77 Woodworking Steambending wood Frequently asked Questions
Description
This article is from the Woodworking FAQ Collection 1, by multiple authors.
77 Woodworking Steambending wood Frequently asked Questions
From: roche@cs.rochester.edu (James Roche)
Newsgroups: rec.woodworking,news.answers,rec.answers
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Reply-To: gregg@clark.harvard.edu
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Archive-name: woodworking/wood-bend
Last-modified: 2/28/96
Copyright (c) 1997 by James J. Roche. All rights reserved.
This FAQ on bending wood is provided courtesy of Gregg Germain. Any
comments would be welcome. Comments should be directed to
gregg@clark.harvard.edu.
I've been in the business of steambending wood for about 10 years now.
I've built a variety of steamboxes and tried a number of boiler
systems. What you see written here is a combintation of reading and
actual experience. Mostly experience.
All of my steam bending has been with either Oak or Mahogany. I've
never tried any other wood as I do this work in my
boatbuilding/restoration. So I cannot comment authoritatively on
bending other woods like cedar, pine, poplar etc.
And if I haven't actually DONE it, I will not comment on it. I will
not state anything here that I have ONLY read out of a book and not
tried.
With that in mind, let's fire up the boiler....
To start with there are several rules of thumb which work quite well.
What you are doing when you are steaming wood for bending, is
softening the hemicelluloses. The celluloses are polymers that
behave the same as thermoplastic resins. [My thanks to John McKenzie
for the last two sentences].
And you need BOTH heat and steam for this. I realize that some people
in Asia "fire" bend their wood but invariably, that wood is quite wet
- typically quite green. The Norse boatbuilders used to get their
planks out for shipbuilding and sink them into a salt water bog to
keep them limber until the time came to use them.
However, we are not always so lucky as to get green wood for our
bending and you can have great success with kiln dried wood. It's
useful if you have the ability to soak your wood for a few days so
that the moisture content rises - those Vikings knew what they were
doing.
You need heat and you need moisture.
The primary rule is the one for steam time:
One hour of steaming per inch thickness of wood.
I have found that you can OVERSTEAM as well as understeam. If you
steam an inch of wood for an hour, try to bend it, and it cracks,
DO NOT assume that you haven't steamed it enough. There are several
factors involved which could explain the result - but we'll get to
those later.
It is smart, however, to have a piece of stock in the steam box that
is the same thickness as the piece you wish to bend, and that is
expendable. PREFERABLY a piece taken from the stock itself. Steam that
with the target piece, and after the requisite steaming time, take the
test piece out and try to bend that to the mold. If it snaps, then
give your piece MAYBE 10 minutes more. But no more.
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