As the name implies, vacuum pressure impregnation of timber involves placing wood in a pressure vessel and applying a combination of vacuum and pressure to achieve complete sapwood penetration. Apart from envelope treatment for termite protection, all specifications in Australian Standard AS1604.1 (the solid wood standard) require that all sapwood be penetrated.
Sapwood and heartwood are described in TPAA’s Technical Note 3, but an easy way to think about how a tree is built is to think of wood as a clump of drinking straws. The straws around the outside are unblocked, allowing dissolved food and water to travel up and down the stem. This is the sapwood, and in a freshly felled tree, it is full of liquid sap. The straws on the inside or the heartwood zone are blocked with resins and waxes that stop the passage of liquid.
Logs are most often cut up when they are green or full of sap, and the resulting sawn product is called ‘green-off-saw’. Because the sapwood is full of sap, green-off-saw material is extremely hard, some would say ‘impossible’ to treat properly to meet the specifications in AS1604.1. It is impossible to get a preservative solution into the cells or fluid pathways that are already full of liquid. It is like trying to pour a beer into a glass that is already full of water.
The best practice to treat green-off-saw material is to dry it before preservative treatment. This is more important with sawn pine compared to sawn hardwood.
However, by far the majority of treatment plants that treat products such as fence palings, fence rails, or landscape sleepers do not pre-dry the material. Why…… because “if I don’t treat it the bloke down the road will” and we wonder why we are losing market share to plastic and the alternate building materials. Why does one supplier condition timber before treatment and another does not?
Often, sawmillers shipping green-off-saw timber know that the material they are supplying to timber treaters will undergo a treatment process, but justify their action by saying: “we haven’t done anything wrong because we have marked it as green-off-saw.” Whilst this may be true (and legal) is it the right thing to do??
I wonder who will get to the bottom first???
