Effects of kiln schedules and sticker variables on board color and sticker stain in hard maple

In spite of kiln schedules and techniques which have been suggested for controlling board color and preventing sticker stain in hard maple, many firms continue to experience problems. While solutions to stain generally rest with maintaining low relative humidities and temperatures during drying, the empirically derived recommendations for the prevention of sticker stain have not been thoroughly investigated in a laboratory experiment. In this study, hard maple lumber was dried under precisely controlled laboratory conditions with several important processing variables. Three drying schedules were compared, T8-C3, T3-05, and a modified schedule. An additional drying assembly was placed outdoors. Logs were cut and sawn in each of the four annual seasons for a period of one and a half years. Lumber was sawn from both fresh and seasoned logs. Stickers used for this study included solid and grooved hard maple and red oak, and southern pine LVL. Prior to drying the lumber, stickers were conditioned to three relative moisture contents (MC): dry (6 to 9% MC), conditioned (11.5 to 14% MC), and wet (30-50% MC). The typically recommended schedule for drying 4/4-inch hard maple, T8-C3, produced a “normal” red-brown-pink color, whereas specimens dried by the low-temperature, low humidity schedule, T3-05, and the air-dried boards were white. The low humidity, high temperature, modified schedule produced a pinkish color midway between the other two schedules. Visual and machine analysis showed sticker stain to be more prevalent in lumber from seasoned (three to six weeks) logs than in fresh (less than five days) logs. Solid, wet oak stickers produced the most severe sticker stain whereas grooved dry maple stickers produced the least. This relationship holds true for old and fresh logs and all drying conditions. Solid dry maple and LVL stickers produce similar discolorations. In the T8-C3 and modified schedules, sticker stain appears less intense as the pinking effect of the schedules masks the stain. Reverse sticker staining occurs more frequently in the pinking schedules (T8-C3 and modified). Increasing the moisture content of the stickers from dry to 12% EMC to wet, increases sticker stain intensity and area.

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