it is not absolutely critical the impregnation is preferably carried out at about 60 u00b0 c in the case of an industrial application of the process this time could be reduced by a few hours to a few minutes by means of an impregnation with steam under elevated pressure and temperature a better yield may be obtained by introducing between the step of impregnation and that of refining a step of treatment of the softened chips with vapor saturated steam at a temperature ranging between about 170 u00b0 c and 210 u00b0 c preferably about 195 u00b0 c for example during about 4 to 8 minutes followed by an explosive decompression at atmospheric pressure to give wood fragments the steam cooking explosive decompression technique is for example that described in u s pat no 4 798 651 mentioned above the extraction of preservatives from the particles of wood is carried out by any means known in the art preferably the chemical preservatives and the oil which are present in the wood are partitioned in the aqueous phase which is in contact with the particles at all stages of the process for example during the impregnation of the chips during their steam cooking explosive decompression and during their refining for example the initial chips containing 8 7 mg g of pcp and 99 mg g of oil loose the equivalent of 2 mg g of pcp and 7 7 mg g of oil during the impregnation 2 6 mg g of pcp and 18 mg g of oil during the steam cooking explosive decompression and 3 7 mg g of pcp and 27 5 mg g of oil during refining while 97 of pcp which is extracted from the chips is recovered in the different aqueous.