Solvent Drying of California Redwood with Methanol

The extremely long time involved in drying redwood lumber, particularly sinker stock, and the problems of collapse, honeycomb, and chemical seasoning stain encountered in drying by conventional methods, suggested an experiment in drying. heavy segregated redwood lumber using the solvent process. Using methanol as the solvent, the experimenters found that the drying schedule for 4/4 heavy segregated redwood could be reduced to about 114 hours using 100 hours for the extraction cycle and 14 hours for solvent removal and recovery. The solvent drying of redwood was found to be controlled essentially by internal liquid diffusion. The solvent-dried stock was relatively free of seasoning defects such as honeycombing, collapse, case-hardening, and free of redwood seasoning stain. The solvent process makes available various silvichemical extractives, from 100 to 300 pounds per 1,000 board feet; these are mainly tannin, other polyphenols, and cyclitols. Full immersion, higher temperatures, and solvent-operating ratios increase the rate of extraction of water from the green lumber and thus reducing the time. Internal liquid and vapor-diffusion resistance constitutes most of the total resistance encountered in removing the solvent from redwood, and higher temperatures reduce the time necessary for solvent removal. Though freshly surfaced boards were lighter in color than kiln-dried stock, solvent-dried material approached the normal color of redwood after short exposure. Economic analyses of the method, the drying rate of redwood using methanol at high temperatures, and other solvents, are planned.

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