[Woodcarver] Why is birch stable?

[Joe Dillett]

Hopefully this won't be asked and answered in the following emails from
earlier this week but... what do you mean ... "how stable is it (the
wood)... ? Thanks for explaining
Merrilee

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Hi Merrilee,

Here in the Midwest U.S.A. a piece of wood that is sealed, like your cabinet
doors, will change +/- 3.5% moisture from summer to winter. This moisture
change causes the physical size of the wood to change in width and thickness
but not in length. That why some of your doors stick in the summer and work
fine in the winter. Different orientation of the grain also has an effect on
how much the size of the wood will change from season to season. Birch, like
most hard woods, changes more than softwoods. A 12-inch wide birch board
that is quarter sawn will change .22 of an inch from summer to winter. That
mean if you cut a dry birch board to exactly 12-inches wide in the winter
that in the summer it will be about ¼ of an inch larger in width. This is
because the fibers take on moisture and enlarge so just the width and
thickness of the board change, not the length.

What makes birch so different is that its size change is about the same
percentage from length to width, where most wood changes about twice the
rate in the flat grain (plain sawn) as it does in the perpendicular grain
(quarter sawn) direction. This means less cupping problems with birch than
with a wood like oak.

For that +/-3.5% moisture change, a 12-inch wide board will change:

Birch quarter sawn (Radial expansion) is .22-inch and plain sawn (tangential
expansion is .28-inch.

Oak quarter sawn is .15-inch and plain sawn is .31-inch

Basswood quarter sawn is .19-inch and .28-inch plain sawn. (also fairly
stable)

Butternut quarter sawn is .10-inch and plain sawn .19-inch. (lower change
but more cupping problems.)

These numbers come from a chart that Tom Liebl, who designs furniture and
boats in Madison Wisconsin, had published in the Fine Woodworking book on
Wood and How to Dry it (available at most good woodworking stores).


Joe Dillett
The Carving Shop
645 E. LaSalle St. Suite 3
Somonauk, IL. 60552
(815) 498-9290 phone
(815) 498-9249 fax
http://www.thecarvingshop.com
jdillett at thecarvingshop.com
http://www.carvingmagazine.com Carving Magazine web site and Readers Forum
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