Back & sides wood - durability

[Scallywag]

Really random question - just saw another thread where a nice rosewood guitar developed a crack - the back, along the grain. No idea how. At the same moment, I rudely knocked my mahogany OM into god-knows-what, for the 1000th time. Once again, right as rain. Thing is a tank, wrapped in Valerian steel

With that, what are the more durable backs & side woods? Contrarily, which are most fragile, once transformed into an acoustic guitar??

Lots of variables here - understood. Curious about the generality in this case.

Let me re-premise. In your opinion, what are the most worry-free woods that still make for a fine sounding instrument? Which should I handle like a sea-sick newborn baby with the flu?

Thanks to you fine people, as always.

[HikariGuitars]

Originally Posted by Scallywag

Really random question - just saw another thread where a nice rosewood guitar developed a crack - the back, along the grain. No idea how. At the same moment, I rudely knocked my mahogany OM into god-knows-what, for the 1000th time. Once again, right as rain. Thing is a tank, wrapped in Valerian steel

With that, what are the more durable backs & side woods? Contrarily, which are most fragile, once transformed into an acoustic guitar??

Lots of variables here - understood. Curious about the generality in this case.

Let me re-premise. In your opinion, what are the most worry-free woods that still make for a fine sounding instrument? Which should I handle like a sea-sick newborn baby with the flu?

Thanks to you fine people, as always.

If you are including only solid woods I I have no idea, however, laminates are definitely sturdier than solids

Also, if that helps, most guitars with cracked backs I have seen were made of Brazilian rosewood , some would take decades to crack , others the slightly change of humidity would give in .

Indian rosewood guitars I have seen hardly crack even when older...

[charles Tauber]

Cracks in guitars most often occur from either mechanical impact - hitting the guitar against something - or due to changes in environmental humidity. Resistance to cracking from impact is increased by using physically harder woods, and, in some cases, by the selection of how the wood has been cut and reinforced/braced. Regardless, the large, flat surfaces of guitars are easily damaged by impact. Cracks due to changes in humidity are more complex.

Wood is a hygroscopic material, meaning that it absorbs and desorbs moisture in response to its environment. As it absorbs and desorbs moisture it changes size and shape. Relative to the orientation of its grain (or fibers), most woods change size differently in different directions. That is, when wood expands or contracts, it doesn't do so evenly in all directions. Wood changes length negligibly. Mostly, it changes in thickness and width. These are generally referred to as the tangential and radial orientations when looking at the end-grain (cross section) of a tree. The radial orientation is from the center of the tree towards the bark. The tangential orientation is along the length of the annual rings.

The amount that a particular wood changes width and thickness in response to changes in moisture content has been measured and tabulated for many different species of wood. (The ratio of the change in thickness to width in response to a change in moisture content is also tabulated and provides a relative measure of how "stable" is a particular wood species.)

The table, below, gives a "factor" for how much a unit-width of different species of wood will change size per unit change in moisture content. It gives values for both tangential and radial change. The larger the number, the more that wood will change size. The larger the difference between radial and tangential for a specific species, the more prone that species is to changes in shape/distortion.



To further complicate things, the orientation of how a board is cut from a tree greatly influences how a particular board will change size and shape.



Note that, for the same change in moisture content, quarter sawn wood will change less in its width than a flat-sawn board.



Guitar tops and backs are 15 to 16" wide, usually, and are rigidly glued all around their edges, preventing them from changing overall size in response to changes in moisture content. Although applying a finish to a board slows the absorption and desorption of moisture, it does not prevent it. When a guitar top or back shrinks, and cannot change size (width), when the stresses due to shrinkage are higher than the strength of the wood, the wood splits. Thus, a flat-sawn board, with its greater shrinkage per unit change in moisture content, is more likely to split than a similar quarter sawn board.

Another very important factor is the humidity level (moisture content) at which the guitar was assembled. For example, if a guitar is made and assembled at a relative humidity of 90%, say in parts of Asia, and then shipped to a location that sees winter humidity levels of 20%, that guitar is going to split: the change in size resulting from the change in humidity level, will exceed the strength of the wood.

From the above, you can understand why it is so important to control the relative humidity of the environment in which the guitar lives. The further one diminishes the humidity from the level at which the guitar was made, the greater the chances that the top and back will crack. The technology is well understood.

[Rodger Knox]

Nice post Mr.Tauber!

[hat]

Pattern makers used to use mahogany because it was the most dimensionally stable, so I would say those same characteristics would make it one of the most resistant to cracking - if properly seasoned.

[Ned Milburn]

Originally Posted by Scallywag

Really random question - just saw another thread where a nice rosewood guitar developed a crack - the back, along the grain. No idea how. At the same moment, I rudely knocked my mahogany OM into god-knows-what, for the 1000th time. Once again, right as rain. Thing is a tank, wrapped in Valerian steel

With that, what are the more durable backs & side woods? Contrarily, which are most fragile, once transformed into an acoustic guitar??

Lots of variables here - understood. Curious about the generality in this case.

Let me re-premise. In your opinion, what are the most worry-free woods that still make for a fine sounding instrument? Which should I handle like a sea-sick newborn baby with the flu?

Thanks to you fine people, as always.

Charles gave you some very detailed information.

All things equal, the more movement a wood expands and contracts due to humidity gain/loss, the more potential it will have to split. That said, certain woods will be more brittle/fragile than others. Balsa wood, for example, will split a lot more readily than Mahogany with the same cut (quarter sawn) and thickness. So, yes, certain species will have greater splitting propensity than others. Sorry I can't give a table of woods and their ease of splitting. But even within the same species, certain cuts of wood (again see Charles' explanation) as well as certain areas of certain trees will have grain that to the discerning eye will have a greater propensity for splitting.

Okay, all that aside...

I just worked on a 30-40 year old Martin. Mahogany sides and back. Splits in the side near the waist (treble). The wood was thin. Very thin. I expect well less than 2.0mm.

A year ago I worked on a L'Arrive guitar. Mahogany sides and back. Splits in the side on the lower bout. Very thin sides. Much less than 2.0mm.

So, if sides are thinned too much, no matter what builder, no matter what wood species, splits will likely occur in the guitar's life.

I saw a brand new hand made guitar with zebra-wood sides & back with severe back splits. Look at zebra-wood's shrinkage rate. It is what we call "unstable". It was not over-thinned.

There are reasons why certain woods have been used for centuries now for guitar building - availability, price, and stability. It will be interesting to see how a lot of newer hand-made guitars built with non-standard and beautiful woods that have greater shrinkage rates survive half a century down the line.

Another great resource for reference is "The Wood Database": http://www.wood-database.com