Evaluating Wood Preservatives

While laboratory and accelerated field tests are useful for evaluating preservatives, they cannot duplicate the combination of exposure conditions for wood in service, and thus they cannot replace the more time-consuming service tests. A wood preservative must be toxic to organisms which destroy wood, be relatively permanent and able to penetrate, and be safe to handle and harmless to wood and metal. To be permanent, the rate of evaporation, leaching, or chemical change must be slow enough so that the preservative is effective for a long time. Colored materials are useful in determining the degree of penetration of a preservative. To be safe in use, a material must not present danger of poisoning, fire, or explosion. Preservatives which reduce the strength of wood or corrode metal are unsuitable. In some applications, such as mines, combined protection against fire and decay is necessary. Preservatives may affect paintability, and this can be tested quite simply. Common tests for fire retardance include the fire-tube test, the crib test, and the vertical and horizontal flame spread tests. Biological assays of preservatives include malt-agar tests, wood-block and soil-block tests. In accelerated field tests, stakes of softwood, such as southern pine sapwood, with large ratios of surface area to volume, are exposed to field conditions. Service tests consist of a systematic study of the performance of full-sized products identified and installed under condition of actual use.

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