Joey -
Let's take the '92, as the others will be further along the trail, so to speak.
Born in '92, likely started in late '91. It's a given that Alembic knows what they're doing with wood, so the raw stock was stable in the storage barn for at least 6 months, more likely 3 years before being cut.
(Take a look at some of the stuff that's still available in the Wood Bank, then work backwards to see the dates Mica first posted the info. There're boards available that she first posted in November '2002; they were at least a few years old at that point!)
Anyway, at the minimum, the wood for the '92 was shop-ready around the beginning of '91. Now, they generally use 4/4 stock (that is, timber that's 4 quarters of an inch, or 1 real inch thick). They often get billets in that are 8/4 stock, which is 2 inches thick, then split them several times to get to the final 1/4 thick bookmatched pair for the top and back, or, for mahogany cores, down to about 3/4 thick.
The key is that the raw stock is generally either 1 or 2 thick when they start working it.
Woodyard rule-of-thumb is that it takes 1 year per inch of thickness for an average hardwood do dry out from full moisture/just cut down to shop ready. Somewhat longer in the Northwest, where it's a bit damper. Alembic gets a reasonable amount of lumber that's sourced from NorCal, Oregon and Washington. (We're a great source for Walnut and Big Leaf Maple.)
So, let's assume 4/4 stock, cut in this region. Figure 18-24 months to dry down to shop level (stock doesn't get into humidity controlled storage right away; it can take a few weeks to get from the forest to the mill, then a few more before it's milled for drying). Call it 2 years. That backs us up to early '89 as the earliest harvest date.
So, the wood in your '92 is probably no less than 16 years old, at this point. The instrument, if left unfinished, would have reached an initial equilibrium point by late 1990, but would still be susceptible to humidity-based dimensional changes for about another 7 years. The wood was finished in 1992, however. This both protects the wood from moisture, but also slows the final equilibrium-reaching stage (because the wood can't breathe as well, after a polyester finish). So, figure the wood was 2 years into its 7-year settling period when it was finished, and that it would take twice as long to settle afterwards. 5 years x 2 = 10 years later, the wood would have reached it's final equilibrium point, where you can pretty much forget about humidity-based dimensional changes.
Call it 2002, or thereabouts.
Now, the wood can still shift a bit, but not traumatically so. Rember, it's _all_ hardwood of varying degrees, and Alembic is careful about grain orientation and glues and properties-of-motion. So the neck and body will generally expand and contract in complimentary--if not relatively identical--ways.
Probably the weakest joint on the bass, as far as two pieces of wood wanting to go in different directions, are the headstock laminations. But they also have a lot of mechanical help in retaining their relationships: the tuning pegs. The mass of the pegs helps stabilize the interior temperature of the headstock (think heatsink), which in turn helps keep the wood's interior moisture level constant. The pegs also act as continuing clamps for the headstock sandwich. So the head isn't going to de-laminate. I might expect to see some lifting of the to lamination at the tip of the standard crown peghead, but probably not for at least 20 years, and that only if the instrument is left outside most of the time. For a regular gigging instrument that travels in a case and stays indoors 95% of the time? Headstock lams should never be a problem. And, as I say, those are the joints _most_ suseptible to dimensional creep.
So, yeah, I'd say your three basses have reached their point of maximum stability, and should keep that for at least 50-100 years or so, assuming the finish is not excessively damaged and the instrument is reasonably well cared for.
By comparison, we're seeing some classical guitars that are sonically dying, because the woods have dried out to the point where they're losing flexibility. They're still structurally sound; the instruments are in no danger of losing structural integrity, but they're simply losing their voices due to age. These are mid-19th century instruments that, generally, have french-polish finishes externally and no finish internally. The hide glues used for the construction are still sound as adhesives, but they are drying out as flexible membranes that allows microtesimal movement between the braces and tops and backs. So the mechanical impedences are changing, and the instruments' voices are shifting (and being dampened out).
But these guitars are 150+ years old and, in many cases, have been played a lot for their entire lives. And the voicing effects are more due to the glue joints drying out than to the wood losing moisture equilibrium.
But it is possible that sometime around 2125, some of the earlier Alembics may start to lose compliance in their necks. Maybe sooner; maybe much later. Hard to tell, because the surface area of ther glue joints involved are so much larger.
Last thought on the wood: there are 14-15th century lutes that, while not playable because the glues have all dried out, still have perfectly solid wood in the necks and bodies. That's pushing 500 years.
If you can still find 6L6GC power tubes in 2505 AD, we'll worry about your Alembic's neck crumbling. ;-)
nic
