Retention of Preservative
Retention, usually expressed as kilograms of preservative per cubic metre of wood (pounds per cubic foot), is the amount of preservative retained in the wood after completion of the treating cycle and is one measure of the degree of protection provided.
Plywood can be penetrated by preservatives more readily than solid wood of the same species because the veneer cutting process opens the wood grain. These minute fissures are hard to detect witht the naked eye but are readily penetrated by preservatives under pressure.
Penetration of Preservative
Penetration is the depth to which preservative chemicals are forced into the wood. It is an indication of the amount of protection provided. The amount of penetration is determined by the qualities of the species and the treating process. The greater the depth of penetration, the less likely it is that the protected boundary of pressure-treated wood will be breached.
In some cases the penetration of chemical can be improved by incising the surfaces of lumber with knives to create artificial openings through which chemical preservative can enter the wood (see Figure 9.9 below). With new incising technology the appearance of a board is not substantially altered by incising.
Figure 9.9 Cross Section of Preservative-Treated Lumber