Sorting sugi lumber by criteria determined with cluster analysis to improve drying

Sugi (Cryptomeria japonica D. Don) is one of the main species used for structural lumber for Japanese wood housing. The initial moisture content (MC) of sugi is relatively high and varies from board to board. This makes sugi lumber difficult to dry. It has been said that sorting sugi lumber based on the initial MC estimated from the green weight is useful for efficient drying. On the other hand, the MC and drying properties of the heartwood in sugi is extremely different from those of the sapwood. So we assume that the heartwood-sapwood proportions affect the drying process. In this study, the heartwood ratio was determined from the cross sections at both ends of sugi green boards using digital image analysis. Furthermore, heartwood color is quite different from board to board in sugi lumber, and there seems to be a relationship between heartwood color and drying characteristics. So, green sugi lumber was kiln-dried to examine the effect of initial MC, green weight, heartwood ratio, and heartwood color as sorting parameters for drying, and cluster analysis was applied to separate those parameters. The results of this study show the relationship between initial MC, heartwood color, green weight, and heartwood ratio to final MC. Sugi green lumber should be sorted by initial MC; in this study, the threshold value of initial MC was 113 percent. Instead of sorting by initial MC, first, green lumber must be sorted by heartwood color: red and non-red. Next, non-red heartwood lumber may be segregated by heartwood ratio, which in this study comprised approximately 60 percent.