Control of Brown Stain in Kiln-Dried Eastern White Pine

An ammoniacal zinc oxide system which imparted good light and water resistance to wood was examined for its effectiveness in the control of brown stain which can develop during the kiln drying, of eastern white pine. Freshly sawn white pine boards containing both sapwood and heartwood with an initial moisture content of from 180 to 210 percent were obtained from a sawmill and kept under polyethylene for 8 days in a conditioning room maintained at 41?F and 95 percent relative humidity. The treating solution was prepared by dissolving 5 percent zinc oxide and 6 percent ammonium carbonate in 6 percent ammonium hydroxide. Forty boards (6 feet long, 4 to 6 inches wide, and 1 inch thick) were dipped in the solution for 30 seconds and excess solution was drained off. The average retention of chemicals was 0.1 lb/ft3. The treated boards were close-piled for 7 days at 70?F and 90 percent relative humidity, along with the same number of untreated boards in a second pile under similar storage conditions. At the end of this period, the boards were kiln dried and examined for the incidence of brown stain before and after skip-planing. Severe brown stain was characteristic of the untreated boards, while in contrast all the treated boards appeared in very good condition. Slight brown stain appeared where stickers had been in contact with the boards in the kiln but it could be removed by skip-planing. Over 80 percent of the rough treated boards were free from brown stain, while over 80 percent of the untreated boards showed moderate to heavy staining in the rough sawn state; the staining was reduced to about 60 percent by skip-planing.

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