What back and top woods are most resistant to changes in humidity?

[dbradfie]

I know a lot of the luthiers on the forum are busy at the guitar show, but I decided to ask the question anyway.

Bruce Sexaur in a recent thread mentioned he had a pernambuco guitar crack in the low humidity of the southwest. It made me think of the question:

What top and back woods would be least resistant to extreme changes in humidity? What if you were in a position where you knew you wanted a nice solid wood guitar, but it would be subjected to wide swings in humidity.

Would cedar, redwood, or spruce be more stable?

Would walnut, rosewood, or mahogany, be more stable?

Would the source of the particular set of wood be the key factor? (dead tree in forest for many years, etc)

Could a guitar be built to hold up the changes, but still perform tone wise?

Thanks in advance for your thoughts.

[Mark Hatcher]

I think your best bet would be mahogany with a cedar top.
And yes if you can get wood from standing dead trees or wood that has been dry but not protected from humidity and temperature changes that is very helpful. The older the better, preferably dead and unprotected for over 30 years.

Mark

[Tim McKnight]

About any wood species [can be] stable between 40%-55% RH if the wood was dried properly before being shaped into a musical instrument. You should also factor in the cut of the wood as an important factor as well. Zircote is the least stable IME outside of that range. Mahogany and Walnut are more stable than many.

[cogito]

I had two guitars with claro walnut back and sides (one with cedar top, the other with redwood top), both of which held up tremendously well in the N.E. without proper humidification. Based on experiences with other instruments, I would never again fail to provide appropriate humidification, especially with a high-end guitar. But I remain very impressed with how those two guitars held up to the elements.

[Bruce Sexauer]

Originally Posted by dbradfie

I know a lot of the luthiers on the forum are busy at the guitar show, but I decided to ask the question anyway.

Bruce Sexaur in a recent thread mentioned he had a pernambuco guitar crack in the low humidity of the southwest. It made me think of the question:

What top and back woods would be least resistant to extreme changes in humidity? What if you were in a position where you knew you wanted a nice solid wood guitar, but it would be subjected to wide swings in humidity.

Would cedar, redwood, or spruce be more stable?

Would walnut, rosewood, or mahogany, be more stable?

Would the source of the particular set of wood be the key factor? (dead tree in forest for many years, etc)

Could a guitar be built to hold up the changes, but still perform tone wise?

Thanks in advance for your thoughts.

It is extremely important to understand two things. First, the pernambuco that cracked was flatsawn, and the crack was expected and exactly where expected.

Two, I took the guitar with me on the trip specifically to make sure it would in fact crack. If it hadn't, I would be charging $5k more for it than I am.

I do not believe there would have been any trouble in a 00 sized guitar in a mere two days of low humidity IF it were properly cut wood, pernambuco specifically.

I made this guitar from this wood because there was no way I was ever going to be able to recommend this set to a customer as the wood they should choose. Instead, I built with it because I knew it would make a great guitar and I would never get to use it if I didn't make a spec guitar with it. The fact that it is cracked means someone will get a $15K guitar for $10K that has a nearly. Invisible crack in the upper bout that will NEVER affect the sound, playability, or reliability of what will likely be the best guitar they will ever own. If you a think I am overplaying the guitar, ask any of the dozen or so forum items who have played it so far.

If one really want to abuse a guitar by camping in the desert as I did, there is no better natural material than Honduras Mahogany.

[dekutree64]

I live in a very harsh humidity climate for guitars, and have made a study of this in hopes of creating guitars that don't need babysitting. The 3 primary variables are what humidity % you brace at, the expansion rate of the wood (many are listed here http://www.wood-database.com/), and brittleness of the wood. Also the cut of the wood (quartersawn or flatsawn), but if you're concerned with humidity, it should always be quartered.

Bracing humidity sets the center point. Expansion rate determines the absolute dimensional change (and thus how much stress is generated from a given humidity % change). Brittleness determines how much shrinkage stress it can take before cracking. The result of expansion stress is popped glue joints.

The lower the expansion rate, the lower RH you can brace and not have problems in high humidity. This is the key to making humidity-proof guitars. If you use a low expansion wood, but brace in medium/high RH, much of the potential is wasted because it will likely never reach its humidity ceiling.

Cedar and redwood are more brittle than spruce, but their humidity expansion rates are about half as much as spruce. All 3 crack at about the same humidity % drop, but braced in low humidity, cedar/redwood will tolerate a larger range than spruce ever can.

Honduran mahogany is not brittle at all, and has fairly low expansion rates, so it can be braced in 40-45% and survive just about anything.

Rosewoods (including African blackwood) are more brittle, but do have fairly low expansion, so brace in 30-35% and they'll tolerate a lot. I'd edge lower with Brazilian because of its extreme brittleness.

Ziricote is one of the most brittle of all woods, and its expansion is medium. I have an ukulele with flatsawn ziricote back braced in 30-35%, which seems to tolerate the full range here pretty well. The back seam peels open a bit in low humidity, but the cross grain reinforcement inside holds it together so it's not really an issue.

Ebony has extremely high expansion, and flatsawn ebony is perhaps the highest of any wood, so expect cracks and popped brace joints with it. I'm currently building one with flatsawn Malaysian blackwood (an ebony), which will serve as a test subject over the next few years. It will be braced in 30-35%, with wide braces that can hopefully hold on in high humidity.

Flatsawn pernambuco does indeed have very high expansion.

Mesquite has perhaps the lowest expansion rate of any wood. Desert ironwood and Texas ebony (not a true ebony) seem similar, but I don't have any real data on them to be sure.

Bottom line... if you want a humidity proof guitar, use a cedar or redwood top braced dry, Honduran mahogany back braced medium, and something other than ebony for the fingerboard and bridge.

[Kent Chasson]

Originally Posted by dbradfie

...

What top and back woods would be least resistant to extreme changes in humidity? What if you were in a position where you knew you wanted a nice solid wood guitar, but it would be subjected to wide swings in humidity.

Would cedar, redwood, or spruce be more stable?

Would walnut, rosewood, or mahogany, be more stable?

There are several factors involved. Using wood that is dimensionally stable is a good start but some species are more brittle and will split more easily with less movement. That's why mahogany has been mentioned. It moves less than other woods but is also less brittle.

As for top woods, cedar is more stable than spruce but it's easier to split so it may be a wash there. The stability would help keep it playable though. (more on that later).

Another factor is the speed of humidity changes. Faster changes can cause a split in short order but slower changes allow the wood to absorb more stress without splitting.

Extremes in one direction are better than extremes in both. Places like the NE US where the humidity goes from the 90's in the summer to single digits in the winter are worse. If guitars experience prolonged high humidity, they will be more split-prone when the humidity drops due to a characteristic of wood called "compression set".

During construction, you can account for humidity at one extreme or the other. I have built for people who live in constant low or constant high humidity with the clear understanding that these guitars will stay in these extremes.

Originally Posted by dbradfie

Would the source of the particular set of wood be the key factor? (dead tree in forest for many years, etc)

It's generally believed (maybe proven but I haven't seen it) that "seasoning" for many years will make wood less hygroscopic and therefore more stable. It has been pretty well proven that "baking" wood will do the same. I think the idea is that the hemicellulose changes structure over time but I don't understand it at a chemical level.

Originally Posted by dbradfie

Could a guitar be built to hold up the changes, but still perform tone wise?

Tone is one issue, action another. Most guitars do have a sweet spot for tone where humidity is concerned. As humidity increases, the plates expand and any radius or dome increases. This effects tone and action. Smaller guitars will perform better on both counts as there is less width to expand.

I think an all mahogany parlor with an adjustable neck is what you're looking for

Originally Posted by Bruce Sexauer

I do not believe there would have been any trouble in a 00 sized guitar in a mere two days of low humidity IF it were properly cut wood, pernambuco specifically.

I bet you're right. I'm always experimenting with my personal guitar. I take them to the desert almost every year and leave them unhumidified until I start to see too much deformation. This year, I waited a full week and quartersawn brazilian was fine.

[printer2]

Not as much experience here as compared to the rest but I have a few guitars that have survived the RH going down into the teens and up to 70%. They were built around 30%, I think the wood can handle a greater range above the humidity they were built at than a lower humidity. I am guessing the wood takes compression better than being pulled apart. My guitars at the lowest humidity had the back and tops lost most of their arc. While I have built some guitars with flat sawn wood, just as learning builds, I have bought wood that is mostly quartered.