Vacuum Dried Red Oak

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Achieving quality and color. May 19, 2004

How many of you have used red oak lumber that was dried in a vacuum kiln? How much stress do you run into? Nothing to worry about? Enough to be a problem? Is there anything you like or dislike about lumber dried in a vac kiln?

Forum Responses
From contributor D:
Not all vacuum kilns are created equally.

Besides better color, I often hear that it's more machinable.

From the original questioner:
I don't feel that our vac kilns cause a great amount of stress. All I am trying to do is prove it. Years ago, when they dried 200,000 BFT of lumber through our vac kilns, the greatest complaint was stress. That was then. This is now. Some things have changed.

I should also add that color isn't a problem with maple, but it is with the sapwood in oak and cherry. It works well to steam the cherry. Steaming red oak for color change is a pipe dream.

From Gene Wengert, forum technical advisor:
The picture below shows two pieces of red oak lumber that were cut in half. The left half was steamed (Elder Steaming Process) while the right ends were not processed (N.P.). After steaming, the lumber was dried identically and then planed. The lack of gray stain and the brightness of the sapwood can easily be seen.

From the original questioner:
How was the oak dried? When I tried steaming red oak and then drying in a vac kiln, all I got was a lot of bow and honeycomb.

P.S. I started my kiln charge of 4/4 RO at 100/98.

From Gene Wengert, forum technical advisor:
The oak was air dried and then kiln dried.

From contributor D:
Have you ever thought about steaming inside a vac kiln?

From the original questioner:
How the heck would you get steam to the center of the load?

From contributor K:
The only experience I have had with a vac kiln was with a company who wanted to sell us a RF vac kiln. They told us they could dry 8/4 red oak in a couple days with no degrade. They even went so far as to dry some 8/4 for us for free. After drying there was degrade due to checking, along with a variance in MC among the boards, and stress in the lumber. Needless to say, we were not impressed and did not purchase one of their kilns, nor do we plan to.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor H:
We have dried 35 mm French oak in our WDE/Maspell vacuum kiln in five days without defects. We usually do 4/4 or 2/4 for parquet boards and when properly stacked and air dried, we have no stress, no cupping, excellent and fast drying capabilities. It's using superheated steam as a heating medium at 150 mbar.