High air velocity and relative humidity reduce development of kiln brown stain in Pinus radiata

Kiln drying of Pinus radiata D. Don sapwood causes the development of a brown sub-surface coloration, commonly called kiln brown stain. Planing exposes the stain and this can spoil one of the appealing features of P. radiata—its light colour. Kiln schedule modifications—for example, drying with low kiln temperatures— reduced the frequency and intensity of kiln brown stain but increased drying times. An attempt was made to shorten drying times and produce an acceptable colour in the dried lumber by using high air velocities as well as high relative kiln humidities in conjunction with medium kiln temperatures. Depth (below the wood surface) and thickness of kiln brown stain were also determined in the dried P. radiata. Using stepped kiln schedules with either high air velocities or high relative humidity reduced development of kiln brown stain compared to accelerated conventional temperature 90760°C drying, but considerably longer drying times were needed. Furthermore, stain formed about 0.5 mm underneath the surface with a thickness of up to 2 mm. The best method for countering the effect of kiln brown stain would be to plane 2 mm off the surface of kiln-dried lumber during remanufacturing