Conditioning lumber -- The final step in drying

Why conditioning is the last but crucial step in the drying process. December 12, 2000

What is meant by “conditioning” at the end of the drying cycle? Is it needed with a dehumidifier kiln?

Forum Responses
Conditioning is the process of removing drying stresses that were created in the early stages of drying when the shell tried to shrink, but the wet core prevented such shrinkage. In conditioning, the shell has moisture added to it quickly, so that the shell tries to swell but the dry core prevents it. The net effect is that the attempted swelling cancels the attempted shrinking. Lumber without adequate stress relief is called "casehardened." Such lumber will pinch the saw when ripping and will have immediate warp when machining.

Conditioning is needed for all products that require no or minimal stress. Air dried lumber has minimal stress already so little conditioning is required. DH has no special key on eliminating stress, however.

Gene Wengert, forum technical advisor

I am also looking at building a DH kiln for my green lumber using a Nyle DH200 unit. I am unsure what is the best way to dry my green lumber to insure a quality product for customers. Gene, are you saying that DH kilns are not the best way to dry green wood quickly and get repeatedly excellent results?

"Are you saying that DH kilns are not the best way to dry green wood quickly and get repeatedly excellent results?"

NO. DH kilns are (as many drying techniques) excellent. However, without adequate stress relief (conditioning or casehardening relief), the lumber may not be suitable for all users. You can use steam in the DH kiln (remove the unit from the system by blocking it out so you do not steam the DH unit); some people have tried water spray (SII Mystifier), but I have not heard from a DH user about this.

All drying systems drying from green create drying stresses (casehardening)--it is a natural event. The amount varies from system to system, depending on the speed of drying, mainly (faster is more) and the temperature used (lower is less).

Gene Wengert, forum technical advisor