wood products undergoes vaporization and is released from the wood products in the form of steam this steam is continuously removed from treatment zone 10 by conduit 26 through which it is conveyed to condensing zone 28 within condensing zone 28 the steam is converted to water which is withdrawn from said condensing zone 28 through conduit 30 for subsequent treatment and disposal not shown following completion of the drying of the wood products the heat transfer medium is withdrawn from treatment zone 10 and returned to htm holding storage zone 14 by way of conduit 18 a naphthalene rich wood preservative agent then is introduced into treatment zone 10 from wood preservative w p holding storage zone 20 through conduit 22 the naphthalene rich wood preservatives agent can comprise any of the naphthalene rich preservative described hereinabove once again a sufficient amount of this preservative agent will be introduced into treatment zone 10 to completely submerge the dried wood products contained therein once the naphthalene rich preservative agent has been introduced into treatment zone 10 the temperature within treatment zone 10 is raised to a value averaging not less than about 175 u00b0 f 79 u00b0 c and not greater than about 225 u00b0 f 107 u00b0 c also in order to force the preservative agent into the open cells of the dried wood products a pressure will be applied to treatment zone 10 useful pressures will range about 115 pounds.