Compression-rolling reduces kiln brown stain in radiata pine sapwood

Kiln-drying of radiata pine (Pinus radiata) sapwood often causes the formation of a brown discoloration, commonly called kiln brown stain. Kiln brown stain develops just under the wood surface in a thin layer but subsequent machining of the lumber exposes the stain. The occurrence of kiln brown stain has caused substantial loss in revenue in New Zealand's high-value radiata pine export markets. In this study, the effect of compression-rolling of radiata pine prior to kilning was investigated as a potential method to control the formation of kiln brown stain, along with its effect on thickness shrinkage and drying time. The results of the study demonstrated that regardless of kiln schedule, compression-rolling significantly reduced the formation of kiln brown stain in the kilning of radiata pine, but increased drying time. In the drying ofradiata pine at 90/60°C, rather than at 71/60°C, compression-rolling significantly increased thickness shrinkage from 4.9 to 6.0 percent. The mechanisms of compression-rolling on kiln brown stain formation in the drying of radiata pine are discussed.