Abstract: An apparatus for treating work pieces in a closed space having a collapsible material. The treatment utilizes a period of subpressure and a period of pressure, in which cavities in the pieces are more or less filled with liquid. Prior to this the cavities are wholly or partially filled with a surface-tension reducing gas and/or solution.

Abstract: Unit loads of wood, surrounded by a small quantity of liquid, are impregnated in sturdy vessels provided with a cover. After or during an impregnation period under vacuum, the impregnation is completed by subjecting the liquid and the wood to pressure surges or impacts of such an intensity that the wood is deformed in the direction of length of the fibers or in the radial or tangential direction thereof within a range between the average pressure causing breakdown of the wood material and one-third of that average pressure. Intensity of pressure surges increases as the vessels are filled with impregnation liquid. Pressure waves may be produced by means of either compressed air, steam or explosive gas. A connecting chamber is provided for preparation and storage of the impregnation liquid.