Lumber Discolorations: Causes and Prevention

摘要

Increases in lumber exports, together with the increased popularity of clear finishes, has dictated that, regardless of structural grade, both hardwood and softwood lumber producers must offer their customers discoloration-free materials to maximize their value. Aesthetic quality (appearance grade) often is of greater value than that based on conventional lumber grades. Some discolorations (eg, mineral streak) occur in living trees and cannot be prevented by logging, milling or drying practices. However, discolorations caused by the growth of microbes (e.g., mold and sapstain fungi); enzyme-mediated reactions within the sapwood of freshly-sawn lumber; or some non-microbial, non-enzymatic reactions (eg., iron stain) can be prevented (Table 1). Anti-sapstain formulations may contain only a biocide or a biocide plus iron-stain inhibitor, but none that are commercially available contain components that will prevent enzyme-mediated discolorations. These formulations are designed to protect freshly-sawnlumber from discolorations by forming a protective envelope on lumber surfaces. Assuming that the lumber and/or logs from which it was sawn are not colonized by mold, sapstain, and/or decay fungi prior to application of the envelope, these treatments arevery effective in protecting lumber from fungal discolorations while it dries. However, if logs are not utilized in a timely manner and/or protected from fungi during transport and storage (e.g., treating log ends and debarked areas with biocide; water-spray storage) mold and sapstain fungi will colonize them and continue to grow in lumber sawn from them beneath the anti-sapstain protective envelope and may break through the barrier to colonize the surfaces.