On the next heating not so much heat can be stored in the wood, because of its lowered moisture content; and this effect is shown in the following tabulation of several successive moisture stages (Table 3), based upon heating to 131° F. (not assuming any addition of moisture to the wood in heating) and cooling to 50° F., or boiling point, at a vacuum of 98.8, the initial moisture percentage of the wood being 68 per cent.

TABLE 3.-Tabulation of several successive moisture stages, based upon heating to 131° F., and cooling to 50° F., or boiling point, at a vacuum of 98.8

1 In this table it should be noted that allowance for considerable amounts of heat, which is impractical to estimate, may be required to produce a movement of moisture through the wood.

It is, of course, obvious that, since the heat in the wood does the work, to reheat the wood if steam were used would require condensation of practically the same quantity of steam that had been evaporated in the previous stage of evacuation; and as this condensation would occur on the wood, it would result in the restoring of a like amount of moisture. The heating of the wood must therefore be done by a mixture of steam and air with a humidity which will not result in changing the moisture of the wood in either direction, which means that the air will contribute most of the heat.

When the boiling point is reduced by the vacuum below the exist ing temperature of the wood the effect is to cause evaporation throughout the interior of the wood as well as on the surface; but if the steam can not pass off freely, a local pressure accumulates which raises the boiling point locally to the existing temperature of the wood, when local evaporation can no longer occur. If the wood is heated to 131°, this represents a boiling point corresponding to 5 inches of mercury, or about 211⁄2 pounds pressure per square inch. This pressure, of course, tends to drive the steam out of the wood and also to drive out the interior moisture, and thus hastens diffusion.

At the stage of vacuum used a pound of water will produce nearly 700 feet of vapor, which has to be removed by the pump in order to maintain the vacuum. This exhaustion of steam produced from all the surfaces of the wood amounts to quite an active circulation of the rarefied atmosphere inside the retort.

It is obvious that the use of higher temperatures in heating the wood must produce higher interior vapor pressures at the vacuum and also tend to greater softening of the wood; and the resulting stresses might produce checking. The foreign specifications, how

ever, state that checking is chiefly produced by failure to heat the wood through; and it is apparent that this would produce faster drying near the surface and the consequent unequal shrinkage, which usually cause checks. If, however, the hea ng is thoroughly done, this system would seem conducive to uniform drying, because the portions of the wood having highest moisture content could store the greatest number of heat units effective for evaporation in the next evacuation stage.

The temperature limit set by the Swedish specification, 131° F., seems very moderate in comparison with maximum dry-bulb temperatures of American kiln-drying schedules, ranging up to 180°. It seems very probable that the damage mentioned as having occurred in using higher temperatures in the vacuum process did not result directly from the temperature but from steam-pressure differences developing in the wood from premature application of a too high vacuum. If, for instance, the wood was heated to 212° and a vacuum of 98.8 per cent were then applied, equivalent to a boiling point of 50° F., the tendency of interior moisture to turn to steam would not be restricted until the pent steam had developed to atmospheric pressure of 15 pounds to the square inch; and this might have a disruptive effect on the softened fibers of the wood at such a high temperature. If, however, a vacuum of 20 inches of mercury were first applied, the maximum interior pressure difference that could develop would be 5 pounds per square inch, probably not enough to cause any checking of the wood; and as the temperature reduced the vacuum could gradually be increased until the maximum was reached. For most woods a schedule could probably be developed of heating to 180°, practically equivalent to boiling point at a vacuum of 15 inches of mercury, and then applying a vacuum of 10 inches, reducing next to 5 inches, and then to the final high vacuum of 98.8 per cent. Using this higher temperature would accomplish more evaporation for each reheating, and 8 stages would be equivalent to the 10 shown in the above table.