...dryness implies having less water and that oil makes it harder for water to get into the wood.
It also makes it harder for water to get
out of the wood.
There is moisture in the air, and there is moisture in the fingerboard. The wood will either
gain moisture or
lose moisture from the atmosphere, depending on the relative humidity and the moisture content of the wood (osmosis- the moisture will move from the greater concentration to the lesser concentration until equilibrium is reached).
In the summer, when the relative atmospheric humidity is as high as 90%, your fingerboard will gradually absorb moisture from the atmosphere until the moisture content of the wood reaches 90% (i.e. equilibrium). Above 65% or 70% moisture content, the wood fibers will swell, and the expansion differential between fingerboard and neck woods can cause the fingerboard to lose it’s relief and create back-bow issues. It can cause fret buzz. It can also cause intonation issues. In some cases, it can even cause glue joint failures.
In the winter, when indoor humidity can be less than 10%, literally drier than the Mojave Desert, the opposite occurs. Moisture will move
out of the fingerboard and into the atmosphere until the moisture content of the wood matches the relative humidity of the room, in this case, 10%.
Below 30% moisture content, the wood will begin to shrink, the fret ends will protrude*, causing sharp fret ends, the fingerboard could develop cracks, the neck can bow, and again, in extreme cases, it could create de-lamination issues.
When the wood is expanding and contracting due to moisture fluctuation, it can cause the neck to warp, twist, or crack. Ebony, in particular, is prone to cracking due to moisture fluctuation.
The purpose of fingerboard oil is to create a semi-permeable moisture barrier. It won't prevent osmosis from occurring, but it will slow it down considerably.
The oil will also displace some of the moisture in the wood. Oil will still evaporate, but it evaporates at a much, much slower rate than water (months rather than hours).
All these rants about how oiling a fingerboard is somehow wrong, or that anyone who thinks they need to put oil on their fingerboard is somehow stupid, are coming from an incomplete understanding of the function of oiling a fingerboard, at best. They typically argue that oiling a fingerboard does not moisturize the fingerboard, since moisture is water, and oil is not water.
This is a straw-man argument, a logical fallacy. It's based on a false premise. The oil is not to "add moisture" to the fingerboard. Adding moisture to a fingerboard will do more harm than good in most cases. It's the
opposite of what you want. Oil is used to
slow the transfer of moisture, both
to and from the fingerboard.
So, is it necessary to oil an unfinished fingerboard? It depends on the circumstances.
When I was a musician, I was traveling with my instruments most of the time. They were exposed to frequent temperature fluctuations, humidity fluctuations, and barometric pressure fluctuations (especially when flying).
It was pretty important to keep the fingerboards oiled.
I am no longer a musician. I keep all of my instruments in a temperature and humidity controlled environment. Since fluctuations in temperature and humidity are not an issue, I don't bother oiling any of my fingerboards. They don't need it.
...They probably also put oil or wax on their lips when they are dry during the winter. Just one of those things where everyone understands what you mean, but they want an argument.
Exact same principle. A semi-permeable barrier to slow osmosis. In the case of a fingerboard, an impermeable barrier can be used (such as nitrocellulose lacquer on a maple board), which renders oil unnecessary. An impermeable barrier would be less than optimal on skin (witness the ignominious death of Jill Masterson).
* That's right. All these people complaining about sharp fret ends are complaining about a
storage issue, i.e. relative humidity and the moisture content of the wood, and
not a manufacturing issue. The instrument was not made with sharp fret ends. The sharp ends protruded because the fretboard shrank due to moisture loss.
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