Much has been made out of how wood is dried, its moisture content, and the impact that has on the sound it makes, especially when that wood is used in a soundboard. Different sellers tout the advantages of various methods, so what's a guzheng buyer to do? Read this page, of course!
There are many methods to season and dry wood but only three seem relevant to guzheng.
Air drying: Slow ( on the order of years), can reduce moisture content to about 10%, doesn't use much energy. The wood's internal forces are well balanced leading to better acoustic properties. Prices tend to be higher.
Kiln drying: Fast (months or less), can reduce moisture content below 10%, uses more energy. The wood's internal forces can be balanced but flaws can be hidden. Too high of a heat or too short of a time can degrade the wood’s musical properties. Prices tend to be mid-range.
Flame Drying: Fast (minutes), can get moisture content low, uses less energy. The wood's internal forces are difficult to balance leading to less appealing acoustics. Prices tend to be low. Note: This might be a form of aggressive heat treatment rather than a full drying process. The rest of this page is written as if it is a drying process but further information is welcome.
Seasoning Wood, Details
The first thing to know is that drying method alone will not tell you if the wood will produce a good sound. How the tree grew, what it experienced in life, how it was cut, how the wood was handled in storage, the particular quirks of that particular plank, how it was worked to become the instrument all impact the final product. You need a skilled instrument maker applying their skills if you want the highest quality instruments.
If two pieces or wood are of different quality then comparing drying styles can only alert you to a problem, it can't guarantee the instrument is what you want. The safest way to evaluate an instrument is to play it and the safest way to evaluate an instrument maker is by their work.
Now let's talk science.
Why Wood is Seasoned
Trees contain water. Lumber starts with that water evaporates over time. As the water departs the wood shrinks. The wood becomes stronger and its sound qualities improve. The last two items are great reasons to dry it but the largest concern is the first. If an instrument is constructed from wet wood it will pull apart its joints or crack as it dries. So, wood is dried before it is worked so that it won't shrink later.
Here's the thing though: wood continues to gain and lose moisture even after the instrument is made. That is why musicians are instructed to store their instruments in humidified environments. A properly dried instrument could still crack if it is kept in an even drier environment. The term for how much water is in a piece of wood at any given moment is called "Moisture Content" (MC).
The amount of water in wood is described as Moisture Content and is given as a percentage. Moisture content is determined in various ways too detailed to get into here, but our frenemy Wikipedia has a decent article on the topic. Woods used in western musical instruments are typically dried to between 8% and 12% moisture content, though it's a broad range and individual makers have their own preferences for the different woods they use. They do this by leaving the wood in dry and/or hot environments.
The thing is, you don't just dry a wood to a specific Moisture Content and then magically lock it in place. During storage, crafting, and then in its life as an instrument, the wood will increase or decrease in moisture content by gaining water from or losing it to the air. The measure of water in the air is called "Relative Humidity" (RH). The wood will vary its moisture content until various chemical/physical aspects are balanced. That is called reaching equilibrium, so the moisture content at this point is called "Equilibrium Moisture Content" (EMC).
Let's look at a graph:
This graph shows that wood will (generally) seek an Equilibrium Moisture Content of 12% (left side) when the Relative Humidity of the air is at 65% (bottom). If you dried an instrument to a lower MC and then put it in that RH 65% air the wood will absorb moisture and swell. If you had only dried it to, say, 18% MC, then the wood would decrease to 12% EMC and shrink, possibly cracking. So on top of everything else in their demanding profession, instrument makers have to remove enough moisture so that the wood won't crack in everyday environments, AND they have to construct their instruments in such a way that a reasonable amount of swelling won't hurt it.
If we look at where guzheng are made we can approximate what humidity levels might be a problem for a guzheng. Based on this map from UNESCO's IKCEST and the knowledge that most guzheng are made east and south of Xi'an, we can see average relative humidities sit at over 60%(1). So! Assuming the instrument can stand the wood swelling, drying and manufacturing the instrument in conditions that yield less than 10% Moisture Content would largely prevent later cracking as it finds its equilibrium with the air - at least in China.
Bring an instrument that's perfectly fine in 60% RH air over to the Northern North America or Europe in the winter and you'll have a problem. A home with uncontrolled humidity might hit multiple days of 20% Relative Humidity or less. Allowing for some changes to the above graph if the temperature was different, you'd still need to have created the instrument at 4 or 5% Moisture Content for it to avoid cracking in such dry air. Since instruments aren't dried to that extent musicians of all sorts keep their instruments in humidified rooms and cases to mitigate shrinkage.
Now that the science it dealt with we have three things left to discuss. Why don't instrument makers dry all of their wood as low a moisture content as possible? What are the different methods of drying lumber? What are their advantages and trade-offs?
Why not just make instruments out of 0% Moisture Content wood?
Instrument makers don't dry woods to super-low percentages because: 1) It's nearly impossible to do so if they are just air drying it, 2) it takes more time and energy if you are using a kiln, oven, or other heating method, and 3) the end result will upset their customers. The above graph shows we'd need a Relative Humidity of <15% to create a stable environment for <5% MC wood. Not even deserts stay that dry!! Instrument makers would have to work on their instruments in airlocks while wearing bodysuits if they wanted that dry an environment. What's worse, the moment the airlock was opened the instrument would start taking on water like nobody's business. It would swell and detune, rendering your instrument unplayable. Don't believe me? Let's take a look at another graph:
Changes in Resonances of Wood Across Different Moisture Contents
This beautiful array of colorful lines is from an experiment where spruce planks were dried to moisture contents ranging from 0 to 25%, and then tested to see how they responded to different vibrational frequencies. What this shows is that lower moisture contents raised the pitch the wood vibrated at while higher moisture contents lowered the pitch. There is a lot of fine discussion to be had but the main idea is this: an instrument made to sound a certain way when the wood is super dry would change drastically once it absorbed moisture from the air.
Let's say we have an instrument maker who built an instrument at the impossible 0% moisture content. That poor individual crafts an exquisite instrument in a perfecly dry airlock then sells it to someone who lives in the real world. The instrument starts absorbing water the moment it leaves the airlock, lowering the frequency it vibrates at. All of the careful tuning done by the instrument maker disappears- and it gets worse. As the authors of the study I took this chart from said, "...under extremes of humidity, the... pressure applied to the material can become large enough that permanent damage results... [by] causing the [wood's] cell structure to become permanently crushed, a phenomenon known as compression set. Wood which has suffered compression set will thereafter be even more susceptible to splitting under dry conditions..." (p. 29, Martínez 2011)
As wood absorbs water its cells expand. This would cause joined wood pieces to exert so much pressure on each other they would damage the very structure of the material, degrading its acoustic properties while making future cracking more likely.
Super low moisture contents might sound like a good idea but they are actually disastrous. (2)
So how are guzheng soundboards dried?
There are three methods I've heard mentioned in conjunction with guzheng soundboards: Air Drying, Kiln Drying, and Flame Drying(3).
Air Drying is leaving the wood open to the air for months or even years. The moisture leaves gradually, the cells of the wood have time to readjust slowly, and if monitored correctly, the wood does not develop any internal stresses that might cause problems later. Proper procedures still need to be followed, such as capping the ends so they don't dry unevenly across the length and warp, but the techniques are well understood and generally straightforward to follow.
Kiln Dying (or oven drying) is placing the wood in a heated environment for a short period of time. The temperature and relative humidity of the kiln can be controlled so as to slow or speed up the rate at which the wood dries. Higher temperatures and lower humidity will make the wood dry faster, but if dried too quickly the wood will warp, crack, or weaken its internal structure from the buildup of internal pressures. While it takes more energy than air drying, it also speeds up the drying considerably and is the well-established alternative tot eh time-consuming, if high-quality approach of air drying.
Flame Drying is a method in which a flame is blown over the surface of the wood. It creates incredibly short drying times but induces a great deal of stress in the wood, causing all sorts of changes in the sound it produces. This is inescapable; in order to force the water out in a very short period of time, a high heat must be applied directly to the surface. By the time the internal water has escaped the external surface will be charred.
There are other techniques or modifications used by the lumber industry at large though I don't know if any are involved in prepping wood for guzheng production. They are:
Forced Air Drying is where air is actively circulated around the wood with fans.
Dehumidification Drying is where the air is dried without heat.
Vacuum Drying is where the air pressure is decreased considerably, decreasing the boiling point of water, which turns it into a vapor and allows it to escape faster, also without heat
Microwave Drying is where the wood is irradiated with microwaves to excite the water into a vapor so that it escapes quickly.
What are the advantages and trade-offs of different Drying methods?
Air drying: Slow (years), can only get moisture content to about 10%, doesn't use much energy. The wood's internal forces are well balanced if handled properly, leading to better acoustic properties. Prices tend to be higher.
Kiln drying: Fast (months or less), can get moisture content very low, uses more energy. The wood's internal forces can be be balanced, but variations in the wood, too high of a heat, or too low of a time can degrade the quality of the wood and thus, its musical properties. Prices tend to be mid-range.
Flame Drying: Fast (minutes), can get moisture content low, uses less energy. The wood's internal forces will be unbalanced and its structure very likely damaged, severely degrading its acoustic properties and lifespan. Prices tend to be low. I have yet to find a positive effect of flame drying.
(1) I'm being conservative here because while outdoor relative humidity might stay high during cold months, the heated air inside of homes will be far drier. While we can see that most of China's population lives in areas with an average Relative Humidity above 70%, the above map says nothing about what indoor RH will drop to in colder months.
(2) For those numerically interested, the folks at an instrument shop ran the numbers on how much wood can shrink on a violin and came up with 1mm of shrinkage when the instrument went from normal summer to normal winter indoor humidity. You can go through their calculations here: https://trianglestrings.com/humidityandwood/ The various constants and rates of wood shrinkage would be different in the wood species used for the zheng, but find those and you can calculate that shrinkage yourself!
If you want to learn more about the science behind the math, this website has a reasonable explanation of the effects of grain patterns and the like: http://www.thisiscarpentry.com/2010/09/03/moisture-content-wood-movement/. Keep those instruments humidified!
Overview of wood drying methods and considerations: "Principles and Practices of Drying Lumber" by Lignomat (wood drying product company) and Eugene M. Wengert, from Virginia Polytechnic Institute and State University.
A whole lot of background on drying, including kiln operation: The United States of America's Forest Products Laboratory publication database.
Flame-drying information from Carol Chang at ChineseZither.net. May refer to a heat treatment rather than a drying process.