These are newbee questions I realize, but if I don't ask I won't learn. ;)
I was wondering if plain sawn hardwoods will still cup despite being properly kiln dried to 6-8%. I suppose that wood varies from one species to another and from one tree to another of the same species, but in general will red oak, for example, inevitably cup simply because of way it was sawn? Or can we say that if plain sawn wood dries flat it will always remain flat? Should I always quarter saw when it comes to hardwoods if the wood is intended for indoor use?
Yes, kiln dried wood can cup if it gains moisture. If it was kiln dried to 8% moisture, and you keep it at 8% moisture, it will remain stable and not cup. If you put it outside where it gets wet and gains moisture, it can cup. It is a reaction to a change in moisture content, generally more on one side than the other. If wood is not in equilibrium with the surrounding environment, i.e. the wood is wetter or dryer than the environment that it is in, it can cup or warp as the moisture content changes. It is actually a common problem that woodworkers have.
As to your quarter sawing question......it depends on the species. Walnut, cherry, oak, yellow poplar, and maple and are generally stable wood once dried. You might quarter saw to expose figure like in oak, but in most cases, it does not matter if the wood is quarter sawn or not for interior use. Some bad acting woods like sweetgum, sycamore, and elm that have spiral grain, behave much better if quarter sawn. The flat sawn boards have a propensity to warp as they gain or lose moisture.
One reason that boards tend to cup opposite the "smile" is because there are generally more open pores on the top side to absorb moisture. But, more absorbed moisture on either side will swell that side causing it to cup in the opposite direction, just as WDH said.
I was just adding the "smile" as additional information. :)
QuoteOr can we say that if plain sawn wood dries flat it will always remain flat?
If it stays flat during the drying process (kiln or air drying) then chances are it's going to stay flat after that as well. If the wood went from green to 8% without moving significantly, then it's not going to move much going from 8 to 6 or 8 to 10 either.
Now if it did cup significantly during drying, and was then machined flat again, you know that piece is prone to movement with a change in moisture. It wont be as much, just a fraction of the amount it moved when drying from green, but it may be enough to notice in seasonal variations.
Wood generally moves as the moisture content changes. and it will move by different amounts depending on the grain direction. So a flat sawn board will have the growth rings changing direction across the board, usually from true flat sawn in the middle, to rift sawn grain on the edges. These different grains move differently, so the board can cup.
A quartersawn board pretty much has the same grain orientation right across the whole board, so you don't get that change in grain that would cause cupping.
Ian
To echo Ian's point, it does not make a hill of beans difference if the board was stable in drying, then moves a little bit after changes in moisture if it is used for trim, paneling, etc. However, if you are building fine furniture, even a little bit of cup can drive you crazy.