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 impregnating properties. (See also ‘Recipe 2: Bark stain’ p. 425.)
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.