WHY DOES DOUGLAS-FIR HEARTWOOD TURN BLACK WHEN TREATED WITH AMMONIACAL COPPER PRESERVATIVES

Douglas-fir heartwood often turns very dark in color when-treated with ammoniacal copper preservatives. The cause of this black discoloratio n is unknown. Examination of the acetone-soluble extractives present i n Douglas-fir heartwood led to the recovery of a white solid, which pr oduced a black reaction product with ammoniacal copper solutions. An u ltraviolet, infra red, mass spectroscopy, and proton nuclear magnetic resonance spectroscopic analysis of the white solid confirmed that it was taxifolin. The black complex was only formed with ammoniacal coppe r solutions, and was not produced during reactions with acidic, neutra l or basic copper solutions, or ammonium hydroxide alone. Analysis of the copper complex confirmed the presence of nitrogen. It was not poss ible to determine from the spectroscopic studies whether the nitrogen was retained as an ammine ligand bound to copper or reacted with the t axifolin to form an imine group. Research is continuing to identify th e copper complex formed.