Besides its low mammalian toxicity and a broad range of activity towards both fungi and insects, boron shows a high diffusibility, encouraging it to treat wood species of low permeability. However, its difussibility is at the same time responsible for a high risk of leaching, known since long. Nevertheless quantitative data about this process are only rarely available. Therefore, the leaching of boron under different conditions and possible means of hydrophobising boron treated wood in order to reduce leaching were investigated by laboratory and field tests. For this purpose paraffin and a primer have been applied to protect the inner surface and alkyd-resin and a varnish as surface coat. The results demonstrate that boron diffuses even at moisture contents below 20%. Thus leaching can not be affected by hydrophobic agents placed on the inner surface of wood because diffusion still takes place within the cell-wall. Surface coatings have some protective effect but only during a distinct periode which is depending on the thickness of the coat. With time leaching increases with increasing moisture content underneath this coat. The best way to prevent leaching is the logistical protection by storing and using boron-treated wood exclusively under cover. Only for a short periode, for example during construction, a surface protection with waxes or resins will be effective.

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