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Wood for the Neck
On a classical guitar the traditional choices of wood for the neck are mahogany and Spanish cedar. Both species are very stable when quarter sawn. The neck on a classical guitar must be stable and strong without being heavy. Since the classical guitar is generating sound without a great deal of string tension as compared to a steel string guitar, it is desirable to keep the instrument light. Other woods have been used for necks on classical guitars but in my opinion they are less than optimal. This is one of those traditions that became a tradition for a good reason - it works. On steel string guitars I have seen walnut, maple, and even nara used with success. On a steel string guitar weight is not such an issue - but strength is everything as the tension of the strings is far greater than that of a classical guitar.
Choosing Wood for a Neck
Mahogany is more dense than Spanish cedar and generally a bit darker in color as seen on the right, but both carve equally well. The difference in density will have an effect on the sound of the instrument. I find that mahogany because of its weight generally produces an instrument with deeper basses. Spanish cedar on the other hand has more of an effect on the trebles and help make them very clear.
One of the most important features of the neck is stability. The neck should not warp or twist as it ages. The way to check for these tendencies is to mill the neck blank taking some wood off on both sides, but leaving it thicker than you need. Then let it sit for a few days. Come back and put a straight edge on it to see if there is any bowing or twisting. If there is a bow of more than 1/32 nd of an inch I would consider finding another piece for the neck blank. And if it has twisted at all I would not use it for the neck blank. As long as the changes have not been too severe you can use this piece for the heel stack since those pieces will only be about 5 inches long and glued on top of each other so movement is unlikely.
The wood for the neck blank should also be strong. To assure the neck is strong you need to select a piece that has minimal run out. Run out can be seen in the side grain of the neck blank. The grain lines should run parallel to the surface. Perfectly parallel grain lines are difficult to find so it is likely that the lines will angle toward the edges. If this is so you want to make sure that if the grain lines run from one face to the other, they do it over a distance of 5 inches or more. If the side grain runs from top to bottom in a short distance, say an inch or two, this is what is known as short grain and is weak to shear force here. Any piece with short grain should not be used for a neck because with a neck its all about strength. The strings will generate a good amount of shear force, therefore the neck wood should have as little run out as possible.
Wood for Fingerboards
Guitar fingerboards are usually made of ebony or rosewood. Higher end guitars tend to use ebony exclusively. Ebony is very dense wood and has excellent wear resistance. Since the fingers are constantly being hammered on and with fingers sliding around on them, ebony seems to be an excellent wood choice. The type of ebony used on fingerboards is Gaboon ebony which is very dense and will hold up to this abuse very well over time. It has been used on all types of stringed instruments for hundreds of years.
Good ebony is becoming harder and harder to find. Jet black quarter sawn pieces of ebony for fingerboards are rapidly becoming a thing of the past. Today if you can find a good quarter sawn piece you more than likely will have to settle for some streaks. If forced to choose I will always choose visual flaws over inferior cuts of wood. Some types of rosewood will also work for fingerboards as well as Macassar ebony but this wood can also be difficult to find. If given a choice of a bad piece of ebony and a really good piece of an alternative wood such as a rosewood or Macassar ebony for a fingerboard I will choose the good piece of alternative wood every time.