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Steam-Bending
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Steam-bending wood is a very simple operation that you will need to
learn to apply chines, transom bows, forward bottom planks, covering
boards, and topsides planks where extreme bends occur. Once a
piece of wood is steam-bent into shape, it will not fight to spring free
of its fasteners and will be far less likely to cup or crack in the
future.
To begin with, there is nothing magic or necessary with steam.
Steam does not add moisture to the wood; in fact, it dries it
out. Many commercial kiln operations us steam heat to remove
moisture from wood in the kiln-drying process.
All steam does is use molecular water to transfer heat. The same
effects can be achieved by boiling the lumber or placing it in a very
large oven. The moisture in the wood simply acts as a conductor
for the heat.
In kiln-dried wood, however, 6 to 10 percent MC acts more like
insulation than like a conductor. Air-dried wood of about 20
percent MC is preferred.
The cell walls contain nearly moisture saturation levels, aiding the
heat transfer and plasticizing, yet not enough moisture to
 host rot
spore. The fiber saturation point at which the cell walls are
still saturated with
moisture and transmit the heat easily to the
interior of the piece of wood, is approximately 25 percent
MC.
Whatever lumber I attempt to bend, I soak it at least a day before
heating. This soaks the entire surface, which aids greatly in
transferring heat to the interior of the piece. I have no trouble
bending kiln-dried 1/2-inch mahogany plank stock this way. Soaking
the wood, even for a short time, will cause it to expand somewhat, so
let it dry for at least one week for every day of soaking before
trimming it to fit.
When enough heat is applied, the cell walls plasticize and can be
compressed into smaller cells, as long as the original free water is
gone. To a lesser extent, the lignin also plasticizes and allows
the cells to slide alongside each other, as this compression occurs.
Almost all cell deformation in bending occurs on the inboard, concave
surface of the piece being bent. When hot enough, these cells can
compress in length some 20 percent without cell wall fracture.
When clamped in place and left to cool, the compressed cell walls
solidify and hold their new dimension. The measured length of the
plank on this inboard, concave surface will be less than it was before
bending, due to the deformation (compression) of the cells.
Very little (about two percent) cell deformation occurs on the outboard,
or convex, surface of the wood. Apparently, these cells do not
like to stretch as easily as they compress. The measured length of
the plank on this outboard, convex surface will be nearly the same as it
was originally. If the bent piece of wood is left clamped in place
long enough to cool off, the cellulose and lignin re-solidify. The
plank will now hold that shape, minus some initial "spring
back" because one side of the piece is now shorter than the other.
I have seen some restorers attempt to steam bend 1/2-inch, kiln-dried
mahogany plank stock with a wet towel and household laundry iron held to
the exterior surface of the plank. This method could not possibly
effect the interior and inboard surface of the plank, where cell
compression needs to occur. You have to apply enough heat for a
long enough time to raise the temperature of the entire board, all the
way through, to as close to 212 degrees as possible. In order to
do this, you must be able to maintain 212 degrees in your steam
box. Use a meat thermometer stuck through a hole in the top of the
steam box farthest from steam entry. You may have to wrap
insulation around your box or tube to accomplish this. The
traditional rule of thumb has been one hour of steaming for every inch
of lumber thickness. This must be tempered somewhat by how many
pieces of lumber you put in an how efficient the steamer and box
is. If you are heating a bunch of frames at one time, add a few
test pieces and check them until they are wobbly and somewhat
rubbery. What you expect to take an hour could very well require
two. |
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