the process of the present invention has been described in detail with reference to drying tanoak of a particular size and shape, it is not intended to limit the invention. As explained hereinbefore, when compared with standard non-radio wave evaporating drying, superior yields are obtained in a shorter time and with less equipment and facility expense even when tanoak wood is allowed to dry to an average moisture content by weight of sixty percent by non-electromagnetic evaporative drying or the radio wave drying terminated before its moisture content is reduced to fifty percent. Hence, the present invention is not to be limited except by the terms of the following claims.

What is claimed is:

1. A process of drying tanoak wood to a selected moisture content level which tanoak wood is to be used for manufacturing baseball bats and the like comprising preshaping the tanoak wood into an elongated billet having a cross section in the plane of the growth rings defining a curved outline form, subjecting the preshaped tanoak wood billet to radio wave energy at a selected power before its moisture content is reduced by non-radio wave evaporating drying to less than about sixty percent dry weight, adjusting the radio wave power to maintain the internal temperature of the tanoak wood billet in the range of about 212 F. to 230 F., and continuing to subject the tanoak wood billet to radio wave energy to evolve moisture therefrom at least until subsequent non-radio wave evaporative does not effect excessive seasoning degrade.

2. The process according to claim 1 further comprising maintaining the moisture content of the tanoak wood after harvesting at a level above about eighty-five percent dry weight until subjected to radio wave energy.

3. The process according to claim 1 wherein said tanoak Wood billet is subjected to radio wave energy before its average moisture content is reduced by non-radio wave evaporative drying to less than about eighty-five percent dry weight, and further comprising surrounding said tanoak wood billet with a moist atmosphere at least while subjected to radio wave energy.

4. The process according to claim 3 wherein said tanoak wood billet is subjected to radio wave energy without interruption until its moisture content level is reduced to said selected level.

5. The process according to claim 3 further comprising subjecting said tanoak wood billet to drying air to evolve moisture therefrom after its moisture content is reduced so that non-radio wave evaporative drying does not effect excessive seasoning degrade.

References Cited UNITED STATES PATENTS 2,543,618 2/1951 Wood 34-1 2,567,983 9/1951 Wood 34-1 3,031,767 5/1962 Wood 34--1 WILLIAM E. WAYNER, Primary Examiner US. Cl. X.R.