The seat of this solid wood chair has warped and pulled out the corner screw. Sure I can drill a couple new pocket holes and pull it down, but I am afraid the seat will eventually crack. Any advice?
exactly. that is why it is important to seal all sides. when completely sealed the wood accepts moisture at the same rate on all sides and moves equally (somewhat). some woods just do what they do no matter what.You can read this exhaustive, very nice report by the Forest Service about how finishes affect moisture exchange. But I'll quote the take home message: "Only 11 finishes [out of 91 tested] were found to retard moisture vapor movement into wood with any degree of success over the relatively short time of 14 days, and then only when two or three coats were applied." The only finishes that were effective were a) paraffin dip b)multi-part epoxies and c) solvent-borne enamel paints. An unfinished seat bottom is very unlikely to be the cause of warping. Plus, as Fred points out, the warpage would have been expected in the other direction if it was indeed due to the unfinished bottom equilibrating faster to lower humidity inside.
Aye, I did.I completely disagree that the paper shows finishes have a negligible effect. Did you look at it before making that statement?
Yes, it was a pigmented, flattened shellac that did this, not really a common finish and somewhat of an outlier. Everything that I would consider a typical furniture finish had a very low moisture excluding effectiveness.It clearly shows effects including a MEE (moisture eliminating effectiveness) of 83 for a type of shellac. That is not a negligible effect.
The wood started at equilibrium moisture content for 30% RH, not 30% MC. But yes the test conditions are severe. Agree it's a useful test though. Moisture absorption by wood is a diffusive process, so reasonably approximated by Fick's law: diffusive flux = gradient x diffusivity constant, and changing the gradient to more realistic environmental conditions shouldn't affect the diffusivity constant (a constant), which is what the finishes are modifying.The wood started at 30% moisture content which is higher than I start with and then they exposed it to 90% humidity.
Yep, I certainly grasp your reasoning and it's a convincing argument from first principles. It's just my two cents that, with respect to the USFS report, the authors' data and their own stated conclusions (as quoted above) suggest that this reasoning doesn't really hold up. Also, for centuries handmade furniture was unfinished on non-show surfaces, and furniture manufacturers by-and-large don't bother to finish non-show surfaces.Last, I would say that a big reason to finish both sides is not that we are blocking all moisture changes. We are not. But we are making the rate of absorption similar on both faces.