Please scroll down for part 1, 2 and 3
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About Wood Drying a.k.a. Dehumidifying Part Four
In this part, I will give more attention to two enduring properties of wood that directly or indirectly affect the playability and tonal characteristics of our beloved guitars, i.e. Wood Moisture Content (WMC) and Equilibrium Moisture Content (EMC). Much of the literature is taken from these websites listed below. The content at these websites are great read for guitar players who wonder why their guitars suffered from mood-swings like … I urge those who are keen to visit the listed websites.
In making sense of Relative Humidity, Wood Moisture Content and Guitars, one ought to understand properties like fiber saturation point, WMC, EMC, dimensional distortion of woods.
Lets’ start with learning more about fiber saturation. Adapted from one of the mentioned websites (http://www.globalwood.org/tech/tech_moisture.htm) Fiber saturation point 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.
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On the left is a magnified picture of spruce wood, 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.
Here is the important part; fiber saturation point 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 fiber saturation point.
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, and an accessible source of water is one necessity for decay fungi to start growing. Therefore, decay can generally only get started if the moisture content of the wood is above fiber saturation.
So what’s is the big deal of fiber saturation point? The deal is; wood only shrinks and swells when it changes moisture content below the point of fiber saturation. One may asked; “Where would the water goes when fiber saturation point 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. But don’t need to be confused by one more term, just remember the fiber saturation point.
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. Fiber saturation averages around 28%. Another words, the fiber saturation point 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. Here is a brief elaboration of WMC and the meanings.
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.
Below is a short excerpt from: http://www.rlcengineering.com/wmc.htm
"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."
Relative Humidity (RH) is the main driver for WMC. As discussed in previous sections, fiber saturation averages at 28%, thus it is common for WMC ranges between 0% and about 28% to be quoted for discussions. As the air's RH increases, so does the moisture content of any wood exposed to the air. Wood exposed to air with a RH of about 90% will reach a WMC of about 20%. Above 90% RH or 20% WMC, mold can grow on the wood. If you see mold growing on your guitars, you can be certain that your guitar is carrying a WMC of greater 20%. What a natural way to measure WMC, right?
Wood placed in an environment with stable temperature and relative humidity will eventually reach a moisture content that yields no vapor pressure difference between the wood and the surrounding air. When the WMC of any given wood reaches stability or equilibrium, it is called the EMC. Wood used indoors will eventually stabilize at 8-14% moisture content; outdoors at 12-18%. Big guitar manufacturers have sound knowledge and processes implemented when drying wood blanks and assembling of guitars. They are able to exercise control to the factories’ internal climate at the desired RH which will result in the desired WMC, in this case the EMC. Generally guitars made in temperate countries carry an EMC of 8.5%.
Behavior of wood before and after Fiber Saturation Point
Taken from: http://www.artisansofthevalley.com/ed/wd/ed_wd_moisturecharg1.jpg
OD = Oven Dry, KD = Typical Kiln Dried, AD = Air Dried
If you’re wondering the EMC of guitars in Singapore, 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 controller in our homes but a taboo for guitars.
Till now, we have discussed fiber saturation point, WMC, EMC and a detail revision of bound water and free water. I hope it has been good so far. Lets’ move on to wood distortions.
So why does wood warp? This tree cross-section (below picture) shows the distortion of flat, square, and round wood as affected 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 fiber saturation point; loss of moisture results in shrinkage, and gain in 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. A piece wood experiences shrinkage (dry) on its surface but it is still moist within will result in warping.
The cycle of wet and dry effects on wood in general
Distortions in sawn woods due to shrinkage and swelling
Taken from: http://www.artisansofthevalley.com/ed/wd/ed_wd_britannicawarp.jpg
In Singapore, most guitars experience swelling top, especially at the area around the bridge. Two major factors contribute to the common phenomenon. They are,
- High RH of the local climate
- Concentrated tensile stress at the bridge area
Take a spruce top as an example. It is common to see the wood grains running along the length of the guitar. The grains or the lines are area where bound water resides. The consistent high RH local climate causes the guitar top to take in moisture from the air. The moisture will move into the grains or lines, thus increasing the dimension of the guitar top. Due to the grains direction, the increase in dimension, in this case known as swelling, increases perpendicular to the grain directions. Much stresses are exerted on the guitar sides. As most guitar sides are solidly built to accommodate shrinkage and swelling, it will not barge. Therefore the increased surface area must go somewhere. You guess it right, it all moves to the middle and that happens to be the bridge area. Helped by the constant string tension pulling constantly on the bridge, the guitar top finds itself swelling inevitably. There you go; the mechanics of a swelling guitar top.
All right, that is all for part 4. Apologies, procrastination gets in the way, I am still investigating WMC and EMC of guitars stored in Singapore. Hopefully I can provide some useful data soon. Cheers!
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About Wood Drying a.k.a. Dehumidifying Part Three
For those who have read the first two parts, thanks you for the patience and support. Let me take a moment to summarize the previous parts before I move on to the next. The first part features the three basic wood drying principles adopted by most lumber industries practitioners in USA. As there are no wood drying trades in Singapore, the reference is taken from the Department of Agriculture of USA (the link: http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr118.pdf). In brief, the concepts of free water and bound water within woods are being discussed. In relation to the two types of water, the Equilibriated Moisture Content (EMC) was mentioned. However I have yet to investigate in details regarding the topic on EMC especially for guitars in Singapore ... your patience on that will be appreciated.
In part two, I have discussed about the use of desiccant agents to dehumidify guitars. Through users’ feedback, it was not conclusive that such desiccant agents are effective in terms of dehumidifying but stabilizing the RH level appeared to be plausible. In Singapore, relative humidity (RH) averages at 70%, it is hard to prevent moisture from creeping into guitars regardless we store it in the case or not. Unless the ambient conditions surrounding your guitar(s) are constantly maintained at the same RH level and temperature levels during the guitar(s) building process, you can than worry not about swelling. Below is an example of someone who is willing to go the extra mile for guitar care. Taken from a thread at Acoustic Guitar Forum (AGF).
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| Taken from: http://www.acousticguitarforum.com/forums/showthread.php?t=167815 |
A common household appliance designed to regulate RH level, i.e. the portable dehumidifiers are commonly used to maintain guitars’ EMC (see picture below). An urban myth to using portable dehumidifiers to maintain guitars at their EMC is warming of the room where the dehumidification takes place. We are also aware that such dehumidifiers are not specifically designed to treat guitars. It is merely a way out to high humidity issues in Singapore. The link below leads to a Q&A website regarding a local who uses such dehumidifiers and its woes.
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| Taken from: http://www.allergyconsumerreview.com/dehumidifiers-answer.html |
“If you are having a problem with the kitchen getting too warm, it also could be that the dehumidifier you are using is too small for the job so it is always running. You should be using a dehumidifier that is able to bring the humidity down to between 40 to 50 percent and then have an adjustable humidistat that will shut the unit off. This will allow the room not to get as warm as if the dehumidifier is constantly running.”
If you are using such portable dehumidifiers, it does pay to know its operating principles and capability. Here is a website describing the working principles in details.
http://home.howstuffworks.com/dehumidifier.htm
Portable dehumidifiers when used on guitars are based on a general principle of impinging dry air onto guitars. This is achieved by mean of placing the guitar to be treated before the dry air outlet of the portable dehumidifier, and directing or aiming the dried air at the guitar. Usually, the guitar is placed on a regular stand, with its top facing the dry air outlet directly. In between the two items, a gap of up to 2 ft is maintained to avoid structural or cosmetic damages to the guitar. Although there is no fast or hard rule regarding the duration of this dehumidifying process, the general practice is to allow the dried air to impinge on the guitar for 12 hours. Whilst dehumidifying, all windows and doors are recommended to be closed. Opening the door is discouraged. See picture below for a typical set up.
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| A guitar set up for dehumidification |
- Energy as a form of heat – portable dehumidifiers are heat exchangers by design principles. Heat is certainly provided.
- RH level of the surround air must be lower than the guitar’s one – the dried air that impinges on guitar is without a doubt dryer than the guitar’s moisture content level.
- Air circulation – as guitars are dehumidified in the open, air exchanges freely.
With all three wood drying requirements satisfied, we can be confident about the effectiveness of portable dehumidifiers. At the same time, we should be well aware of the operating requirements when using these portable dehumidifiers.
While we can be confident of the effectiveness, it is important to avoid mis-using this appliance. These are things to take note of,
- Never placed portable dehumidifier and guitar in a tight and small containment in the name of accelerating the process. This is a potential heat trap and irrecoverable damages can happen to the guitar.
- Make sure guitar is not placed too close (less than one foot) to the air outlet.
- When the guitars are in closed cases. It will take a long time to take effect.
- Leaving windows or doors opened whilst dehumidifying can undermine the effectiveness of the entire process.
- Leaving wet items like opened containers filled with fluid within the same room where the guitars and portable dehumidifiers are can decelerate or nullify the process.
In the next part, I will try to provide some EMC data of guitars that reside in Singapore for a period of time and also highlight certain innovations in this area. Stay tuned …
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About Wood Drying a.k.a. Dehumidifying Part Two
There are indeed many commercially available devices or equipment related to humidity control for guitars. Our guitars can experience a dichotomy of dry or wet, largely due to the climate fluctuation that affects the ambient temperature and humidity. These two factors are the main external drivers for moisture retention within wood.
A short reading on humidity: http://www.physicalgeography.net/fundamentals/8c.html
In the previous part, we have also discussed briefly on free water and bound water within wood that are the two internal drivers for moisture retention within wood. The interactions of both internal and external drivers result in the moisture content within our guitars. It also translates to the proportioned amount of dimensional distortions on the guitar. If guitar is too low in moisture content, shrinkage will take place that may result in cracking and warping. When moisture content is excessive, it may loosen glue joints, cause swelling to the soundbox that may result in bulging top. None of these are good for the guitar yet these physical changes are inevitable. The link below offers a brief understanding of various properties related to humidity … it has quite some equations and formula … check it out if you’re keen.
http://www.natmus.dk/cons/tp/atmcalc/ATMOCLC1.HTM#rh
Most guitar owners are constantly challenged by the arduous demands of maintaining moisture content in guitars so as to preserve their best resonance qualities as well as prolonging their life span. While setting up a air-tight room with its humidity controlled at 40 to 50% for all times will help to maintain guitars in their pristine conditions but this option not accessible to many people. As such, product developers have churned out many products that cater to different tiers of consumers. The many products created to serve a common purpose of humidity control does beg the question of which product is most effective or otherwise.
In the era of audio hi-fi, many brands produce audio equipment to enable the users with the sense of superiority and control. A classic example is a spectrum equalizer. See picture below.
Likewise, certain humidity control products in the market are not entirely transparent in terms of their effectiveness and applications. E.g. when you place a pack of desiccant materials into the guitar case, how does one determine its effectiveness? While the act of maintenance is easy to execute but evaluating the outcome is another different aspect.
In North America and Europe, guitars commonly faced dryness issues whereas in equator regions guitars absorb too much moisture, i.e. wet guitars. Since I live in Singapore, it would make sense to focus on wet guitars.
Lets’ begin with desiccant typed dehumidifying products. Some examples of desiccant typed dehumidifying products are Zorb-It, Charcoal Dehumidifiers, Thirsty Hippo, Silica Gel, etc. I believe there are more of such products than the mentioned ones. The most common advocated mode of operation is storing these desiccants products with the guitars to be dehumidified in a closed containment for a predetermined duration. There is little doubt that the desiccant products will take in moisture. In the previous part, I have mentioned about how moisture is carried by wood, i.e. contained within cell lumen and not held by any chemical bonds known as free water and the other trapped within cell walls and held by hydrogen bonding.
Theoretically, when these desiccant products are placed within guitar bodies, they are in fact absorbing the moisture from the ambient air within the closed containment thus reduction in humidity to the ambient air within is achieved. However to be absolute that desiccant products will also absorb the free water from guitars is at best speculative. Much less desiccant products will have effects on bound water that chemically held by hydrogen. It is Physics 101; to break any chemical chain or bond, energy is required, often as a form of heat. Suffice to say that these desiccant products are not known or designed to generate heat. Without the extra energy to break the bonds, the bound water remains bounded. This fact will automatically challenge the claims of such products that they can obtain dehumidification effects on guitar when applied.
Does it mean that such products are irrelevant to guitars? I reckon not because such products are likely more effective in maintaining the humidity level of the ambient air within the closed containment. That means guitar stored with such desiccant products should retain its resonance qualities for longer period as compared to guitars leave in the open, in theory that is.
So how can we ascertain if the products are effective? There are many ways to skin a cat so let me proposed the compare and contrast method. First, assuming your guitar is properly dehumidified. Record its string relief at 12th fret (see picture below).
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| Measuring String Relief |
Thereafter leave the guitar in the open for three days or more but not direct exposure to sunlight. Take measurement of the string relief at 12th fret. If the guitar has absorbed excessive moisture, you should expect an increase in string relief. Next activate the desiccant product you have to treat the guitar according to its instructions. At the end of the treatment, take measurement of the string height at 12th fret. If it works, you should expect a decrease but if it doesn’t, quickly contact a friend who owns a dehumidifier and restore your guitar or drop me an email and I will try to assist. Personally I have tried it and I have observed very varied results. Disclaimer; it is entirely your free will to try it and I shall not be liable for any mishaps or damage to your guitars.
Next part, I will discuss other dehumidifying methods and their effectiveness. Thanks for reading!
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About Wood Drying a.k.a. Dehumidifying Part One
After reading in this area, also relating the various dehumidifying methods we used commonly, I will share my thoughts on this topic. Hope you find it beneficial reading it.
Three basic requirements for wood drying or drying wood ...
(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 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.
How wet a piece of wood 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 EMC of 8.5%.
Would the guitar (wood) continue to take in moisture until it drips or drools?
Unlikely to happen unless the guitar is placed in a shower room filled with steam from the hot bathing water, you will probably see droplets on the guitar. But you will not have a case of water oozing out from the guitar (wood).
This means an environment with RH level from 40 to 50% can never evaporate water molecules into vapor beyond what it is capable of... eventually the wood stops taking in anymore from the atmosphere ... therefore the MC of the guitar reaches equilibrium. Many guitar-makers use the term 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 properties of top plate stiffness instead of thickness.
In Singapore, what is the EMC(s) of our guitars? I am still investigating ... will share this information once I have determined it.
Water within woods; as wood is hygroscopic, it is natural for wood to absorb moisture or water. But the reverse will prove to be difficult, isn't it? I am sure some of you can understand ...
Within any piece of wood, there are two classes of water or moisture. One of it, 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, 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 evaporate fast, back to the atmosphere but the bound water held within the cell walls require extra energy to evaporate.
That is all for part one ... In the next part, I will try to use the three basic wood drying requirements to evaluate the various dehumidifying methods we have been using ...
