ARTICLES & REVIEWS
The Practice of Steaming Walnut Lumber
by Bill Tindall
Every pack of commercial walnut lumber that I ever saw for sale was steamed. Yet, steaming is a controversial practice among some woodworkers. So, why is commercial walnut lumber steamed? The sapwood of walnut is white and most woodworkers and all furniture factories find it an unattractive addition to furniture made from walnut. The clearest wood in a log will be found on the outside of the log because this is the wood laid down as the tree matures, its young limbs fall off, and the knots heal over. The outside of the log is also were the white sap wood is located. The practice of steaming was developed to turn sapwood into the color of heartwood.
It has been found that when fresh cut walnut lumber is heated in wet steam for a sufficient length of time the sapwood turns nearly as dark as the color of the heartwood. Once the sap wood gets to a temperature around the boiling point of water in the presence of wet steam the sapwood will turn black as a result of a chemical change (you can replicate this by experimenting in your pressure cooker). The process does not involve diffusing dye from heartwood to sapwood.
A typical steamer is shown in this picture. The silver things on each side are huge aluminum doors—aluminum because the iron doors corroded away in a few months from the wood extracts. One dead pack (a pack of lumber without sticks between the layers) of walnut has been put in the steamer. Once this chamber is filled it will be closed up and steam from a boiler introduced for a couple of days until everything is hot and soggy. A pleasant odor of furfural seeps out of the steamer indicating some decomposition of the cellulose. One furniture factory owner told me that steamed walnut processed better, perhaps due to some alteration of the wood during steaming.
What goes into the steamer brown and purple and white comes out of the steamer looking like it has been blackened with soot. This change is the objection of some woodworkers. Their complaint is that kiln dried (really steamed) walnut loses it color character, and they are correct in purist’s terms. Once the soot is planed off, the walnut will be a uniform brownish-black, and the fact that it was steamed may be irrelevant in the long run.
Most woods get darker with age, but walnut gets lighter unless it is stained. Walnut will become almost blond after years in a sunny room. This picture shows two pieces of wood split from the same air dried plank. One was steamed in my pressure cooker, the other was not. At time zero the steamed piece was almost black. The other piece was shades of lighter brown and purple.
This picture shows these pieces after a few months in my sunny dinning room. In the best light I can still detect slightly more black in the steamed piece, but forget the purples in the air dried piece—they left almost immediately upon sun exposure. I will leave it to the reader to judge if it makes any difference whether one uses air dried or steamed walnut in their projects. There will be a lot of difference in the sap wood, of course.
. . . Bill Tindall
© 2007 by Bill Tindall. All rights reserved.
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