Mike, you need to think. You cannot compress a tenon into a mortise so
tight as to remove all the air from the tenon. Period, end of sentence.
Since there is available air space, water, an incompressible, will expand
the cellulose to compress the air spaces.
This is so basic, I don't understand what you might be thinking.
Going back to my original point - I stated that while yes, the tenon will
swell some with higher humidity, so will the mortise, and the mechanical
constriction placed on this joint will serve to limit the expansion from
changes in humidity. This is not to say that the joint members will not
take on the moisture, but that they will not take it on in the same manner
as a plank laying unencumbered in the same environment. My point was in
response to the number of posts which proclaim "explosions" of MT joints.
This simply does not happen on a regular basis and there is more to wood and
its reaction to its environment than simple moisture levels.
Mike, you're starting to change your tune now, so I assume you've finally
caught on to the fact that moisture changes within a M/T joint, and can
cause compression sets to the fiber which will become gaps later as the
joint dries. The joy of the joint, as mentioned way back, is that it holds
mechanically even when loose in the load direction, when pinned to keep the
shoulders registered against the face of the mortised piece, it even
survives the other five load directions when glueless.
As to your latest - once again your knowledge base is too narrow. Man has
been pouring water onto wooden wedges to break blocks from quarry walls for
thousands of years.
Only if I didn't understand wood. I heartily recommend the study of the
material whose name is in the title of this group to you.
The incompressible fluid replaces the air, then the adsorption at the
molecular level produces the rest of the hydraulic force to split. This is
what happens to wood with humidity cycling. It adsorbs moisture, bonding at
the molecular level. Whether or not there was expansion of the fibers
themselves was the original bone of contention.
If you're in the temperate zone you can get some extra oomph by waiting for
nighttime and the freeze....
By making no distinction between the two cases, you appear to be suggesting
that the magnitude of the force exerted is the same when a small amount of
moisture is absorbed from the atmosphere, as when a large amount of moisture
is absorbed from immersion in liquid.
If this is the case, I heartily recommend to you the study of logic; if this
is not the case, I heartily recommend to you the study of the English
language, in particular William Zinsser's book "On Writing Well" in which he
makes the point that the purpose of writing is not to be understood, but to
make it impossible to be misunderstood.
Doug Miller (alphageek at milmac dot com)
think about it like this- wood compresses. if it is compressed very
much, it crushes. once crushed, it will not return to it's original
size. water causes wood to expand. the force exerted by the water is
very high (remember hydraulics?). given enough water and a tight enough
joint, the expansion force of the water exceeds the crush limit of the
wood. remove the water and you have a loose joint.
items that will have to endure repeated wet/dry cycles need to be held
together without relying on constrained jointery.
You are assuming that expansion of the wood in the mortice will crush the
tenon locally..It would seem to me it might deform a little but basically
this would be within the "elastic limit" of the wood. Bare in mind wood is a
very complex material regarding it's engineering properties and has a
different moduli depending which grain direction you are considering ..
Getting back to the crushing situation again from my standpoint and even
considering the preceeding and even realizing that wood is not a homogenious
material iof it cannot move in one axis I would at least some movement in
another unconstrained axis.resulting in a reduction of pressure [stress].
Again considering the "hydraulic" aspects the pressure in the joint is
pretty much the same as atmospheric and always will be regardless of how
well the finishes seal the wood . mjh
I realize I'm speaking a bit of heracy here, but think about the large
number of MT joints we are surrounded by on a daily basis. How many of
those do we find loosened by humidity? By racking, as in a chair that gets
rocked in, sure, but by simple humidity? We just don't find that to be a
huge problem. While I agree that wood compresses and crushes, it takes
significant pressure to do this. My point is that the mechanical
constraints caused by the joint itself make normal humidity cycles unable to
consistently cause this type of pressure. We're not talking about putting
the joint under water here, we're talking about what furniture is exposed to
every day. I have personally brought furniture home from the Orient where
humidity levels are monsterous, and some of that came back to the States, to
Utah where humidity is near zero. No problems. That same stuff then made
it across the country to NY where humidity does vary considerably throughout
the year. Still tight as the day I bought it. Don't misunderstand what I'm
trying to say here - I'm not arguing that tight jointery alone is
suffiecient for long term survival of the joint, such that glue would not be
necessary. I started this thought in response to the posts about humidity
causing MT joints to explode.
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