A. Gene Wengert responds: Most treated lumber carries two stamps: The grade stamp indicates the condition before treatment; the treatment stamp indicates the level and type of treatment.
Before treatment, the lumber (southern pine, in most cases) is typically dried to approximately 19% MC (moisture content). This initial drying removes the "free water" from the lumber, making room for the preservatives. After pressure-treating with chromated copper arsenate (CCA), the moisture content will be 75% or higher.
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