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 simply large ovens used to accelerate the drying of wood without producing defects. The tempera-
ture, humidity and air velocity in the kilns are controlled.
Steam Conditioning. Wood in a treating cylinder is subjected to live
steam at 220-245F for one to twenty hours followed by a vacuum. Moisture is removed during the
vacuum period. Unfortunately, steaming can also reduce wood strength. Therefore, the maximum al-
lowable steaming temperatures are defined in standards.
Vapor Drying. This method exploits the principle that energy lost in
the condensation of hot organic vapors on wood can be used, to evaporate water from wood in a
treating cylinder. The drying agent, an organic compound, is pumped into the cylinder and heated,
water from the wood and organic (drying agent) vapors are transferred to a condenser where they
are separated and the drying agent is recycled.
Boultonizing. This drying method is accomplished in a treating
cylinder by heating wood in creosote or an oil type preservative while subjecting it to a vacuum.
Water will boil at less than 212F because the wood is under vacuum. Therefore, this process uses
temperatures below those required in the steaming and vacuum process.
Incising. Incising is the production of small slits or holes in the
surface of wood products to facilitate deeper and more uniform penetration of preservative (Figure
5-2). Incising requirements for different species and commodities are given in AWPA standards.
Machining and Cutting. Where possible, all machining such as
boring, framing or dapping of timber, gaming and boring pole tops, cutting, etc., should be done
prior to the preservative treatment to reduce exposure of untreated portions of the wood to decay
fungi and insects.
5.3.3 Treating Methods. Pressure treating is the most commonly employed method to ob-
tain deep penetration of wood preservatives. Other methods include hot-and-cold baths, diffusion
and cold soaking.
Pressure treatments are done in retorts, or closed cylinders (Fig-
5.3.3.1 Pressure.
ure 5-3).
Pressure processes can be divided into two categories.
Full Cell Process. This process is commonly used with oil borne treatments for marine
piling where high retentions are required and with water borne solutions of preservative
salts.
Empty Cell Process. The empty cell process impregnates and coats the cell walls with
preservative without filling the cell lumens. This process provides the deepest penetration
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