files and other web site information. as wood absorbs
moisture it gets wider and contracts as it loses moisture.
if one side of a board absorbs more than the other side it
is gonna bow. (the dry side "cups") and there are other
plain sawn boards are less stable than quarter sawn. and so
on and so on.
A very much simplified explanation:
Wood is hydroscopic in nature and almost always contains water to some
extent, more as a growing plant, less as a dead plank. Wood fibers exchange
moisture with the surrounding air until an equilibrium is reached between
the air and the wood. These fibers, being oriented lengthwise for the most
part, expand across their width to a greater extent than they do length when
they absorb water. When moisture content is high, once again generally
caused by the relative humidity of the surroundings, and depending upon
species and how it was cut from the log, wood will swell across its grain,
sometimes as much as 1/4 ", or more, in a foot.
Conversely wood will shrink for the same reason when exposed to low relative
humidity for any length of time.
When building furniture, or anything made of wood or wood products, you must
take this usually seasonal change in humidity, and subsequent dimensional
instability, into account.
Much of the art and science of joinery deals with the propensity of wood to
be dimensionally unstable due to the relative humidity of its surroundings.
http://www.fpl.fs.fed.us/TMU/publications.htm Where wood is always the
Swingman's description is right on. The parts of the wood which expand most
are those generally lighter colored, softer earlywood areas. Broader the
earlywood areas on the visible surface, more the wood can expand. Latewood,
the dense stuff, much less.
Changes with humidity. There's some change with temperature, but this
is trivial and can be ignored.
Moisture related changes are really important for serious woodworking.
If you're interested in timber drying and movement, read Bruce
Hoadley's "Understanding Wood"
(Amazon.com product link shortened)
or the US Forest Products handbook that George posted the link to. Lee
Valley also sell a nice printed copy of this.
Wood is full of water. Freshly cut oak might be 80% moisture NB - as
this is quoted as relative to the _dry_ weight, then this means that
only about 45% of the log's weight is water, not 80%. Some softwoods
are even wetter - maybe 120% when freshly cut.
This water is in two places within the timber; in the large spaces
inside the vessels, and inside the cells themselves. Drying it down to
about 35% water (or simply squeezing it !) removes the water from the
large vessels first. The timber doesn't move during this process.
Further drying (which needs time, although warmth and vacuum can speed
it up) can remove (most of) the rest of this water from within the
cells. During this process, the timber shrinks.
Looking at the moisture content for timber, the concept of EMC
(equilibrium moisture content) is very important. Leave a piece of
timber in the open air for a long time and it exchanges water with the
air until each has its relative humidity / moisture content in
equilibrium with the other. This is the EMC, and you can plot a graph
of it for timber against air humidity. It's a well known graph, with
a slightly S-shaped curve to it. Surprisingly (to me anyway) we
discover two facts about it; it's almost the same graph for any timber
species, and it's also independent of temperature. That's such a
surprising result, but it's also useful because it means there's only
one graph and we can make use of it. I can roughly draw it from
memory (but not in ASCII), although the only points you really need to
remember are that EMC for 35% RH (driest part of normal English
Summer) is 7% EMC and that for 70% RH (damp English winter, when not
actually raining) is 14% EMC.
So indoor timber "in service" will move its moisture through the year,
every year, between 7% and 14%.
Semi-green timber stored in my workshop in Summer will get drier, down
to about 7%
Kiln dried timber at 6% MC will _gain_ water when it arrives in my
workshop. In Winter it might gain an appreciable amount and doubel its
water content !
Now let's look at movement. All the movement is "sideways" to the
central axis, and timber expands with increasing MC. There's
negligible lengthways expansion with moisture. Tangential movement is
greater than radial movement.
Movement behaviour is less consistent between species than the EMC
curve, but we can make some approximations. All timber moves approx
10% maximum from "fully wet" to "fully dry" - although the moisture
content to be regarded as "wet" varies between species.
Timber has approx twice as much tangential movement as it does radial,
i.e. there's a maximum of 5% radial movement from our "fully wet" MC.
This ratio also varies with species.
The tangential/radial ratio is what causes warping, cupping etc. If it
was always 1:1, then timber would simply pump in and out in a linear
manner. Unfortunately it isn't, but species with lower ratios will be
less prone to it. Considering the geometry will allow you to work
this out, but a rule of thumb is that "rings tend to straighten on
drying". A quartersawn board is thus pretty stable, but a flatsawn
board tends to cup with a concave outer face.
With age and number of moisture cycles, timber stops moving so much
and settles down around the lower end. This takes 10-20 years,
depending on species and extent of the cycles it goes through.
In the meantime, we have to allow for shrinkage. Read Hoadley et al
for just how to do this, but the good news is that some very simple
maths allows you not only to expect this shrinkage, but to _predict_
just how big it will be. Imagine making something, like a big
greenwood timber frame, then knowing just _how_much_ to make it loose,
knowing that it will shrink to fit perfectly over the next ten years.
Well, I was impressed anyway.
Some timber species are stronger than others at resisting tensile or
compressive stresses (forces). However they all fail at around 2%
strain (change in length, as a percentage). Now if we compare this to
our "10% total shrinkage" figure, it's a lot smaller. And _that_ is
why you can't dry disks from the ends of logs without getting radial
cracks, no matter how carefully you do it. Consider the log as a
series of hoops, which try to shrink shorter, but can't get any
smaller radius because there's another hoop inside. That's also why
boring a big central hole allows you to dry these disks and avoid the
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