Civil Engineering Reference
In-Depth Information
Extraction of wood tar in a charcoal stack
The stack is dug out in a sloping piece of ground. The bottom is shaped like a funnel
and covered with birch bark. A pipe made out of a hollowed branch is placed in the bot-
tom of the funnel. The timber is split into sticks about 18-20 cm long and 1 cm thick and
they are stacked radially round a strong central log. The stack is then covered with earth
and turf, and lit at the bottom. The stack is allowed to smoulder for up to 24 hours,
depending upon its size. The tar gathers in the funnel and can be removed through the
wooden pipe.
Wood tar can be used pure or mixed with boiled or raw linseed oil in a propor-
tion of 1:1; pigment can also be added. Wood tar extracted from pine trees con-
tains considerable amounts of polycyclical aromatic hydrocarbons (PAH) sub-
stances, for example benzo-a-pyrene, which is a well-known mutagen and car-
cinogen. Tar from beech is almost free from these substances.
Bark extract
Bark extract often has borax and soda salt added to increase its antiseptic effect.
The extract is poisonous to insects and fungus, even though somewhat weak. It
is not dangerous to humans. Bark extract is not waterproof, and is most useful on
exposed materials indoors. Extract based on birch bark has the best impregnat-
ing properties. (See also 'Recipe 2: Bark stain' p. 425.)
Wood vinegar
Wood vinegar is corrosive and is not used as a preventative but for treating
materials that have already been attacked by rot and insects. Wood vinegar is
extracted by distillation from deciduous trees, although even coniferous trees
contain wood vinegar, but in smaller quantities.
Soda and potash lye
These have been used for surface treatment in many Swiss villages for hundreds
of years, and the buildings have kept very well. A drier climate is, of course, part-
ly responsible for their success, but this treatment deserves discussion.
Impregnation with lye brings the resins and tar to the surface of the wood in the
same way as burning. The lye also has an antiseptic effect. The treatment has to
be repeated every two to three years. Gloves and glasses should be worn during
the treatment, as the material is very alkaline.
