Cause and Control of Brown Stain in Western Hemlock

Western hemlock wood from Vancouver Island, British Columbia, was studied to determine the cause of brown stain in material which is manufactured and shipped unseasoned. The stain appears in wood from sawmills along the Pacific Coast of Oregon, Washington, and B.C. It was not widely recognized until the spring of 1960 and probably appeared then as a result of sawing logs that had been stored for excessive time periods. This stain in hemlock is not the oxidation stain that has been identified in pine. It is light and dark brown, no other color. It occurs in lumber cut from the heartwood-sapwood boundary, not the outer sapwood. Rarely is it visible on freshly cut lumber. It develops during drying and is most severe when drying is most rapid. Brown stain penetrates less than 0.2 inches below the surface. It will dress off of most lumber that is kiln-dried in rough dimensions. It does not harm the wood structurally. Enzyme poisons would not prevent this stain. Bacterial infections were present in stained sapwood. Heat would not sterilize the wood and would not control the stain. Thiourea in 0.1 percent aqueous solution was an effective treatment in laboratory screening tests. Field tests of material treated with a fungicide showed an effective treatment with 50 lb./100 gal. solutions of thiourea. The application of solutions of between 6 and 25 lbs. per 100 gals. water was effective on previously untreated lumber. Fungicides can be added to the solution to control fungus stains. Chemical and bacteriological causes were studied. It is believed that leucoanthocyanins are the sap constituents responsible, and soluble polymeric leucoanthocyanins are the precursors. It is further suggested that these polymers form under a bacteriological influence. They develop in logs which are exposed to long periods of warm temperature in open yards. Pond storage is not a cause.

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