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 bundles of exterior millwork. DOD specifications usually require at least a one-minute dip treat-
ment.
5.2.2.4 Vacuum. The preservative is applied in a chamber in which air has been
exhausted with a vacuum pump. After the preservative enters the chamber and the vacuum is
released, air at atmospheric pressure increases penetration.
5.3 IN-DEPTH TREATMENTS. Pressure and other in-depth treatments are used for wood that is
subject to severe exposure.
5.3.1 Treatment Quality. Standards for pressure treated wood define minimum depths of
preservative penetration and minimum levels of preservative retention.
5.3.1.1 Penetration. Penetration is the depth to which the preservative is forced
into the wood. Penetration is the simplest measure of treatment quality because deep penetration
contributes to good protection. If penetration is shallow, the preservative shell is easily broken by
checking or other mechanical action and the unprotected wood beneath the shell is subject to
deterioration (Figure 5-1).
5.3.1.2 Retention. Retention is the amount of preservative within a specified
volume of wood. Retention is usually defined for wood at specific depths from the surface of the
treated wood product. This zone is commonly referred to as the "assay zone".
5.3.1.3 Standards. The American Wood Preservers Association publishes stand-
ards for pressure and thermal treatment of wood. These standards vary by commodity type, in-
tended end use, and species. The amount of retention and penetration are specified for each
commodity in specific standards. Minimum quality control requirements are cited under each stand-
ard.
5.3.2 Pre-treatment Processing. The ways in which wood products are processed prior
to preservative treatment can influence the penetration and retention of the preservative.
5.3.2.1 Debarking. Removal of bark before treatment is essential. Remaining
bark prevents preservative penetration and encourages wood boring and decay organisms.
5.3.2.2 Drying. Wood must be dried before treatment to obtain satisfactory
penetration and retention of preservative. The most common techniques are:
Air Drying. Air drying is the simplest method. Wood is stacked with
adequate room for air to circulate around each piece allowing water to evaporate from it.
Kiln Drying. Wood products are also dried in kilns. Dry kilns are
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