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Abstract
Specific drying conditions for control of discoloration in pitch pine were analyzed and evaluated to determine best drying temperature and moisture content (MC) to maximize high value-added uses. High-temperature kiln-drying with restraint effectively reduced twist but also caused an excessive darkening stain. Further experiments showed that this stain generation is controlled by drying at conditions exceeding 100 °C dry-bulb only until before wood core temperature reached 100 °C. Results also showed accurate control of MC and finishing the drying process at the most suitable time to be important. Boards of excellent color were produced by applying a newly developed reducing temperature drying schedule for stain control. It is anticipated that higher additional value can be obtained from pitch pine, with continued studies on stain control, resin exudation, and twist restraint.
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Pitch pine (Pinus rigida) has been widely used for structural lumber in construction, interior wood, and packaging materials throughout the world. For several decades it has been widely planted in Korean forests, initially primarily for erosion control and use as a fuel supply. Currently, the Korean wood industry sector mostly uses pitch pine logs as raw material for such low value uses as nonstructural boards and where visual appearance is not important, or as furnish for such process industries as particleboard, fiberboard, and pulp and paper.
Efforts to use pitch pine as a higher value lumber product have continued through wood-drying research in Korea. Due to the lack of pruning, pitch pine trees typically have big and crowded knots. Unfortunately, the sloped grain caused by these knots commonly causes excessive twist during drying. Also, if much of the resin in pitch pine is not eliminated or set during drying, it can be a principal factor obstructing adhesion of glue and paint. It is well known that suppression of twist and reduced resin exudation from products in use are among the benefits of high-temperature drying. While this supports high-temperature drying as a suitable drying method, generation of an undesirable discoloring stain during high-temperature drying can decrease the economic value and price of pitch pine products.
Jung et al. (1986) estimated moisture content (MC) losses of air-drying related to meteorological variables, and developed an air-drying calendar for Korean pitch pine...