Guitar making is a mystery to many of us and embellished with lore. I once spoke to wood technologist Bruce Hoadley, author of Understanding Wood, and mentioned that we guitar builders think that as wood gets older the resins break down, thus causing a guitar to sound better and to maintain stability, to which he said, “That’s great. What resins are you talking about?” To which I answered, “Well, I don’t know.” I’m a dork.
He went on to say that he thinks it’s like
chicken noodle soup, which people say
makes them feel better when they’re sick,
and it probably does, but let’s just go with
the idea that it does, and not try to explain
it because we don’t know what we’re talking about. So, he believes that older guitars
sound better, but he cautioned me to be
careful about saying why.
That was about the time I wanted to find
out about spruce shrinking and if there was
something I could do about it. So, I’m going
to get all technical on you and tell you about
some experiments I conducted about 15
years ago and how those led to my current
techniques for drying wood.
Testing the Field
The scientific method is to have a hypothesis,
to test it with an experiment, and then to
accept the results of that experiment. That’s
hard to do because often the results aren’t
what we want to see and so we often are
tempted to ignore them. But this was a dispassionate idea, which was to simply find out
how much spruce shrinks after being dried,
either naturally, or through a forced process
(thus the title of this article).
Step one was to take a group of tops, air
dried and kiln dried, equalize them to 47
percent relative humidity (RH), then cut and
mill them to the exact thickness of my dreadnought model. They were then measured
for width to the thousandth of an inch and
weighed on a gram scale. Next step was to
equalize them to 30 percent RH. “Equalizing”
expresses the state where wood has
absorbed or given off the necessary moisture
to be even with the surrounding RH.
Next was to weigh and measure each piece.
The results were that they lost between a
1/16" and 1/8" in width and lost significant
weight due to water being removed from
the lower humidity. An eighth of an inch is
enough to crack some guitars, so what to do?
During this time, I also asked for the help of
every guitar factory you can imagine. I asked
them to send a top to me that was ready
for bracing, seal it in layers of plastic wrap,
and then overnight it to me. I measured and
weighed each piece as soon as I opened the
pack, and proceeded to equalize it to my
factory, then measured again. By doing this
I was able to compare the humidity levels of
all the shops and factories around the U.S. at
the time. Then I proceeded to find out if any
of their drying methods worked better than
those I had been using. They didn’t, and their
spruce exhibited the same shrinkage factors.
This sampling included 20-year-old tops and
tops which were salt-water cured, a method
we guitar makers have heard about that some
people say Stradivarius employed. None of
them performed better than the others.
Lessening the Shrinkage
To keep this article short enough to read,
I’ll fast forward to the final drying method
that I came upon. That is to heat the spruce
to 200 degrees for 30 minutes, or in other
words, bake it. I call it oven-roasted. I do
know that the pitch in spruce will “set” at
180 degrees, but more than that, nearly
every bit of water is driven out of the wood
through this process. The next step is to re-
equalize the spruce to 47 percent RH, which
hydrates it back to “normal,” but at this point
it is permanently smaller than it was before
it was heated. Upon exposing these pieces
of spruce to 30 percent RH, they now only
shrank between 1/32" and 1/16"! That is half
as much as before and enough to eliminate
most of the cracking that might occur on a
guitar that is dried due to exposure to low
I shared with the results with all the builders
who contributed tops to the experiment. I
immediately employed this method of drying
and have done it ever since.
Did it help the sound? I can’t tell, but it didn’t
hurt it. It certainly helped the shrinkage factor.
Does it still shrink? Yes, but less than before.
Point is, there was enough scientific method
employed to come up with a drying method
that proves to be better than air drying when
it comes to the stability of the wood.
When it comes to the mechanical performance of a guitar, I think that an approach
like this works very well. We use a similar
method for designing the sound (making
samples with small changes), but the results
are much more subjective because it’s a value
judgment in the end. I suppose we could use
some kind of meter to tell us what the guitar
sounds like, but that just makes me cringe, so
I’m not signing up for that. I suppose that’s
why guitar making is considered a blend of
art and science.
Bob Taylor is the co-founder and president of Taylor
Guitars. He built his first guitar as a teenager and has
since gone on to establish Taylor Guitars as one of the
world’s premier acoustic, acoustic/electric and electric
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