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During the last decade, the volume of treated sawnwood has expanded dramatically, due to the marketing of preservative-treated decking and fencing through retail outlets. Most of this sawnwood is impregnated with chromated copper arsenate (CCA). In western Canada, hemlock (a commercial mixture of western hemlock (Tsuga heterophylla Sarge.) and amabilis fir (Abies amabilis Forbes.)) is used for this market, because it is generally regarded as being relatively easy to treat (17). Lumber of other species, such as spruce (Picea sp.), pine (Pinus sp.), and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) are considered more difficult to treat (5,16). Indeed, Douglas-fir heartwood is so difficult to treat successfully with CCA that this combination is often avoided by treaters.
Improved treatment of refractory wood species can be achieved by ammoniacal copper-based wood preservatives (13,15), such as ammoniacal copper zinc arsenate (ACZA), ammoniacal copper quat, (ACQ), and ammoniacal copper citrate (CC). While this development offers clear benefits for treatment of refractory wood species, there are other aspects of ammoniacal copper preservative treatment that require consideration. One of these is the dark color encountered in treated products of some wood species, e.g., Douglas-fir (14). This dark color is not desirable for many sawn products where appearance is important. Since the reaction does not occur in all wood species, it is likely that the cause of the reaction is related to the heartwood extractives present in certain species.
The objective of the current study was to identify the extractive causing the dark color in Douglas-fir treated with ammoniacal copper preservatives together with the nature of the complex formed.
MATERIAL AND METHODS
PRELIMINARY ANALYSIS OF EXTRACTIVES
Blocks of Douglas-fir heartwood were chipped and ground to 20-mesh sawdust for use in the extraction experiments.
Two 20-g samples of Douglas-fir heartwood sawdust were extracted with either acetone (100 mL) or diethyl ether (100 mL) for 24 hours at 20degC. The two solutions were filtered, concentrated under vacuum at 20degC, and the residues were dried under vacuum at 30deg to 40degC to constant weight. The acetone-extracted wood sawdust was re-extracted with distilled water (100 mL) at 40deg C for 24 hours. The extract was filtered and concentrated at 60degC under reduced pressure to produce a third residue.
Solutions containing 2 percent (w/w) of each of the...