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 discoloration is unknown. Examination of the acetone-soluble extractives present in Douglas-fir heartwood led to the recovery of a white solid, which produced a black reaction product with ammoniacal copper solutions. An ultraviolet, 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 copper solutions, and was not produced during reactions with acidic, neutral or basic copper solutions, or ammonium hydroxide alone. Analysis of the copper complex confirmed the presence of nitrogen. It was not possible to determine from the spectroscopic studies whether the nitrogen was retained as an ammine ligand bound to copper or reacted with the taxifolin to form an imine group. Research is continuing to identify the copper complex formed.

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