Shop-Drying Eucalyptus

      Thoughts on how to dry and condition Eucalyptus for use in a table top. December 31, 2012

I have some 6/4 eucalyptus (E. globulus) live edge slabs to be glued up into a Nakashima-style (including contrasting butterflies) table top. The top will finish out at 1-1/4” thick. It will be attached to its trestle base via two cross-grain stretchers using sliding dovetails, center-fixed to allow for seasonal movement, leaving the underside as exposed as possible to ambient conditions.

The eucalyptus was air dried; stored outside, covered up when weather demanded, for many years. Current MC is 10-12%. My gut tells me that if I can get the MC down to 8 or 9% before starting this project, I’ll be okay. My concerns are, of course 1) warp and 2) the possibility of the butterflies splitting open my splices (the table top grain that a 3” cross grain butterfly is let into will shrink about 1/32” as MC goes from 12 down to 7 or 8% in service, right?).

If I can maintain 40% RH in my shop, how long should I expect it to take for the wood to reach its EMC of 8%? If we are talking months rather than weeks, any simple ideas for speeding this up? Maybe just increasing air movement with a fan?

Forum Responses
(Sawing and Drying Forum)
From contributor A:
I just picked up about 500 bd. ft. of 8/4 and 4/4 KD eucalyptus. I believe it is one of the red gums. Typical of eucalyptus, the wood experienced severe cracking and some warping during drying.

My guess is that your air drying has removed most of the tendency to warp. However, it is likely that either kiln drying or exposure to very dry indoor conditions (like Minnesota in winter) will cause some cracking. This is far more likely to be a problem than your butterfly wedges. You'll need to season the wood in your shop at lower than normal RH to force some cracking. Backfill these small checks with colored epoxy and your wood should be stable against low moisture from then on.

As for shrinking butterflies, use a very low shrinkage wood like walnut or mesquite and don't make the butterflies wider than 2" and they shouldn't be a problem.

From the original questioner:
I'm not worried about the butterflies shrinking - they will be 6% MC walnut. My concern is that they will be cross-grain to the slab splices and will not shrink when the eucalyptus does. How low do you think I would need to get the RH and how long should I expect this to take (weeks, months, years)?

From contributor A:
If your butterflies are less than 3" long overall, they are very unlikely to create a problem. The slab will simply expand outward from the centerline where the butterflies are.

As for stabilizing the eucalyptus slab, it's going to be a crapshoot, as eucalyptus is highly unpredictable. Simply get the MC down as far as you can, build the table, and hope for the best. Checking can only occur if MC goes lower. Your air drying has already done a lot to prevent warp, which would occur if the MC rose significantly (like in a humid, uncontrolled indoor environment).

From Professor Gene Wengert, forum technical advisor:
Keeping your shop at 40% RH (8% EMC) when the ultimate use might be 25% RH (5% EMC) is not a good idea. Try to get your shop to match the end use, as a mismatch can stress the piece more than seasonal changes (faster and larger).

Because of internal stress, eucalyptus movement when the MC changes can be a problem… a crap shoot is indeed accurate.

The 3" cross-grain butterfly should be good if the MC change is not too big and not too fast. It is well to remember that a hole will get smaller as the wood around it shrinks.

From the original questioner:
Thanks for the responses. I really want to try to get the best odds I can in this crapshoot. Gene, let's say ultimate RH will be 35% (7% EMC). Do you think I can get by with a home dehumidifier?

From Professor Gene Wengert, forum technical advisor:
Yes. Maybe two or three weeks. Measure to be sure.

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