Re: David Marks And Loose Tenons - Revisited

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.

"Mike Marlow" theorized incorrectly:

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

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.

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.

That's correct. I never implied it could be done.

To some point, but we're talking about humidity here George, not water under pressure.

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.

Pouring water directly onto a piece of wood is a bit different from exposing it to atmospheric humidity, don't you suppose?

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.

That was your idea. Mine was that the principle of wood adsorbing moiture and exerting force was well-known

Sorry that your were ignorant of it.

Nah, after grade three, the reader is supposed to know what's on the page and what's in their imagination.

That explains why so much of what you say makes so little sense.

of course. and the force required to put a little compression set on a tennon is a bit different than the force required to split a slab of granite off of a quarry face, don't you suppose?