The lumber industry producing spruce-pine-fir (SPF) is interested in marketing a termite-resistant wood product in regions where subterranean termites adversely affect consumer confidence in wood-frame construction. The use of borate treatment appears to be a promising approach to overcoming the difficulty of pressure treating unincised Eastern SPF, particularly black spruce. Recently at a commercial trial at the Légaré plant in Quebec, Canadian SPF was treated with borates and met the AWPA standard. Washboarding was however noted after treating and storage in red spruce, white spruce, balsam fir and alpine fir. The effect was even more pronounced after kiln drying. This experiment was designed to determine whether washboarding could be reduced or eliminated by modifying the treating schedule used at the Légaré plant. Seven charges were carried out with various modifications to the Légaré schedule. From these results an eighth charge, combination treatment, was devised using all the modifications that appeared to reduce washboarding.
The modifications that significantly reduced washboarding were: reducing solution temperature from 60oC to 50oC, adding a 15-minute pressure rise, and using constant pressure. The modifications that did not reduce washboarding were: 60-minute pressure relief and no final vacuum. However, eliminating the final vacuum did not affect, uptake or kickback of treating solution thus this modification was added to the combination treatment to compensate for the added time. The combined treatment schedule included a solution temperature of 50oC, a 15-minute pressure rise, constant pressure and no final vacuum. The combination treatment reduced the washboarding effect to a similar degree to the reduced temperature. Washboarding appeared to develop mostly in post-treatment drying, and mainly (if not always) on the heartwood face of the lumber. Washboarding was much lower for end-sealed samples than for full-length material.
The modifications that substantially reduced preservative uptake were: no initial vacuum, 15-minute pressure rise and constant pressure. The last two of these in the combined treatment produced the greatest reduction in uptake. Measurements of preservative penetration were affected by treatment schedule and storage time prior to drying, however, the penetrations were not out of line with those in the commercial trial at the same storage time where red spruce met the AWPA standards.
Washboarding can be reduced to an acceptable level without compromising treatment quality by reducing the solution temperature to 50oC. If it proves necessary to eliminate washboarding altogether, further work could include further reduction in treating solution temperature, the addition of ethanol or surfactants and the use of end seals.