Pressure treated wood faces strict standards

IrwO FACTORS that assure the I durability of treated wood are the depth of penetration of preservative chemicals into the wood and the amount of preservatives retained in the wood.

Treatment of wood by the pressure process is the best technique yet devised to assure consistently adequate penetration and retention of preservatives in a wide variety of species. Non-pressure processes used to treat wood include brushing, dipping or spraying. Unfortunately, these are not as effective as pressure treating, and often do not provide wood with a significantly greater life span than no treating at all. Surface applications of preservatives tend to weather away, leaving the wood unprotected.

Pressure treating assures durability because the process drives preservative chemicals deep into the wood cells and "locks" them in permanently.

The treatment process begins with either air-dried or kiln-dried lumber since drying permits better penetration of preservatives and reduces checking of lumber.

Then, most lumber is incised to permit deeper and more uniform penetration. Sharp knives, or teeth, sink into the wood to a depth of between Vz and 3A of an inch.

Next lumber is loaded on a tram which is run on tracks into a long steel cylinder. The cylinder is tightly sealed and filled with preservative chemicals. Pressure is applied inside the cylinder, forcing preservatives into the wood.

The amount of pressure and the length of time it is applied affect the penetration of the preservative. After the pressure period, the unused chemical is pumped back into a storage tank. Quality control is important. To measure the preservative penetration and retention, samples are laboratory tested for conformity to specification.

Representatives from the American Wood Preservers Bureau visit member plants from timetotime to repeat these tests. Periodic samples also are sent to the headquarter labs of AWPB in Virginia. This independent agency performs quality control testing to assure the wood meets the standards set to permit it to carry the appropriate Bureau marks.

These tests are performed to determine if the pressure process performed the desired result as well as to determine if the amount of preservative in the wood is adequate for the intended use. Wood to be used in ground contact requires a greater quantity of preservatives per cubic foot of wood than that used above the ground.

The mark of an approved standards-setting and quality control organization may appear on the lumber, indicating that it meets the penetration and retention requirements for the indicated usein ground contact or above ground.

Story at a Glance

Pressure forces chemlcrls deep into wood.. tests deter. mlneadequacy. .stemp shows Intended uso.