Wood Drying - Part Four
The science of wood drying. We will discuss the importance of Fiber Saturation Point (FSP) in woods.
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In this part, I will give more attention to two enduring properties of wood, Wood Moisture Content (WMC) and Equilibrium Moisture Content (EMC). They affect wood directly and indirectly, and eventually our beloved guitars. I have referred literature from the websites listed below. The content in these websites are great read for guitar players who wonder why our guitars suffered from mood-swings.
To make sense of Relative Humidity (RH), Wood Moisture Content and Guitars, one ought to understand properties like Fiber Saturation Point (FSP), WMC, EMC, dimensional distortion of woods.
Fiber Saturation Point
Lets’ start with learning more about fiber saturation. FSP is an important benchmark for both shrinkage and for decay. Measured in percentage (%). The fibers of wood (the cells that run the length of the tree) are shaped like tapered drinking straws (a.k.a. Xylem). When fibers absorb water, it first is held in the cell walls themselves. When the cell walls are full, any additional water absorbed by the wood will now go to fill up the cavities of these tubular cells.
A a magnified picture of spruce wood (second-left), clearly showed the hollow and elongated fibers or xylems. It is easy to imagine how tree branches and leaves get their water, right? Not all xylem looks just like this, and this is a picture of softwood xylem. Hardwoods’ fibers arrangement tends to be more sporadic.
Picture is taken from:
Here is the important part; FSP is the level of moisture content where the cell walls are holding as much water as they can. Got that? The total volume within the fibers is finite; therefore it can get filled up. We called the percentage of moisture within these fibers or cell walls at the maximum of any given wood can accommodate the FSP.
Still remember bound and free water? Water held in the cell walls is called bound water, while water in the cell cavities is called free water. As the name implies, the free water is relatively accessible. The growth of decay fungi will flourish with such water source. Therefore, decay can generally only commenced if the WMC is above FSP.
What is important about FSP? The deal is; wood only shrinks and swells when moisture content changes below the FSP. One may asked; “Where would the water go when FSP is reached?” My understanding is water will first fill up the fibers or cell walls to it maximum capacity (max bound water). The wood swells as a result. The excess water (free water) will be trapped in cell cavities but it has no effect on the wood dimensions. Due to this behavior, the fiber saturation point is also referred as the limit for wood shrinkage. There is no need to be confused by one more terminology. Just remember FSP.
19% and 28%
Two important numbers to remember are 19% and 28%. We tend to call a piece of wood dry if it is at 19% or less moisture content. FSP averages around 28%. Another words, FSP is also a measure of WMC. Wood Moisture Content (WMC) is the weight of water in a piece of wood expressed as a percentage of oven dry weight of wood.
Fresh cut trees can have a WMC over 200%, while completely dried wood will have a WMC of 0%. Wood in buildings usually has a WMC of 5% to 15%. That is where our guitars are stored, right? Anyone owns an outdoor guitar? These percentages can mean nothing to us if we don’t know the context that they are being defined.
The context of WMC
"Wood destined to become lumber is deemed as "green" when the tree is first felled. A freshly felled tree may have a WMC anywhere in the range 30% to over 200%, depending on the species. That is very wet and heavy because the amount of water within the wood adds to the eventual weight." Taken from:
Below 12% - Readings in this range are common to kiln or oven dried woods and furniture grades of wood, and represent dry conditions. Most interior wood is in this range.
12% - 16% - Readings in this range are common to lumber during construction, air dried lumber and "healthy" residential substructures (beneath first floor in crawl spaces). These are typical readings for exterior wood.
16% - 20% - Readings in this range indicate a possible elevated level of wood moisture. Such readings should alert the homeowner to look for a source of excess moisture. The excess moisture source should be corrected if found.
20% - 28% - Readings in this range indicate that conditions are border-line for decay. Surface molds may develop. The excess moisture source should be corrected immediately, and monitored until the WMC returns to the 12-16 range.
28% and above - Readings in this range are often accompanied by decay damage. Substructures with WMC in this range may show decay or rot in floor joist, sills, and subflooring. Repair is often required when WMC readings are in this range.
How is RH connected to WMC?
Fluctuations in WMC is largely due to changes in RH. If RH has caused WMC to drop below 28% a.k.a. FSP, there will be physical changes to wood. In essence, WMC can be predicted by RH. Wood exposed to air with a RH of about 90% will reach a WMC of about 20%. Beyond 90% RH or 20% WMC, mold can grow on the wood. If you see mold growing on your guitars, you should know what that means, right?
Equilibrium Moisture Content (EMC)
Woods in a controlled environment with constant temperature and relative humidity will eventually reach moisture content that yields no vapor pressure differences between the wood and the surrounding air. Wood has equalized with its environment and it is known as EMC. Indoor woods stabilizes at 8-14% moisture content; outdoors at 12-18%. Big guitar manufacturers are able to implement controls during drying wood blanks and assembling of guitars such that the factories’ internal climate is at the desired RH which will result in the desired WMC, in this case EMC. Generally guitars made in temperate countries carry an EMC of 8.5%.
If you’re wondering the EMC of guitars in high humidity regions, yes I think many will share this curiosity. Do note that even a piece of totally dried wood still exhibits the residual effects of Hygroscopicity. It means the wood releases moisture when it surrounding is dry and do the reverse when the surrounding is moist (high RH). It isn't necessarily a bad thing - this allows wood to function as a natural humidity detector in our homes but a taboo for guitars.
Till now, we have discussed FSP, WMC, EMC and a detail revision of bound water and free water. Lets’ move on to wood distortions.
So why does wood warp? A tree cross-section (fourth-left picture) illustrated different types of warping. The variety is accounted by the direction of the annual growth rings. The clear space surrounding each wood section is the shrinkage in drying from green to oven dry condition. Wood undergoes dimensional changes when its moisture fluctuates below the FSP; loss of moisture results in shrinkage, and gain in moisture causes swelling. It is characteristic that these dimensional changes are anisotropic, another words different in axial, radial, and tangential directions. These differences are great potential for uneven tensile or compression stresses across any conceivable axis and directions. The result is distortion or warping. A piece wood experiences shrinkage (dry) on its surface but it is still moist within will result in warping.
What gives "WET" guitar problems
In Singapore or countries of similar climate characteristics, most guitars experience bulged top, especially at the area around the bridge which is excerbated by these two factors,
1. High RH of the local climate
2. Concentrated tensile stress at the bridge area
Take a spruce top as an example. The straight grains run from one end to another. The grains or the lines are area where bound water resides. In regions of high humidity climate, the guitar top to take in moisture from the air. Take a spruce top guitar. Moisture will move into its grains or lines, thus increasing its size.
Wood grains for guitar top wood are oriented to run with the guitar length. However top wood swelling goes perpendicular to the grain directions. It is easy to imagine how much stress is exerted on the sides. As most guitar sides are solidly built to maintain the guitar in shape, the swelling must go somewhere. Eventually it all moves to the middle and that happens to be the bridge area. Exacerbated by the string pull, the guitar top swells at the bridge area inevitably. That's sum up the mechanics of a swelling guitar top.
This isn't the final part because there is so much to be learned about wood and its drying process. When I have gathered more findings, I will publish another part.