Drying Western Juniper
There are many challenges to making value-added products out of western juniper. Warp and splitting of seasoned wood are two frequently mentioned. Many in the wood products industry believe these occur because the wood is difficult to dry.
Dr. Mike Milota, Wood Drying Specialist at Oregon State University, conducted several studies to assess drying characteristics of western juniper, and investigate alternative drying techniques to improve recovery for value-added products.
Recent Drying Trials
In his first study, Dr. Milota used both slow and fast schedules to dry 4/4 and 7/4 juniper lumber. Air drying, finished off by kiln drying, was one of the schedules evaluated. Dr. Milota examined drying stresses during kiln cycles and calculated moisture meter correction factors. Effects of moisture cycling on splits and cracks in coated and uncoated edge-glued panels were also assessed. Key results were:
- Knots, grain, pith, and treatment of lumber before it is kiln dried (for example, how it is stored), are probably more important to recovery than drying technique.(1) Basically, common lumber defect affects juniper drying results similar to other species.
- Different drying schedules had little effect on panel performance (measured by splits and cracks) during moisture cycling.
- Causes of defect during moisture cycling were, in decreasing order of occurrence: Knots, grain, weakness along growth ring, weakness along ray cells, manufacturing snipe, and weakness near pith.
- Commercial lacquer finish appears to reduce splits and cracks.
- Knots greater than half-inch in diameter should be avoided if possible. Splitting and cracking can be reduced or eliminated by cutting-out larger knots and finger-jointing.
- Juniper air-dries comparatively quickly.
- Moisture meter correction factors are significant. They varied from +1.3% to +2.2% for resistance ("pin") meters and +1.8% to +5.2% for capacitance meters.
In another study, Dr. Milota conducted preliminary tests to determine if high temperature and steam would help "relax" growth stress in juniper lumber, similar to what is done with Pinus radiata (Monterey pine). Results indicated that the process would require kilns capable of higher temperatures (in excess of 190 degrees F) than what are currently available in the Pacific Northwest.
Dr. Milota also tried pre-steaming juniper lumber before kiln drying. Initial results did not indicate a significant difference between pre-steamed lumber and lumber which was not pre-steamed.
Dry Kiln Schedules for Commercial Woods provides a kiln schedule for drying western juniper (listed under "cedar"). Based on his experience, Dr. Milota believes the published schedules will work well if adjusted for individual kilns.
Given current knowledge and experience, it appears that short, narrow, thin boards (such as 4/4 by three-inch by four feet ) represent the best approach to reduce growth stress effect in western juniper lumber and improve recovery for value-added products. It also is critical to take care of logs before they are sawn. For example, a significant amount of splitting appears associated with end checking. End-coating and timely processing of logs (within one week of falling) should reduce this problem.
These projects were funded by Oregon lottery dollars, through the Regional Strategies Program. Support and technical assistance were provided by the U.S. Forest Service, OSU Extension and numerous private businesses.
For More Information Contact: Scott Leavengood, OSU Extension Service (541-883-7131)
1. At least three manufacturers with hardwoods experience say juniper machines and dries like bigleaf maple. They point to similar "figuring" and "character wood", and growth stress issues.