Quality Control and the Improved Seasoning Processes of the Redwood Industry

Before drying, redwood lumber must be sorted according to weight, rate of growth (rings per inch), and intensity of color. Boards of high initial weight, which may be high in either moisture content or specific gravity or both, require a longer drying time; boards of high specific gravity have slow transfusion rates, while boards of high moisture content have transfusion rates proportional to their moisture content. Light redwood weighs less than 3.5 lbs./bd.ft.; medium redwood 3.5-5 lbs./bd.ft.; and heavy redwood weights over 5 lbs./bd.ft. Fine-grained wood has a lower drying rate than wood with fewer rings per inch. Pieces on the borderline: between two weight categories should be put into the heavier category if they have a deep color or 50 rings or more per inch. Sorting should improve final quality as much as 10 percent. To prevent warps and splits, boards should be stacked carefully on stickers of uniform width (2 inches) and thickness (1 inch), and aligned as accurately as possible in the air-drying yard and kiln. Preliminary air seasoning to a moisture content of 50 percent prior to kiln-drying produces the highest quality product at the lowest cost, reducing kiln time and the natural variability of moisture content. Footages between 750,000 and 1 million per acre are recommended for maximum air circulation. In the kiln, the air circulation must be uniform and the velocity not less than 250 ft./min. Rates between 250 and 500 ft./min. do not decrease time, but they do improve quality; at 500 ft/min., degrade is less than 0.5 percent, and the savings justify the additional power cost. The heating system should be able to produce a range from 110?F. dry-bulb on green lumber to 180?F. on dry lumber. As redwood is very sensitive to variations in temperature, lack of uniformity results in collapse, split, and stain. The venting system should be adequate to produce a wet-bulb depression of 20?F. on green lumber with a dry-bulb temperature of 110?F. Humidifying sprays should be adequate to hold a 5?F. wet-bulb depression at 165?F. dry-bulb temperature when the lumber is dry; if this cannot be reached, all the stresses may not be relieved, and losses may occur in remanufacturing.. The kiln schedule is the most difficult to determine. The physical law that loss of moisture content per unit of time (drying rate) is proportional to residual moisture content is used to derive a theoretical schedule. However, the theoretical schedule must best modified because as drying progresses the residual moisture content decreases/ and the drying rate can then be increased. The schedule arrived at using the drying-rate method will give a uniformly falling drying-rate curve dependent upon the residual moisture content.

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