Author Topic: Nic's Post on Wood and Moisture Content  (Read 1166 times)

David Houck

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Nic's Post on Wood and Moisture Content
« on: May 25, 2005, 05:39:22 AM »
I've extracted the following from Nic Grabien's (sfnic) posts on wood and moisture content.
In general, the main thing any woodworker looks for is stability. Green wood, that is, wood that's been recently harvested, still contains a lot of moisture from the growing process. Some logs measure out as almost 30% water. As that water evaporates out of the wood, the wood shrinks. The general goal is to get the lumber down to between 4% and 7%. By the time it gets to be that dry, it's shrunk about as far as it's going to.  
Of course, just sitting there, it'll absorb moisture like a sponge. Right up until it hits either its maximum capacity or the ambient humidity. In the Bay Area, the RH generally sits between 40-60%. Higher in parts where it gets foggy (annual averages can hit 85% RH in some spots); lower in the more sheltered valleys in the south bay (20-30%).  
So, it's kinda hard to air dry lumber to less than about 18% here. A lot of stuff is kiln dried which, as the name implies, uses an oven to bake the wood down to the desired moisture level.  
Once you get the wood dry enough to work (4-7%), you build the instrument. Then you have the problem of absorption. Left unfinished, a guitar will suck in moisture until it hits equilibrium, back up in the 14-20% range. Pieces will expand, glue joints will be stressed, etc.  
Also note that the opposite can occur. I've talked with a few Asian builders who have sold guitars to customers in Saudi Arabia, Israel and Egypt, who have had their instruments implode because the ambient air's too dry. The guitars literally shrunk themselves to death by pulling all the glue joints open. Of course, these guitars were built in shops where the ambient RH was around 90%...  
Anyway, the older a piece of wood is, the lower it's moisture equilibrium point becomes. It's not dramatic, and does take a while, but over time the cellular structure of wood gradually looses its ability to retain moisture. Which is why we tend to look for older pieces of timber.  
It's also why we finish the things. Finishing seals the wood and, for all intents and purposes, prevents the wood from absorbing or shedding moisture. Oh, there's _some_ vapor-barrier effect and any finish is really a semi-permeable membrane, but the rate-of-change is miniscule.  
Acoustic guitars are a bit more vulnerable to moisture changes, if only because they're rarely finished on the inside. The exterior of the guitar is sealed, but the interior usually isn't. It's a trade-off between stability and environmental compliance (in that the instrument can more easily adapt to the local conditions if the wood can breath a bit). Cost is also a factor, as is the desire to not add too much extra mass to the soundboard by coating the underside with finish.
Nic discusses a 1992 Alembic as an example.
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. Remember, 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 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_ susceptible to dimensional creep.  
So, yeah, I'd say your 1992 Alembic has reached its 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 impedances 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 their 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.