Hydrostatic compression method for producing a fancy log from a primary wood

A hydrostatic compression method for producing a fancy log with a decorative and complicated external appearance from a primary wood. In the method, a primary wood having a water content adjusted in the range of 10-80 wt % is brought into a softened state, then the softened wood is compressed with hydrostatic pressure by means of liquid as pressurizing medium. Next, the compressed wood is treated with a fixation means to fix the compressed state. The fixation means can be a shaping jig, a mold, heating in a particular temperature range conducted while constraining the volume relation of compressed wood, cooling down below the softening point of the wood while under pressure, compact-packing together with hard particles into a vessel followed by heating, or a primary wood is chemically treated to form a localized wood-plastics composite before applying hydrostatic compression.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to a novel forming method for wood to obtain a
 unique ligneous material having high hardness, a beautiful grain and an
 excellent external appearance. More specifically, this invention relates
 to a compression forming method for shaping a wood by using hydrostatic
 force of a pressurizing liquid.
 This invention can be applied to a manufacturing process to economically
 produce, for example, a shaped ligneous material with improved surface
 hardness, or a sawn lumber with a beautiful grain and improved physical
 properties, or a so-called "fancy log" with decorative and complicated
 external appearance, without using a mold. The application of this
 invention is, however, not necessarily limited to these, and this
 invention is useful in other ways of application in industry.
 2. Description of the Prior Art
 Forming method, so-called "densification" for soft wood such as cedar, to
 improve the physical properties as well as its shape is known in the art.
 In the densification, a primary wood is heated in order to bring it into
 softened state, then compressed by using a mold to form it into a desired
 shape to obtain a shaped ligneous material with high surface hardness. The
 word "primary wood" means a log, lumber, or any other form of wood used as
 raw material which is to be treated in the process of this invention.
 For example, a forming method has been practiced commercially, in which a
 timber of cedar is heated by hot water or saturated steam to around
 100.degree. C. to bring the wood into a plasticized state. It is then
 compressed with the solid surfaces of a mold to form a pillar having
 polygonal cross section. Next, the shape is stabilized by cooling or
 drying the compressed wood and the wood is kept in the mold for many
 hours.
 Also, another forming method is known in the art, in which a primary wood
 is plasticized by means of steam at a high temperature and pressure. Next,
 the plasticized wood is loaded between a pair of mold plates and
 compressed while steam under high or atmospheric pressure is applied. Then
 its shape is stabilized by leaving it in the mold under an atmosphere of
 high temperature steam for period of hours.
 In either case, a shaped ligneous material having a higher density and a
 higher hardness in comparison with a primary wood is obtained. It should
 be noted that compression of wood by mold surfaces, when carried out to
 excess, can cause damage to the wood tissues resulting in local
 deterioration of hardness. Typically, however, compression of wood has
 been conducted almost exclusively by using the solid surfaces of a mold.
 Moreover, as described above, it is absolutely necessary for a compressed
 ligneous material to be held in a mold closed under mechanical force for
 many hours, in order to stabilize the shape of the compressed ligneous
 material.
 Up to this time, a few methods have been practiced commercially with the
 above purposes in mind. For example, cooling water is supplied through an
 inner path of the mold to cool down the temperature of compressed wood
 below the softening point, for a period of time. Then, the shaped material
 is taken out of a mold. In another way, after compressing a softened wood,
 the mold holding the compressed wood is treated in steam at around
 180.degree. C. for a period of time, then the shaped material is cooled
 and removed of a mold.
 Without said treatment by high temperature steam, the stress generated by
 the forming is retained in a compressed wood and will cause relaxation of
 compressed state of the wood when the shaped ligneous material is heated
 to a temperature above its softening point. That means, the volume of a
 compressed wood is recovered up to nearly the original volume, (i.e. this
 is so-called volume relaxation), and also a rebound in its shape takes
 place until it approaches to original shape.
 It is evident from the above discussion that a long residence time in the
 mold is required for wood in the conventional densification methods, which
 will inevitably push up the production cost of shaped articles since
 manufacturing cost of a mold is generally high.
 For the purpose of reducing a residence time in a mold, a method was
 proposed in an earlier development. That is, the use of a jig with
 sufficient strength to withstand a volume relaxation (called a shaping jig
 hereinafter). The jig is usually installed inside the mold, and softened
 wood is compressed inside the jig. When compression is over, the jig parts
 are firmly mechanically connected together along their edge lines, and the
 jig holding the compressed wood inside is immediately removed from the
 mold.
 This method may be useful for manufacturing smaller shaped articles. But it
 can not be practical for production of large material, such as lumber for
 building a frame house, since rather thick, heavy metal walls are need for
 the construction of the jig.
 In practice, after all, there has been almost no method, except use of
 molds or jigs, useful for the compression of wood and for the fixation of
 the resulting compressed structure or state. At the same time, no method
 has been proposed so far to utilize hydrostatic pressure for direct
 compression of plasticized wood in a liquid.
 SUMMARY OF THE INVENTION
 The object of the present invention is to provide a method for obtaining a
 shaped ligneous material with improved hardness, density, without
 utilizing a mold.
 A further object of this invention is to provide a method for fixing the
 compressed state or internal compressed structure of densified wood
 obtained by aforementioned forming to prevent it from volume relaxation or
 dimensional change during its use.
 A still further object is to provide a shaped ligneous material produced by
 employing the above method having a decorative external appearance and
 beautiful internal grains which become visible when sawn. Other objects
 and advantages of the present invention will become apparent from the
 detailed description to follow taken in conjunction with the appended
 claims.
 According to the first invention, a primary wood, such as a log, sawn
 lumber and the like, is brought into softened state by heating it above
 its softening temperature by means of, for example, hot water or steam at
 high temperature. Then, the softened wood is compressed to a desired
 compression ratio, for example, roughly 50% by cross-sectional ratio, to
 shape a densified wood by means of hydrostatic pressure of a pressurizing
 liquid in replacement of a mold used in conventional technologies. By an
 effect of the hydrostatic compression, the physical properties, such as
 density and surface hardness, of a primary wood, in the form of log,
 column, plank, square lumber and the like, increase with the decrease of
 its volume, and a densified ligneous material with improved physical
 properties is obtained.
 Features of hydrostatic forming method of this invention will be explained
 as follows, taking a log as an example of a primary wood. At the moment
 when the pressure of pressurizing liquid exceeds the yielding stress of
 the softened log, compression by hydrostatic pressure starts. Generally,
 the compression is considered to begin at the surface layer part of a log
 (i.e., sap wood) which is softer than the heart wood. At the first stage
 of compression where the sap wood is compressed, a log is compressed by
 isostatic force exerted by pressurizing liquid in a direction
 perpendicular to annual rings. Consequently, the diameter of the log
 decreases due to collapse of cells in the early-growing part (i.e., early
 wood). But, the late-growing part of a log (i.e., late wood) which cells
 are stronger in mechanical strength due to small cell size and thick cell
 walls, tends to resist deformation by compression.
 Therefore, the length in the tangential direction of each tree ring (i.e.
 the contour length), is hardly changed by compression. As the result,
 individual annual rings display a wave-like pattern at the surface layer
 part and a complicated undulation, reflecting the internal deformation,
 appears on the side surface of the compressed log. At the stage of this
 surface layer compression, a log having a decorative appearance of a
 so-called "fancy log" is obtained as the shaped ligneous material.
 At the later stage of compression, the surface layer part, which has
 already been mechanically strengthened by preceding compression, is forced
 to cave in towards the softened heart wood. As a consequence, the cross
 section shows the highly deformed annual rings consisting of non-circular
 closed curves with many large bends. The highly compressed log is in
 itself useful due to its decorative appearance. It is also a useful
 starting material for production of boards, pillars and other sawn
 material with beautiful grains.
 On releasing the pressure of pressurizing liquid at the same temperature
 used during hydrostatic compression, a volume relaxation of the compressed
 log takes place immediately, and the log recovers the volume by a recovery
 ratio of about 90% . However, a densified wood obtained according to the
 compression forming of the present invention has tendencies to allow a
 lower incidence of cracking by drying and to reduce the size of cracking,
 though a substantial expansion by volume relaxation is inevitable.
 That is to say, one of the advantages of this invention is that a densified
 ligneous material in the form of logs obtained as mentioned above does not
 need a processing called karfing for aging, though a timber usually needs
 karfing to prevent it from cracking, by drying before it is normally used
 without compression.
 To bring a primary wood into softened state, it is necessary to heat it
 above the softening temperature of lignin and hemicellulose. The softening
 point of a primary wood is dependent on the water content of the wood, and
 is generally around 100.degree. C., if a wood contains moisture above its
 fiber saturation point. The softening temperature rises with a decrease in
 moisture content below the fiber saturation point. The moisture content
 quoted above means the percentage by weight of total water existing in the
 ligneous tissue versus total weight of the wood. Total water comprises
 free water existing freely in the cell cavity and combined water bonded to
 components of ligneous material by a hydrogen bond and so forth.
 Hydrostatic compression becomes substantially difficult if the free water
 content is exceedingly high, as the space in vascular tissue and cells is
 then almost filled with free water. Hydrostatic compression is also
 difficult if water content is so low as to make a primary wood extremely
 dry. Many cracks develop on the surface of wood where the pressurizing
 liquid breaks into the wood. Further, the aforementioned rise of the
 softening point of wood along with drying causes inconvenience for
 softening the wood. For these reasons, the water content of a primary wood
 is desirable in the range between 10% and 80% by weight.
 Furthermore, in the aforementioned forming method, softening a primary wood
 and compressing the softened wood by hydrostatic pressure can be done
 simultaneously by using, for example, hot water at a high temperature,
 preferably at a temperature above the softening point of the primary wood.
 According to the second invention, a shaped ligneous material stabilized
 against temperature in various uses through restraining the volume
 relaxation is obtained by treating the densified wood with fixation means
 to fix the compressed state of wood.
 Fixation of the compressed state is defined as a semi-permanent retention
 of the compressed state in terms of volume, dimension, shape, and internal
 structure units like tracheas, pits or cell cavities and so forth, in the
 compressed and temporarily stabilized ligneous material, irrespective of a
 change in humidity and ambient temperature imposed on a shaped ligneous
 material.
 Conventionally, fixation of compressed state has been achieved by heating
 densified wood in the mold for hours, in case of a compression shaping by
 recourse to a mold. A combination of an upper concept regarding the
 fixation of compressed state and the hydrostatic forming is not known in
 the art. The hydrostatic compression forming of wood, as the forming
 method itself, is quite novel.
 According to the third invention, a shaped ligneous material fixed in the
 compressed state in a desired shape is obtained by loading a densified
 wood, produced the aforesaid hydrostatic compression, in a shaping jig
 under the compressed condition. Next, the densified wood is relaxed
 slightly in its volume by reducing liquid pressure, so that the densified
 wood presses its surface against the inside wall of the jig. The shape of
 the densified wood will be defined by the shape of the jig cavity.
 By this method, a shaped article, such as a column, with excellent surface
 hardness and high accuracy in circular cross section, or a pillar with a
 desired geometrical pattern on its surface, for example, can be prepared.
 This method has no recourse to any mold but makes use of only a shaping jig
 fabricated at low cost, resulting in economizing fabrication cost of a
 mold and its associated apparatus, as well as running cost. Further,
 loading the compressed wood in the jig is much easier than loading a log
 in a mold in the air, as it is carried out in the pressurizing liquid.
 Consequently, manpower can be cut down as well.
 According to the fourth invention, a shaped ligneous material fixed in the
 compressed state is obtained by compressing a softened wood using the
 aforesaid hydrostatic pressure of a pressurizing liquid, then cooling down
 the densified wood by lowering the liquid temperature while maintaining
 the liquid pressure. In the method by the fourth invention, the cooling
 temperature is desirable to be chosen between ambient temperature and
 softening point of a primary wood.
 As a way of cooling, a pressurizing liquid of low temperature can be
 charged into the vessel used for hydrostatic compression under high
 pressure while discharging hot liquid to exchange the hot liquid for cold
 liquid in a short period of time.
 According to the method of the fourth invention, a compressed state of
 densified wood shaped by hydrostatic compression is fixed without using
 any shaping jig. The shaped ligneous material obtained by this method has
 a decorative appearance on the whole surface, displaying furrows which are
 characteristics of hydrostatic compression, originating from selective
 compression of softer parts of the primary wood.
 According to the fifth invention, a shaped ligneous material fixed in the
 compression state is obtained by compressing a softened wood using
 aforesaid hydrostatic pressure of a pressurizing liquid, then heating up
 the densified wood by elevating the liquid temperature while holding the
 liquid pressure.
 Upon heating the densified wood, while keeping the liquid pressure as
 described above, a compressed state is presumably fixed by the effect of
 hydrolysis of hemicellulose and lignin contained in ligneous tissue. This
 results in the elimination of internal stress generated in a ligneous
 material during compression.
 The heating temperature is desirably in the range of 140-180.degree. C.
 wherein the abovementioned change takes place. The shaped ligneous
 material obtained by this method also has a decorative appearance on the
 whole surface, displaying characteristic furrows which are attributable to
 hydrostatic compression.
 The advantage of the fifth invention is that the effect of fixation of a
 compressed state by heating of a densified wood is sustained permanently,
 on the contrary to the fact that the effect of fixation by cooling of a
 densified wood lasts more or less temporarily.
 According to the sixth invention, a shaped ligneous material fixed in the
 compressed state is obtained by compressing a softened wood using the
 aforesaid hydrostatic pressure of a pressurizing liquid and by stabilizing
 the compressed state temporarily, then releasing said liquid pressure. The
 temporarily stabilized densified wood is then loaded in a treatment vessel
 and the space between the surface of densified wood and the inner wall of
 the vessel is filled up with heat-resistant hard particles in a state of
 compact-packing, and is nearly in the state of so-called "closest
 packing". The contents in the vessel are then heated to fix the compressed
 state of the wood.
 In the method of the sixth invention, hard particle having the particle
 size in the range of 0.3-4.0 mm can be used. A small particle size is
 desirable for fixation of compressed state of a densified wood having fine
 undulations on its surface with a decorative appearance or for a fixation
 of a densified planks formed by compression between heated mold plates for
 which smooth surface is required. A particle size less than 0.3 mm is not
 desirable, as it is difficult to the remove particles from the decorative
 surface of shaped ligneous material after fixation is completed.
 On the other hand, the particle size in the range of 2-3 mm is convenient
 in case of forming a densified log for sawing to produce sawn lumber after
 the densification. The particle size beyond 4 mm is not desirable, as the
 surface of the ligneous shaped article becomes rough and pneumatic
 conveyance of particles to and from the vessel becomes difficult.
 In the method of sixth invention, filling up the space with particles is
 necessary to the extent of minimum occupation of the vacancy in the vessel
 or filling to the state of compact-packing by means of, for example,
 vibration of the vessel. At the state of compact-packing, as described
 above, volume relaxation of the densified wood, which is fixed
 temporarily, is restrained; therefore the force to expand in radial
 direction of wood is checked by the effect of frictional force exerted
 among hard particles.
 By heating the whole contents in the vessel under the restrained condition,
 upon relaxation, the compressed state of densified wood is permanently
 fixed. A heating temperature in the range of 180-250.degree. C. is used,
 in case of dry-heating, and 140-190.degree. C., in case of wet-heating.
 Saturated steam, for example, can be used, resulting in the completion of
 fixation in a short time period due to very good heat transfer
 attributable to the heat of condensation of steam, as the heating fluid
 can pass through the layer of hard particles. Superheated steam also can
 be used, resulting in simultaneous progress of fixing of compressed state
 and drying of the shaped ligneous material.
 Furthermore, the fixation method of this invention described above can also
 be applied to a densified wood formed by utilizing a mold for compression.
 In this case, however, the mold is used for shaping a softened wood into a
 densified wood with desired cross-sectional shape only for short period of
 time. The fixation of the compressed state, which needs many hours after
 compression shaping, is achieved by loading the densified wood together
 with heat-resistant hard particle in the vessel used in the sixth
 invention. Productivity per mold for a shaped ligneous material is fairly
 improved, as the residence time of a densified wood in the mold is
 remarkably shortened by this invention.
 Still further, according to the seventh invention, residual stress in a
 primary wood, in the form of a log, lumber and the like, is eliminated by
 loading a primary wood together with heat-resistant hard particle in a
 vessel, then filling up all the space in vessel with the particle to
 become the state of compact-packing as mentioned above. Then the whole
 contents in the vessel are heated.
 By the method of seventh invention, logs and lumber free from dimensional
 changes irrespective of changes in moisture and ambient temperature is
 produced conveniently and with high productivity.
 Lastly, an invention utilizing a chemical means on a primary wood to make
 hydrostatic compression and succeeding fixation easy is described below.
 According to the eighth invention, a shaped ligneous material fixed in the
 compressed state is obtained by treating a dried primary wood by resin
 impregnation using an impregnation liquid containing vinyl monomer as a
 principal ingredient. Next, the vinyl monomer impregnated in the wood is
 polymerized to produce a ligneous material containing a synthetic resin.
 Then the ligneous material is compressed by applying hydrostatic
 compression at the temperature above the softening point of primary wood,
 followed by cooling the compressed ligneous material while maintaining the
 liquid pressure.
 In the method of the above-mentioned invention, the liquid containing vinyl
 monomer penetrates into the vacancy existing in the primary wood. It also
 fills any cracks on the side and end surfaces, and finally polymerizes
 into synthetic a resin. As the synthetic resin exists in the manner of
 plugging the cracks and vacancies, it can hinder the pressurizing liquid
 from penetrating into the wood at the time of hydrostatic compression.
 This is important, as hydrostatic compression becomes substantially
 difficult if pressurizing liquid penetrates into the primary wood through
 cracks on its surfaces.
 In the method by the eighth invention, the monomer used is of the liquid
 type having an affinity for a primary wood. A single substance or mixtures
 chosen from styrene, methyl methacrylate, vinyl acetate, hydrophilic
 acrylic monomers such as polyethylene glycol methacrylate, and glycidyl
 acrylate, unsaturated polyesters, and so forth, can be used in the present
 invention, although the invention is not limited to these examples cited
 above.
 Further, in the method by the eighth invention, the impregnation liquid can
 contain, as one of the principal ingredient, at least one kind of high or
 medium molecular weight compound with high or medium degree of
 polymerization selected from high polymers, pre-polymers or oligomers,
 along with the aforesaid monomer. These polymers, pre-polymers and
 oligomes need to be soluble in the vinyl monomer and are the component
 which regulates the viscosity of impregnation liquid. By using the
 impregnation liquid containing the polymer dissolved in the vinyl monomer,
 the penetration of the pressurizing liquid into a primary wood can be
 prevented more effectively.
 The shaped ligneous material obtained by the method of the eighth invention
 also shows fine undulations which are characteristic of the surface of
 shaped wood by hydrostatic compression, indicating that it can be a
 valuable decorative material.
 It should be stressed that the shaped ligneous material obtained by the
 above-mentioned method shows high dimensional stability against changing
 humidity and ambient temperature, since the synthetic resins contained in
 the surface layer of the densified wood effectively prevents moisture from
 penetrating into the wood. In order for a densified ligneous material to
 undergo volume relaxation at the temperature of ordinary use, it is
 absolutely necessary that moisture content increases, for some reason,
 beyond its fiber saturation point. Thus, the eighth invention provides
 with a densified wood having permanently fixed compressed structure
 without recourse to treatment for fixing at high temperature.
 In practice, all the present inventions described above are applied to soft
 coniferous wood, such as cedar, larch, Japanese cypress, Port Orford
 cedar, Douglas fir, Oregon pine, Western hemlock and the like. Basically,
 however the present inventions should not be restricted to particular
 species of wood.

DETAILED DESCRIPTION OF THE INVENTION
 The invention is illustrated in more detail by reference to the following
 examples. However, the present embodiments are to be considered in all
 respects as illustrative and not restrictive. In the embodiments, unless
 otherwise indicated, the percentage of moisture content is by weight. The
 preparative recipes of impregnation liquids are by weight as well. The
 fundamental properties of the wood such as surface hardness and so forth
 were measured according to the method of Japanese Industrial Standard JIS
 Z 2101-1994. Further, the degree of fixation of the compressed state is
 indicated by the recovery ratio meaning the percentage of decrease by
 compression in terms of cross-sectional area or thickness in the direction
 of compression is recovered by relaxation, which is calculated by Equation
 1 or 2.
 EXAMPLE 1
 The first invention is explained referring to FIG. 1 which shows an example
 of the apparatus for practicing hydrostatic compression forming. The
 apparatus 40 is equipped with a pressure vessel 41, a heater 42, a water
 tank 43 and a pump 44. A thermostat, known in relevant industry, may be
 equipped on the heater 42, although this is not shown in FIG. 1. Further,
 reference numeral 45 is for a drain valve furnished to the vessel 41, 46
 is a valve for venting air or nitrogen gas to the atmosphere, 46 a for a
 pressure-releasing valve, 47 for a cover of the vessel, 49 for a gas
 cylinder containing nitrogen which can be substituted by an air compressor
 in certain cases, 48 for clamp furnished to the vessel for tight-sealing.
 As many as 20 pieces of raw bolts of Japanese cedar with bark having a size
 of about 140 mm-160 mm in the diameter at the butt end and 2000 mm in the
 length. The average water content of the raw bolts is 120% and the bolts
 were treated by drying to reduce the average moisture content to 50%, in
 the atmosphere of steam with the pressure of 1.0 kg/cm.sup.2 G at
 104.degree.C. for 3 days. Among these dried bolts, 10pieces were selected
 randomly to be daubed with a polychloroprene based adhesive for wood
 (commercial name; Bond G 17, manufactured by Konishi Co. Limited) on the
 both cut ends and semi-dried at about 100.degree. C. Next, a
 polyvinylidene chloride film with the thickness of 20 microns,
 commercially used for food packaging, was glued on the layer of said
 adhesives, and the film at both cut ends was bound on the bolt by means of
 a heat-resistant rubber string.
 Then, after softening these bolts by heating in an air oven at 95.degree.
 C. for 3hours, the bolts were loaded in a pressure vessel 41 as shown by
 FIG. 1 with the inside diameter of 900 mm and the length of 3000 mm, and a
 cover 47 was closed. Then hot water at 95.degree.C. was filled by using a
 pump 44. The hot water was pumped in for about 10minutes until the
 pressure reached to 25 kg/cm.sup.2 G. After keeping the pressure at
 aforementioned level by means of a relief-valve 46 a set at 25 kg/cm.sup.2
 G, for 10 minutes, the pressure was released and hot water was then
 returned to a water tank 43, and then the bolts were cooled spontaneously
 to ambient temperature.
 The diameter of the bolts at this stage after applying hydrostatic
 compression was smaller by 5% than before the treatment. The bark was
 partially peeled off from the ligneous part. The bolts treated as
 described above were loaded in a dryer working at constant temperature
 with the size of 2000 mm in the inside width, 2000 mm in the inside depth
 and 2000 mm in the inside height, respectively, then dried at 80.degree.C.
 under the control on moisture of atmosphere inside at 80% until the
 average moisture content reached to 20%.
 By observing the shaped ligneous material obtained as aforesaid, cracking
 by drying was noticed on the side surface of 4 pieces out of 10 bolts.
 In comparison to EXAMPLE 1, the other 10 pieces of the bolts with moisture
 content of 50% were dried in the same dryer under the same condition as
 EXAMPLE 1until the average moisture content reached to 20%. Cracking after
 drying was observed on the surface of 9 bolts out of 10 bolts. The data
 indicates an advantage of hydrostatic compression forming by the first
 invention to improve the physical properties of wood, enabling to avoid
 the aforementioned karfing on drying logs by adopting the hydrostatic
 compression.
 EXAMPLE 2
 A bolt of Japanese cedar with the size of 150 mm in the diameter at the top
 end and 1000 mm in the length, with its bark chipped off, was wrapped with
 a commercial polyester film of 100 microns in the thickness, and pinholes
 were opened on the film. Then, the bolt was dried for 3 days in an air
 oven kept at 110.degree. C., resulting in decrease in the moisture content
 to 37%. The bolt was taken out of the dryer to use as a primary wood for
 hydrostatic compression.
 Both cut ends of bolt were daubed with a 20% solution of polychloroprene in
 methylene chloride and semi-dried, then a polyvinylidene chloride film,
 typically used commercially for food packaging, was glued on the both
 ends. Next, the film was bound on the bolt at each end by a heat-resistant
 rubber string.
 Although the covering of cut ends with polyvinylidene chloride film is
 effective in preventing water from penetrating into the wood, it is not
 always a substantial part of the invention. The covering is necessary when
 higher pressure is required, as in the case where the bolts or logs are to
 be compressed to the heart wood. When compression is required to be
 limited to the peripheral part or the sap wood, the liquid pressure does
 not need to be too high. Treating the end surfaces with heat-resistant
 adhesives can be enough in such a case.
 Next, the bolt was loaded in a pressure vessel 41 as shown by FIG. 1 before
 the temperature of bolt being raised below its softening temperature, and
 hot water controlled at 95.degree. C. was pumped in. When the air inside
 was completely replaced by hot water, pressure-releasing valve 41 was
 closed. As soon as the valve 46 is closed, the inside pressure rose to 8-
 10kg/cm.sup.2 G. The pressure stayed in the region for a while and then
 rose rapidly to 30 kg/cm.sup.2 G, and the relief-valve 46a functioned to
 release the excess pressure, resulting in the pressure in the vessel being
 held constant by balancing of the discharging speed from the valve 46a and
 the pumping speed of hot water.
 At this stage, if the compressed bolt were taken out of the vessel after
 stopping and discharging of hot water, a shaped ligneous material by the
 first invention will be obtained by naturally cooling and drying as
 described in EXAMPLE 1.
 In this EXAMPLE 2, however, the pumping in of hot water was replaced by
 pumping in of cold water just at the time when the inside pressure reached
 constant pressure at 30 kg/cm.sup.2 G. The temperature of discharged water
 was brought down to 32.degree. C. in 15 minutes after switching of the
 water supply while the inside pressure being kept constant. The pumping
 was stopped 60 minutes after the temperature of discharged water reached
 32.degree.C. Next, the shaped ligneous material of the fourth invention
 was taken out of the vessel.
 The resulting shaped material was compressed by 50% of its original
 cross-sectional area after the hydrostatic compression forming. The side
 surface of the shaped material was totally uneven, and irregular furrows
 were noticed all over the surface, showing an external appearance
 resembling that of a so-called "fancy log".
 The shaped material fixed in its compressed state by quenching does not
 bring about the volume relaxation in the ordinary environment, since the
 softening temperature of the material is high enough to hinder rebounding.
 The stability improves as the compressed wood is further dried.
 EXAMPLE 3
 A bolt of Japanese cedar with the size of 150 mm in the diameter at the top
 end and 600 mm in the length, with its bark chipped off, was dried in the
 same manner as in EXAMPLE 2, resulting in the moisture content of 25%.
 Then both cut ends of bolt were cleaned with acetone and were coated with
 a commercial silicone coating (Toray Dow Corning Silicone PAX 305 RTV
 Dispersion). Coating was repeated three times with about 1 hour intervals
 between coats. The coating was cured by curing at ambient temperature for
 3 days.
 Next, the bolt was loaded in a pressure vessel shown in FIG. 1, then the
 vacancy inside was filled with silicone oil. The vessel was closed with
 cover 47 using clamp 48, then heated to 100.degree. C. of the inside
 temperature by means of metallic heater 42. Then, the vessel was
 pressurized to 15 kg/cm.sup.2 G by injecting nitrogen gas from the
 cylinder 49. And then, the vessel was heated an inside temperature of 160
 .degree. C. and maintained for 60 minutes while keeping the pressure
 constant. Next, the contents of the vessel were cooled down to room
 temperature.
 The shaped ligneous material obtained as above-mentioned by the fifth
 invention was compressed by 52% in terms of the ratio of volume before and
 after the hydrostatic compression forming. The side surface of the shaped
 material was uneven similar to that obtained in EXAMPLE 2, and showed an
 external appearance resembling a "fancy log". The surface hardness
 increased up to 1.5 kg/mm.sup.2. Further, a test piece in the shape of a
 disk sawn off from the material showed only a small dimensional change
 when immersed in hot water of 90.degree. C. for 20 minutes to assess the
 fixation of compressed state.
 EXAMPLE 4
 The third invention is explained referring to FIG. 2 which illustrates an
 example of the apparatus equipped with an example of a shaping jig for
 practicing the hydrostatic compression forming.
 The apparatus 20 is equipped with a pressure vessel 21, a wood-pushing arm
 22, a water tank 23 and a pump 24. The pump 24 raises the pressure of a
 pressurizing liquid L, in turn a primary wood 10 is compressed by the
 hydrostatic pressure of liquid. A shaping jig 30 is installed inside of a
 shaping part 26 of the apparatus 20. A compressed wood 10A is pushed into
 the shaping jig 30 by the wood-pushing arm 22 which is pushed from outside
 of the cover 27. At the cover opening 28, the compressed wood 10A, as
 loaded in the jig 30, is taken together out of the apparatus 20.
 A bolt of Japanese cedar with bark and having a size of 150 mm in the
 diameter at top end, 165 mm in the diameter at the butt end, 1000 mm in
 the length and its moisture content being 95% was dried in the atmosphere
 of steam of 1 kg/cm.sup.2 G at 103-105.degree. C. for 5days, resulting in
 decrease in the moisture content to 40%.
 The bolt was taken out of the dryer, and both ends of the bolt were daubed
 with the same adhesives as used in EXAMPLE 1. Next, the film and string of
 Example 1 were applied as well. After 2 hours of heating in an air oven
 controlled at 90, the bolt was loaded in the pressure vessel 21 shown in
 FIG. 2, then the cover 27 and 28 were closed. Next, hot water at
 90.degree. C. was pumped in the vessel by means of the pump 24 until the
 inside pressure raised to 25 kg/cm.sup.2 G.
 At this stage again, if the compressed bolt was removed from the apparatus
 21, a shaped ligneous material by the first invention will be available
 after natural cooling and drying as described in EXAMPLE 1.
 In the EXAMPLE 4, however, the compressed wood 10A was pushed into the
 shaping jig 30 by means of the wood-pushing arm 22 under water pressure
 controlled at 25 kg/cm.sup.2 G. Next, the pumping was stopped to release
 the pressure and the hot water was drained. As the result, the compressed
 wood 10A was pressed against the inside wall of the shaping jig 30 due to
 expansion caused by partial relaxation of its volume. Then, the compressed
 wood 10A as loaded in the jig 30 was taken out of the apparatus 20 and
 dried in an air oven at 110.degree. C. for 2days. The shaped ligneous
 material of the third invention was easily taken out of the jig 30, as the
 material shrank slightly during the drying.
 The shaped ligneous material in a shape of column as obtained above has
 high accuracy in circular cross section, and the surface hardness was
 remarkably improved when compared to that of a primary wood.
 An advantage of this method is that pillars with circular or polygonal
 cross-section can be easily produced. It should be noted that the
 production of similar shaped material by means of compression in molds is
 extremely difficult and expensive.
 EXAMPLE 5
 The forming method by the sixth invention is illustrated below.
 Preparation of Densified Wood (No. 1)
 A flat grain board of Japanese cedar with the size of 900 mm in the length,
 50 mm in the thickness in radial direction, 150 mm in the width in
 tangential direction and with moisture content of 23%, was heated in an
 autoclave using saturated steam of 2 kg/cm.sup.2 G for 60 minutes. Then,
 the softened board was removed from the autoclave and compressed in radial
 direction between a pair of hot plates controlled at 120.degree. C., until
 the board decreased in thickness to 22 mm. The hot plates were then
 chilled by circulating water in their cooling pipes, while maintaining the
 pressure, until the board temperature, at its center, reached below
 30.degree. C. A densified wood (No.1) with its compressed state
 temporarily fixed was obtained.
 A test piece with the size of 300 mm in the length, 22 mm in the thickness
 in radial direction and 102 mm in the width in tangential direction was
 machined out of the wood (No.1). To observe transformation accurately, the
 measurement points were positioned in the manner of a grid having 3 points
 (A,B,C) placed in the tangential direction and 3 points (No.1, No.2, No.3)
 in the fiber direction, giving 9 points in total.
 Compact Packing of Heat-resistant Hard Particles A vessel made of stainless
 steel was used for the heat treatment in this experiment. The vessel is of
 a cylindrical type with 2 end plates bolted to the cylinder through a
 flange. The vessel has an inside diameter of 105 mm and a length of 400
 mm. It should be noted that no packing was used at the flange so that the
 vessel could retain hard particles inside but gases, such as steam and
 air, could freely flow into and out of the vessel.
 The vessel, fixed with lower end plate, was placed in an upright position
 and alumina powder with average particle size of 0.5 mm (Morundum A-40,
 No.36, manufactured by Showa Denko Co. Ltd.) was put into the vessel to a
 depth of about 50 mm. The test piece was placed along the center line of
 the cylinder. The vacancy in the vessel was then filled with the alumina
 powder while the side wall of vessel was hammered for compact packing of
 the powder.
 In practice at large scale, it is preferable to separate each the board
 from each other at a distance of a few centimeters to avoid contact.
 Generally, average particle size of 0.3-2 mm is desirable, in case of
 fixing a densified wood with smooth surface. Natural sand with relatively
 uniform particle size, synthetic inorganic particles, like silica and
 alumina, and commercial alumina abrasives can be used as well.
 Filling and hammering was repeated several times until no more improvement
 in packing appeared to be possible. Finally, the upper end plate was
 bolted to the vessel in such way as to squeeze the powder inside. The side
 wall of the vessel was hammered again, and further bolting was done to
 better squeeze the contents.
 As an alternative for compact packing, a vibration rod can be used to
 vibrate the particles by inserting the vibration rod into the particles
 and adding more particles.
 Heat Treatment
 The vessel filled as above was placed in an autoclave and heated in
 saturated steam at 175.degree. C. for 1 hour. The steam pressure was then
 gradually reduced to atmospheric pressure and the whole was left to cool.
 The temperature for heating is desirable in the range of 140-190.degree.
 C. in case of wet-heating by means of steaming, for example, and in the
 range of 180-250.degree. C. in case of dry-heating by means of hot dry
 air, for example.
 The shaped ligneous material by the sixth invention, as above-mentioned,
 was taken out of the vessel. The material showed weight decrease of 5.8%
 due to loss of moisture. Next, the thickness of material was measured at
 the same 9 points as in FIG. 3 and the results were given in Table 1.
 Measurement of the Degree of Fixation
 The test piece of shaped material fixed as above was immersed in a water
 bath controlled at 95.degree. C. for 60 minutes. The test piece was then
 dried completely by heating in an air oven at 105.degree. C. for 3 days.
 The thickness of the dried test piece was measured at the same 9 points
 mentioned above. The percentage recovery was calculated by using the
 equation 1 and the results were given in Table 1.
 ##EQU1##
 t.sub.0 : thickness of test piece before compression
 t.sub.1 : thickness of test piece after fixation by heat treatment
 t.sub.2 : thickness of test piece after soaking and drying
 Tables
 TABLE 1
 Thickness
 after Thickness Thickness after
 Position of Compression increase on hot water Recovery
 measurement Fixation fixation % soaking ratio %
 A1 21.6 21.9 1.4 21.2 -2.5
 2 21.9 22.6 3.2 22.3 -1.1
 3 21.7 22.3 2.8 22.0 -1.1
 B1 22.5 22.2 -1.3 22.1 -0.3
 2 22.4 22.7 1.3 22.5 -0.7
 3 22.2 21.8 -1.8 21.5 -1.1
 C1 21.9 21.9 0 21.3 -2.1
 2 21.7 22.5 3.7 22.0 -1.8
 3 21.5 22.5 4.7 22.0 -1.8
 Note:
 Unit of thickness; mm
 t.sub.0 = 50 mm
 Slightly negative values for recovery are due to complete drying of the
 soaked test piece, which caused an excessive shrinkage in the radial
 direction. This should be taken actually as proof of complete fixation of
 the compressed state.
 EXAMPLE 6
 Preparation of Densified Wood (No.2)
 A bolt of Japanese cedar with bark having a size of 170 mm in diameter at
 the top end and 950 mm in the length, was dried in the same manner as in
 EXAMPLE 4 but for 2 days, resulting in decrease in the water content to
 37%. Next, both cut ends were treated in the same manner as in EXAMPLE 1.
 The bolt was then heated in an air oven at 90.degree. C. for 2 hours.
 Next, the bolt was placed in a pressure vessel of vertical cylinder type
 shown in FIG. 2. The vessel was filled with hot water of 95.degree. C. On
 closing the lid, cold water was pumped into the vessel from the bottom at
 the rate of 2 liters per minutes. The inside pressure reached 30
 kg/cm.sup.2 G in 5 minutes. Then, cold water was continuously supplied
 while the pressure was retained by means of the relief- valve until the
 drain temperature reached to 30.degree. C., which took about 15 minutes.
 Pumping of cold water was continued for another 90 minutes before
 compressed bolt was removed.
 After removing the bark, the bolt was left in a dry environment for a week.
 The densified wood (No.2) obtained had the characteristic appearance
 resembling a "fancy log". The cross sections of densified wood(No.2) was
 photo-copied on paper to measure the area. By the measurement, average
 decrease in the area by 48% was observed due to the compression.
 Fixation by Heat Treatment
 The densified wood(No.2) was split along the fiber direction and the split
 surface of the resulting piece was finished with a plane. By sawing the
 piece at a right angle to the fiber direction, a test piece of 220 mm in
 the length was prepared. The cross section of the test piece is shown in
 FIG. 4(a), wherein the letter A marked on the section means it is the butt
 end.
 Next, the test piece was treated in the same way as described in EXAMPLE 5
 by using the same vessel, alumina and autoclave as described in EXAMPLE 5
 to obtain the shaped ligneous material by the sixth invention.
 Generally, the average particle size of 2-4 mm is desirable in case of
 fixing a log for producing sawn lumber.
 The butt end after the heat treatment was photo-copied to measure the
 cross-sectional area. A comparison between FIG. 4(A) and 4(B) shows that
 details of the section profile are retained except that there is a
 decrease in the area of 1.6% which occurred due to the treatment. It was
 shown that no volume relaxation or expansion of test piece took place
 during the course of fixation by steam heating. A decrease in the weight
 by 5.4% was observed in the treatment.
 Measurement of the Degree of Fixation
 The test piece was treated in the same condition as described in EXAMPLE 5
 by using the hot water bath and the oven. The measurement of the cross
 section showed no change in the area through the treatment. That is, the
 percentage recovery calculated by the equation 2 has turned out to be
 zero.
 ##EQU2##
 s.sub.0 : cross-sectional area of test piece before compression
 s.sub.1 : cross-sectional area of test piece after compression and fixation
 S.sub.2 : cross-sectional area of test piece after soaking and drying
 By applying the above mentioned method to a log, a sawn lumber and the
 like, as a primary wood in replacement of a compressed wood, a residual
 stress existing in logs and sawn lumber can be removed. In case of this
 seventh invention, the temperature of heat treatment corresponding to
 fixation treatment is desirably in the range of 70-150.degree. C.
 EXAMPLE 7
 The forming method by the eighth invention is illustrated below.
 Driving of a Primary Wood
 Two pieces of a log with the length of 800 mm were cut out from Japanese
 cedar with bark, with the size of 150 mm in the diameter and 1800 mm in
 the length, are use as a primary wood in hydrostatic compression and as a
 control, respectively. The primary wood was dried in the atmosphere of
 steam of 1 kg/cm.sup.2 G at 105.degree. C. for 3 days. After drying, the
 bark was removed by means of a metal scraper. Cracking by drying with
 maximum width of around 1 mm and radially directed was observed at the
 heart part of both ends, as well as small cracks in the fiber direction on
 the side surface. The moisture content was calculated to be 29% by using
 the decrease in weight after drying. The weight after drying was 7.34 kg.
 Generally, it is desirable to dry the wood to approximately to the fiber
 saturation point (moisture content of about 28%) to make impregnation
 easy, though excessive drying below that point is not desirable due to
 frequent generation of surface cracks.
 Preparation of Impregnation Liquid
 A dope of polymethyl methacrylate for impregnation was prepared by
 dissolving 30 parts of polymethyl methacrylate of a commercial grade into
 100 parts of commercially available methyl methacrylate of extra pure
 grade. After cooling the solution, 2 parts of benzoyl peroxide were added.
 Further, 0.2 parts of N,N-dimethyl aniline was added to the solution as a
 promoter for polymerization just before using the dope.
 Generally, the volume of polymer to be dissolved in a dope can be adjusted
 to make viscosity of the dope suitable for filling the cracks generated on
 a primary wood. The impregnation liquid can contain any wood preservatives
 as long as they are soluble in the liquid.
 Resin Impregnation
 The dried log mentioned above was loaded in the pressure vessel of vertical
 type with the inside diameter of 200 mm. Next, the dope was poured in the
 vessel up to 1000 mm in the depth. The log, with buoyancy, was submerged
 into the dope by using a weight. Then, the vessel was tightly sealed and
 the inside pressure was lowered by a vacuum pump to 50 mm Hg and kept at
 this level for 5 minutes.
 Next, nitrogen was injected from a gas cylinder to impregnate the dope by
 pressurizing for 10 minutes. The vessel was opened to recover the dope and
 the log was removed and wiped-off to remove the extra dope on its surface.
 The polymerization of methyl methacrylate in the impregnated dope was
 completed by heating the log in the atmosphere of nitrogen at 30.degree.
 C. for 1 hour and at 90.degree. C. for another 1 hour, successively. Total
 weight of log after impregnation was 7.92 kg. Nitrogen can be replaced by
 air in case of the monomer in the dope is polymerizable in the presence of
 oxygen or moisture in the air.
 Hydrostatic Compression
 The log, after treatment, was loaded in an autoclave of the vertical type
 filled with hot water at 95.degree. C., Then the autoclave was tightly
 sealed and heated at 95.degree. C. for 30 minutes. After the heating, the
 valve at bottom of the autoclave was opened to inject cold water at
 15.degree. C. by means of a pump. The log was compressed by the
 hydrostatic pressure at 26 kg/cm.sup.2 G in the same manner as in EXAMPLE
 2, and in consequence, the temperature of water was brought down to
 30.degree. C. in 15 minutes. The operation continued for another 60
 minutes. Then, the shaped ligneous material by the eighth invention was
 removed from the autoclave.
 The cross-sectional area at both ends of log was reduced to 57% in the
 average by compression. The side surface of the log had uneven and
 external appearance like a "fancy log". Penetration of water during the
 hydrostatic compression was concluded to be minimal, as the total weight
 of the shaped material was 7.95 kg.
 Test for Physical Properties
 A 20 mm thick board was prepared from the above-mentioned log by sawing and
 planing. A non-natural beautiful grain appeared on the surface of the
 board reflecting the internal deformation of annual rings by the
 hydrostatic compression. Table 2 summarizes the results of the measurement
 on flexural strength and other physical properties conducted on the test
 piece with the dimension of 20 mm in the width in tangential direction, 20
 mm in the thickness in radial direction and 320 mm in the length in fiber
 direction cut out of the above-mentioned board. The hardness and abrasion
 data are shown also in the table. It was concluded that the shaped
 ligneous material obtained by the eighth invention as mentioned above was
 superior to a dried primary wood in all of the fundamental properties.
 TABLE 2
 Fundamental Shaped material Dried primary wood
 properties of this invention for comparison
 Dried specific gravity 0.71 0.40
 Flexural strength (N/mm.sup.2) 99 78
 Flexural modulus (N/mm.sup.2) 12300 7800
 Surface hardness (kgf/mm.sup.2) 1.26 1.05
 Abrasion loss (mm) 0.17 0.36
 The present invention may be embodied in other specific forms without
 departing from the spirit or essential characteristics thereof. The scope
 of the invention is to be indicated by the appended claims rather than by
 the foregoing description and all changes which come within the meaning
 and range of equivalency of the claims are therefore intended to be
 embraced therein.
 The entire disclosure of Japanese Patent Application No. 8-283181 filed on
 Oct. 4, 1996 and Japanese Patent Application No. 9-60365 filed on Mar. 14,
 1997 and Japanese Patent Application No. 9-106027 filed on Apr. 23, 1997,
 including specification, claims, drawings and summary are incorporated
 herein by reference in its entirety.