I'm going to register a mild disagreement with Tim, who wrote:
" Flat and riftsawn wood exchanges moisture from BOTH of its flat faces where QS wood only exchanges moisture from its end grain (IF) there isn’t any run out. "
Moisture loss from wood depends a lot on the species: for the most part it is highest through the end grain, but some woods can gain and lose moisture rapidly from the side grain as well. So far as I know, side grain is side grain: there's no difference in the rate of moisture loss that I've ever seen or heard of through the radial (quartered) or tangential (flat) surfaces in general. Again, there can always be differences by species.
Figured wood does tend to expose more end grain than straight grained wood, of course, and thus may lose moisture more quickly. Given the thin sections we work with, even in bracing, the large surface areas, and the fact that only the outside surfaces of the guitar are finished, in practice I can't imagine it makes any real difference.
Straight grained wood tends to be more stable, all else equal, than figured wood, simply because the fibers are aligned. Wood shrinks differently depending on the grain orientation: least along the grain, much more radially, and most tangentially. These differences cause a piece of wood to change shape as the moisture content changes. The biggest problems come in when the annual ring lines are curved as seen on the end grain, which causes cupping that can be extreme. Curly wood, which is simply a periodic variation in run out, tends to move over time in ways that accentuate the ripple.
Finishes slow moisture exchange, but don't stop it. I know of only a few guitar makers who actually finish the inside of the box: it leads to problems down the road when repairs are needed. At any rate, given the large surface and thin sections, as I have said, it's best to assume things will move. All the 'standard' designs have been worked out to take this into account to at least some extent, so if you don't get too carried away with the wood things will work. I'm always leery of bird's eye maple, since I've never seen an old one that didn't have lots of nasty cracks. I've also got a burl figured back that I've been trying to work up the courage to use for a decade or more.
|