Hardwood and Pine Fence Posts Treated By the Double-Diffusion Method–Final Report

In 1955, the double-diffusion method was considered an economical preservative treatment process for fenceposts. This treatment offered a potential way of converting low-value hardwood species into higher value, durable fenceposts. Nine hundred hardwood and pine fenceposts, including untreated controls, were treated by the double-diffusion process, installed at an exposure site near Athens, Ga., and inspected annually thereafter. Freshly cut and peeled white oak, yellow-poplar, sweetgum, red oak, hickory, and loblolly pine posts were soaked in aqueous solutions of zinc sulfate and arsenic acid in the first tank, and sodium chromate in the second tank. Each species group was treated according to four soaking schedules–1/2, 1, 2, and 3 days in each of the two treating solutions. After 29 years of ground exposure, it is clear that both the treatment method and the chemicals are highly effective in protecting loblolly pine from decay fungi. When total chemical retention was at least 2 pounds per cubic foot (pcf) and adequate proportions of chemical were present, an average service life of 40 to 50 years could be expected for the pine. However, it was clear that zinc salts did not provide adequate protection for any of the hardwoods species, even at high retentions of chemicals. These findings are consistent with those of other investigators throughout the world–that certain heavy-metal salt solutions protect pine very well, but do not adequately protect hardwoods from decay.

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