Many processes for the preservative treatment of wood have been developed. Swedish Pat. No. 301,870 discloses the preservative treatment of wood with aqueous solutions followed by the drying/coloring of the wood in a hot high-boiling oil. In another treatment, disclosed in Swedish Pat. No. 401,994, impregnation of the wood takes place in an organic solvent containing a perservative agent followed by drying/coloring in hot oil with simultaneous recovery of the solvent. A third process, disclosed in Swedish patent application No. 7901957-6, discloses the preservative treatment of wood with an emulsion followed by the drying/coloring of wood in hot oil. A fourth treatment, disclosed in Swedish Pat. No. 393,676, discloses a process where a wet (i.e., green) wood is dried by treating the wood in a special oil.
In the first and third treatments discussed above, the wood is impregnated with an aqueous preservative in the form of an actual solution or an emulsion thereof. Subsequently, a drying/coloring step in a high-boiling oil is employed. With such processes the drying/coloring step is generally very successful.
However, when organic solvents are employed as the carrier for the preservation agent, it has not generally been possible to carry out the drying/coloring of the wood as successfully. The coloring pigments are difficult to fix and distribute on the wood in the same successful manner as when an aqueous preservative was initially employed.
When using the treatment intended for drying of the wood as set forth in Swedish Pat. No. 393,676, a very fast and mild drying is obtained as compared to the other three treatments. Contrary to the other treatments, no introduction of oil into the wood surface is desired. In order to avoid such an introduction, a somewhat more evaporable drying medium is used which can be evaporated from the surface of the wood after the drying step. The dried wood is thus freed from the drying medium. As a result of the increased evaporability, a portion of the drying medium evaporates together with the water from the wood during the process. This portion is recovered upon cooling and separated from the water. Another kind of recovery is accomplished by the second treatment discussed above during the heating under vacuum. In that case, however, no drying medium is recovered but only organic solvent for the preservation agent.