You don't want to argue because you were mistaken, charitably, either in your wording, or your understanding.
And you are very mistaken in your contention that you "doubt anyone knows for sure" ... a first semester, college level, Botany 101 class would prove to you that this is well understood, and well documented with empirical evidence.
For your future benefit, and so this ends on an instructive note, you will certainly want to explore the definitive work on the matter:
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woodworker can learn something from reading Hoadley, guaranteed.
See - that is why I get pulled back in. Because the logic is so illogical.
If I said I don't believe they really know what is happening - you say take a "first semester, college level, Botany 101 class would prove" - The only thing it would prove is they believe it. Very circular thinking (as the "scientists" would say).
That will be the last think I desire to say on the subject to you. You may continue to deride me if you like.
You are wrong again ... it is not you being derided, It is you showing up here, making a statement that is misleading, and taking exception to being corrected (and very nicely and politely so, if you read my first post)
What we don't particularly care for here is misleading information, so pardon me if I don't feel sorry for your above whining as somehow being a "victim".
Well, that's self-serving to the max. You might as well say that the physicists on believe Newton's Principles; after all, it's "only" a theory.
I suggest you do read Hoadley (or any of several references from the US Forest Products Lab, these can be downloaded at no cost); specifically Chap 4, "Wood and Water".
The short story is---wood shrinks as it dries and it does so non-uniformly. The tangential/radial shrinkage ratio from green to oven-dry has been measured for some hundreds of species and averages about 2 (roughly an average of 8%/4% T/R) but shows a significant variation between species of from just barely above 1:1 to highs approaching 3:1. The higher the ratio the more the particular would will distort as it is dried as the relative shrinkage in the two directions competes at a different rate.
The difference between tangential and radial shrinkage isn't random nor magic; it's caused by the anatomical structure, principally the effect of wood rays whose lengthwise axes are oriented radially outward. Species w/ more predominant rays are more stable.
Over the range of moisture content shrinkage is roughly proportional to moisture loss. This doesn't matter too much as raw lumber is dried; the roughsawn stock is oversize to begin with and if dried uniformly will be stable after it is milled given a uniform environment. This is why it is so important to finish both sides of a furniture panel, say--if one side is finished and the other not, differential moisture absorption is highly likely to cause movement.
Cupping after the fact is owing to one of two causes--either the piece wasn't at equilibrium to begin with and dried after milling with the resultant change in dimension as determined by the species' particular T/R ratio and the percentage change in moisture.
The second is that the piece has subsequently absorbed moisture and therefore grown. The relative amount in the direction is also dependent on T/R and how uniform (or un-uniform) the moisture absorption is.
Cupping in flatsawn boards results in concavity away from the pith, the result of greater tangential than radial shrinkage. The magnitude is greater as the location of the board from the original trunk is closer to the pith on a surface. This face is completely radial while the opposite is tangential in the portion across from the location of the pith and will therefore shrink at twice the rate. Woodworkers tend to say the "rings flatten out" which an easy way to remember the direction the board will cup but the reason for the cup has virtually nothing to do w/ the growth rings themselves.
Cup is reversible on swelling which is why the idea of wetting is given as a cure. Of course, the end then has to be to get the whole board in equilibrium again at that point which goes back to point a) above--if the piece was milled in inequilibrium, it's an insoluble problem w/o mechanical repair.
Whatever will allow air movement and keep it basically flat (flat is relative really - a wal-mart straight edge is "flat" - unless of course you compare it to a $45.00 straight edge - which is more flat :) ).
Let me clarify (at the danger of possibly being accused of trying to change tack) what I meant when you asked "concave moist side". I meant yes - as in moist side that has dried. I did not mean simply the moist side causes a cup - it would have been made moist while compression was placed on it (i.e. the dry side) and then dry out (but it may also take many many times of this before a cup is seen). Information that is only needed if someone is interested - from books - and not even needed to be mentioned to answer your OP - so I guess my headache was caused once again be me :)
Have you ever seen deck boards or a table top that was not attached? Given restrictions a board will bow in the direction that is not restricted. Given no restrictions the surface absorbing moisture will expand and that will not result in a concaved surface.
Absolutely the underneath side. You mop it and water goes down inside the cracks to lower and side sections of of the board. Those surfaces stay wet longer than the top surface. Then the edges expand and they cut up but the surface has long since dried because of exposure.
You are not thinking about all of the surfaces that have actually gotten wet.
Very interesting and valuable information. But I'm not sure it's particularly relavent to my situation.
If it is warped, simply putting under great pressure (I have a few thousand pounds of red bricks at hand), it's probably not pertinent *which* way it's warped.
Placed on small square lumber and weighted, should give some good results.
You can press it flat all you want but unless you change either the moisture content back to equilibrium at the time it was milled and (presumably) flat or, if as noted previously, it was milled while not in an equilibrium moisture content where it now is it will just return to the present shape again.
If you can restrain it sufficiently, you _may_ be able to hold it in place.
This doesn't hold water ;-) when speaking of this table top. The bottom is not likely to be "soaked" in water but free to dry, much more so than the top. The top is exposed to water (rain, spillage), whereas the underside has little exposure to water except for the little that drips around the edges of the table and which gravity will pull it off immediately.
The bottom side of the table top looks nearly pristine, whereas the top looks like it's been through a war: the factory finish (polyurethane?) is still on the underside of the wood but long since baked off the top (from sun exposure).
You obviously did not bother to read the very specific quote at the end of my post that deals with "sun exposure". LOL
You're right that what you quote above does not apply to your table ...nor was it ever intended to.
What does indeed apply to your table is the description you used in your original post ... the term "weathered".
You might want to go back and read the second "example" I used for the effects of moisture on the dimensional stability of wood, as well as the quote at the bottom which specifically deals with wood subjected to sun and "weather".
If you did not accurately describe the condition of your table, then you have no one to blame but yourself for misapplying the examples. :)
Just so there is no mistake how appropriate the quote was to your table suffering "sun exposure" (sic), here it is again:
Wood warmed by the sun experiences a virtual RH far below the ambient RH. The surface dries faster than the rest of the lumber. This is why cupping and checking often occur on decking boards; the top surface is much drier than the rest of the board. Shrinkage of the top surface commensurate with this dryness causes cupping and checking parallel to the grain
It really does help to read an _entire_ post, not just those parts you want to take exception to. :)
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