Insertion and Air Removal

• Dry lumber is placed into a horizontal treating cylinder where the wood pieces are subjected to a preservative liquid under pressure -- hence the term "pressure treated" wood. A vacuum pulls air from the chamber and wood cells to make space for the preservative to absorb.
  
  

Introducing the Preservative

• A diluted preservative solution is pumped into the cylinder until the wood is completely submerged. After chromated copper arsenate (CCA) was outlawed in 2003 due to containing arsenic, new preservative liquids like copper azole, ACQ, borates, cyproconazonle and propiconazole were developed to use in the treatment process as arsenic-free alternatives.
  
  

Ensuring Proper Saturation Levels

• The cylinder is pressurized, forcing between .10 to .25 pcf (preservative per cubic foot) for above ground wood and between .21 to .40 pcf of the preservative liquid into the wood fibers for ground contact uses. The leftover preservative fluid is pumped from the cylinder and into a tank for the next cycle of treatment.
  
  

Re-drying the Wood

• The same vacuum process used to remove excess air from the cylinder and wood cells is activated again to remove excess preservative liquid from the wood cells. The treated wood is removed from cylinder and ready for installation as a weather and insect resistant wood.