Wood tar is usually extracted from the parts of pine which are rich in resin: the bole and the roots. It can also be extracted from other coniferous and deciduous trees.Tar from beech is widely used in mainland Europe.
Modern extraction techniques give a very clear tar.Traditional extraction of wood tar took place in charcoal stacks and high levels of pitch and particles of carbon were in-cluded.The stack was dug outonasloping piece of ground with the bottom shaped like a funnel and covered with birch bark. A pipe made out of a hollowed trunk was placed in the bottom of the funnel. The timber was split into sections about 18-20 cm long and 1 cm thick which were stacked radially around a strong central log. The stack was then covered with earth and turf, and lit at the bottom.The stack was allowed to smoulder for upto 24 hours, depending upon its size.Thetargathered in thefunnel and could be drained offthrough the wooden pipe.
Wood tar can be used pure or mixed with boiled or raw linseed oil in a proportion of 1:1; pigment can also be added. Wood tar extracted from pine trees contains considerable amounts of polycyclic aromatic hydrocarbons (PAHs), for example as benzo(a) pyrene, which is a well-known mutagen and carcinogen.Tar from beech is almost free from these substances.
Bark extract isslightlytoxictoinsectsandfungus, eventhoughsomewhatweak.lt is not dangerous to humans. Bark extract has little water fastness and is most useful on exposed materials indoors. Extract based on birch bark has the best impregnating properties (see recipe on page 405-406).
Wood vinegar is corrosive and is not used preventively but for treating materials that have already been attacked by fungus and insects. Wood vinegar is extracted by distillation from deciduous trees, although even coniferous trees contain wood vinegar, butinsmallerquantities.
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