Pacific silver fir is the more-treatable component of hem-fir from Coastal British Columbia

Preliminary studies at Forintek and elsewhere have shown that preservative penetration in Pacific silver fir is greater than in western hemlock. To justify separating these species for preservative treatment it is necessary to quantify this difference and determine if it is common to the entire British Columbia (BC) coastal region. Sorted Pacific silver fir and western hemlock from five coastal BC sawmills was planed to nominal 2 by 4 inch dimension and cut into one short length and four end-matched sub-samples. Half the end-matched sub-samples were conventionally incised and half were double-density incised. Form each group, half were treated with chromated copper arsenate (CCA) with a 3-hour pressure period and half were treated with a 6-hour pressure period. Without incising, Pacific silver fir exhibited 80 percent deeper penetration and higher preservative retention by analysis than western hemlock. Non-incised Pacific silver fir came close to meeting the proposed Canadian standard for residential decking (5-mm penetration). Conventionally incised Pacific silver fir met the Canadian standard for wood in ground contact (10 mm penetration). In contrast, none of the sorted western hemlock from the five sawmills met the standard even when double-density incised and given a 6-hour pressure period. These results suggest that there is an opportunity to produce a more treatable Pacific silver fir product through species separation prior to preservative treatment. Furthermore, these results suggest that refractory batches of hem-fir may occur when there is a low percentage of fir present.

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