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Wood Drying - Part One
The 3 wood drying principles explained
It has been Guitaring Passionately's obsession to solve the "WET" acoustic guitars issue. We have taken our first step in compehending the science of wood drying.
Taken from a paper produced by the USA's department of agriculture, we have discovered the three key principles of wood drying. They are,
(1) Energy in the form of heat must be provided to evaporate moisture throughout the drying process,
(2) The air surrounding the wood must be capable of receiving moisture from the wood surface, i.e. the relative humidity (RH) of the surrounding air must be below 100% and
(3) During drying, air movement among and around the wood to be dried must be adequate to bring energy into it, to remove evaporated moisture, and to maintain the desired relative humidity.
To learn more of the three principles, click HERE.
About Wood Moisture Content (WMC)
How "WET" is a piece of wood? It is determined by its moisture content. You may have come across the term MC, which refers to moisture content. For wood, it is in percentage. It is very common for guitars made in North America, in an environment with RH level between 40 to 50% to carry an WMC of 8.5%.
Will guitars (wood) continue to take in moisture until they drool? While wood is hygroscopic, it will contain its moisture within itself. There will not be a case of water oozing out from the guitar (wood). This means an environment with RH level from 40 to 50% can never provide water molecules in vapor beyond what it is capable of... eventually the wood parts of a guitar stops taking in moisture from the atmosphere ... therefore MC of the guitar reaches equilibrium.
Many guitar-makers use EMC, a.k.a. equilibrated moisture content... 8.5% for guitars from North America. This MC level seems to allow the wood to be responsive, at least according to the guitar-makers who understand the principles of top plate stiffness instead of thickness.
In regions with high humidity, what could be the EMC in our guitars? Suffice to say that it would be proportional to the environment. As wood is hygroscopic, it is natural for wood to absorb moisture or water. But the reverse isn't.
How does wood keep its moisture within?
In wood, there are two classes of water or moisture. One is found in the cell lumen, not held together by chemical bonds within the cell, called the free water. The other one is found within cell walls, which is held by hydrogen bonding, called the bound water. I think you can already infer which one can be removed easier. The free water not held, can be evaporated but the bound water held within the cell walls require extra energy to purge it.