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December 2014
more... Acoustic SoundboardGearAcousticAcousticGear BlogAcousticSpruceJuly 2010

Drying Spruce: Nature or Nuture?

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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 humidity.

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 guitar manufacturers.
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