Hydrothermic Treatment of Wood

the treatment of wood with hot gas, steam, or fluid in order to alter its physical and service characteristics. The processes are subdivided into three groups: heat treatment (heating or thawing wood), seasoning (removing moisture from wood), and impregnation (introducing various impregnants).

Heat treating is done with hot water (cooking) or saturated steam (steaming) in order to temporarily reduce the hardness and increase the plasticity of the wood, thus facilitating its frame sawing, stripping, planing, bending, and pressing. It is used in sawmills (thawing logs in open tanks) and in the production of bonded plywood (cooking pieces of wood in closed tanks), planed veneer (steaming blocks in steam pits), matches (thawing pieces of wood in steam chambers or pits), bent wood furniture, and pressed wood (steaming work pieces in autoclaves).

Wood is seasoned in a medium of moist air, fuel gases, or superheated steam. The objective of seasoning is to bring the moisture of the material down to a value corresponding to the service conditions of the items fabricated from the wood, thus preventing changes in their dimensions and shape. Wood is seasoned in the form of sawn lumber (in drying chambers and in open storehouses), as bonded and planed veneer (preferably on veneer roller driers), and as chips, shavings, and small semifinished products (in drum, pneumatic, and belt driers).

Wood is impregnated with organic fluids or solutions of mineral and organic substances that favor Hs preservation— that is, substances that provide long-term/protection for the material from decay or damage by insects. Preservation treatment is given to sawn lumber (ties, posts, beams, and boards) for structures that are used in the open and are in contact with the ground. In some cases the impregnation is for fireproofing and also for altering certain physical properties, such as the color or the electrical characteristics. The most effective methods are autoclave impregnation under pressure in special impregnation cylinders (or autoclaves) and impregnation in hot-cold vats. In construction areas diffusion impregnation is sometimes employed (coating posts and beams with preservative pastes or covering them with strips).

The hydrothermic treatment of wood is of great economic significance. Correct and timely treatment, especially seasoning and impregnation, substantially lengthens the life of wooden objects and structures.