Shrinkage
is a factor to be reckoned with in all wood construction.
In simple terms, shrinkage is the contraction
of wood as a result of a loss of moisture. When
a tree is freshly cut, it has a high moisture
content which is evident by its sappy nature and
extreme weight.
Presuming that wood is dried to a point where
it is stable prior to its use (and that's not
always a safe presumption), should additional
problems be expected?
The answer is yes. Wood can be brought to a point
of relative stability, but the environment in
which it is placed is not stable. Invariably wood
will continue to lose moisture from years of being
indoors. What is more significant is that wood
will continue to change with seasons. Wood will
gain or lose moisture according to the moisture
content of the air. Summer provides high moisture,
while winter means low moisture and contraction.
A
piece of furniture purchased in a high-moisture
area such as London, England and brought to a
drier environment such as New York or Toronto
often will start to fall apart through contraction.
The degree of shrinkage varies with the age of
the piece (how much it has pre-shrunk already),
the type of wood, and how it was constructed.
There is no mystery to some pieces falling apart.
The
move to lower humidity (often centrally heated
houses) results in shrinkage. Stress to all the
glue lines causes the piece to sometimes come
unglued.
The first rule of shrinkage is that all woods
shrink across the grain. The second rule is that
all wood will continue to shrink to some degree
after being employed in furniture construction.
The third rule is that
all wood will continue to expand and contract
with changes in humidity no matter what you do
to it (save shrink wrapping it in an air-tight
plastic wrapper!).
The last rule, to complicate matters, is that
uneven absorption of moisture (one surface absorbing
more than the other surface) will result in warpage.
This last point is all too frequently illustrated
with tabletops. A craftsman will finish the top
surface of a table, for example, but rarely the
underside. The finish on the top surface inhibits
the absorption of moisture (the primary reason
for using a finish) forcing greater absorption
from the bottom. The consequence is a warped top
leaf. This effect is known as cupping. Understanding
and observing shrinkage is at the basis of authenticating
antique furniture. Without shrinkage, a piece
simply cannot be antique. A piece exhibiting shrinkage
might not be unique but it must be old. In some
cases, a combination of very well-seasoned wood
and a stable climate will result in a minimum
of shrinkage to a table and no splitting. But
you are likely to encounter more split tops than
not.
You
will find harvest tables in barns, deserted houses,
and old sheds, have problems with shrinkage and
splitting when the items are refinished and moved
into a heated home or cottage. Tables on pedestal
bases are less likely to split because the manner
in which they are attached allows greater latitude
for the movement of the wood.
There are numerous examples of shrinkage. In an
old schoolmaster's desk the cross member shrinks
and leaves a shrinkage ridge where it is mortised
into the 1eg post. With ladderback chairs the
slat will shrink leaving a portion of the empty
mortise showing. In addition, the glue holding
the chair together will dry and crumble with temperature
and moisture change. Everyone has experienced
a chair coming apart.
The important thing with shrinkage is the simple
principal that it shrinks across the grain. The
nailed blanket box will not change because all
sides are continuous and they will shrink the
same. After a hundred years or so each will have
shrunk to approximately the same width. The key
to shrinkage in this case will be the bottom of
the box. Considering the bottom board is fixed
at several points it will either have cracked
or pulled away at one side.
So, the next time your newly purchased antique
shrinks in the winter, don't blame the dealer
- check your heat
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