Kiln-dried or Air-dried Tonewood. What's the difference?

If you’ve seriously shopped for a premium instrument, you have most likely heard the term tonewood applied to the materials coveted by mandolin builders (and even some musicians). It generally applies to the woods used to build musical instruments, usually those with strings such as violins, cellos, mandolins and guitars. The inherent effect of different wood species upon the tonality of an instrument is a never-ending discussion, but little is heard about how wood is prepared.

The idea of old wood has currency among builders, but unless properly seasoned, a board’s age is a worthless statistic. Labels like “air dried” and “century old” are suspect because these designations by themselves mean nothing. As we start to examine the actual science behind how wood gains and loses moisture, the fallacy of these sorts of terms becomes clear. The correct seasoning of wood that’s used for mandolins is important, but exactly why is a bit opaque as viewed through the lens of marketing.

Why We Dry

Builders of all things wooden must be concerned with moisture content for many reasons. Most of the considerations are structural, including the ability of glues or final finishes to bond properly. Wood shrinks in size as it dries, so it is important for pieces to be stabilized before fitting them together in a final form. Unlike a table or door, extremely small changes in dimensions can cause playability problems for a guitar. For all these reasons, it is important to bring instrument wood into a state that will be at rest in “normal” operating situations and environments.

Appropriate Moisture Content

To begin with, wood in its natural state is fairly saturated with moisture in what is referred to as sap. It’s the easy flow of this moisture through the tree that nourishes its extremities, much like our own body’s circulation system. After a tree is felled, it no longer draws moisture through a root system, but can still absorb water from its surroundings. Even after trees are cut into boards, the wood is at the mercy of its environment. Moisture in boards is found in the tube-like rays and vessels that supply food to the wood cells, as well as inside the cells themselves. Transfer of waterborne food to the cells is accomplished via small gateways called pits, which act as valves. The notable thing here is that this system can work in either direction—gaining or losing moisture in an effort to reach what is called equilibrium. This fact is important because it means that a board stored for decades in a humid environment will not lose enough moisture to be deemed usable for most furniture or musical instrument purposes.

On a freshly cut tree the moisture content can be anywhere from 30%-45%, this is called green wood and some chair makers use this wood for their projects, but for mandolin makers, the moisture content is too high to build anything from it, the lumber has to be dried before it is of any use.  The ideal moisture content for furniture making is around 8-9%, some say 7 and some say 6, so let’s just say anywhere from 7-9% is ideal.

How We Dry

Most commercially available lumber is dried to between 8 to 12 percent moisture content, measured by weight in what is called the “dry basis.” The most common method of achieving this is by using a drying oven, or kiln. It’s not just a matter of putting wood into a hot room and waiting because there are a myriad of variables to be aware of. Green (new) wood can vary greatly in moisture content, ranging anywhere from 30 percent to as much as 200 percent as a ratio.

Bound and Free Water 

In the living tree, the wood cells, which are like skinny soda straws, are about 3 to 5 mm long; the diameter is 1/100 of their length. The center of the cell is hollow. Water that is in the hollow space is called free water. It could be removed by blowing it out of the cell. (Actually, we call the liquid "water," but the water contains many other chemicals--just ask anyone who likes maple syrup on their pancakes what the water in wood tastes like!) The cell wall itself (which is actually 1.5 times heavier than water, so wood actually doesn't float--the air in the hollow spaces makes wood float) also can absorb water. The wall can absorb up to 30% of its weight in water. This absorbed water is called bound water, as it is held in the cell wall by hydrogen bonding.

Moisture is divided into two different categories: free water, which is found in the rays and vessels, and bound water, which is held in the cells themselves. Also, the material nearest the outside dries more quickly than the center of the board. The resulting vacuum instigates a capillary action that draws the internal moisture towards the surface. This aspect takes more time and requires more heat. But forcing any of these issues with too much heat too soon can destroy the wood with cracks, or leave the center wet—a structural time-bomb. Some experts insist that fracturing the cells or cooking the resin left behind by overenthusiastic drying can also change the resonance of the wood. Whether or not this is true, the structural reasons are enough to warrant tailoring the drying cycle to each individual load.

When drying wood, the free water leaves first. It requires less energy to evaporate than the bound water. At about 30% moisture content, all the free water is gone and just bound water remains.

Air-drying Tonewood

Air drying is a lengthy process usually each board takes 1 inch (25mm) per one year to dry, the lumber is stacked off the ground 15-24 inch(400-600mm) on stickers which are placed across the boards and inline to each below, and above in between the boards to avoid sagging to gain even weight distribution.  The stickers serve to allow air flow between each board.

A canopy or preferably plywood with cinder blocks or other heavy items should be placed on top.  The canopy or plywood is there to protect the timber from weather elements, plywood is preferred over the canopy because it allows air flow and protection from the sun.  If you use a canopy you should remove it when it’s not raining.  Once the boards have reached equilibrium with its environment which is usually about 15% outside, it needs to be brought into the shop to acclimate, and continue to dry until they reach the 7-9% moisture content.  Depending on your own environment this can take from two weeks up to a month or more.

Air drying needs to be in an open space not surrounded by trees or other plant life, the ground shouldn't be damp either.  If any of these features are present, then the lumber will continually absorb the moisture content from its surroundings and not dry properly and mold may start to form, mold is notorious in pine, spruce and other woods.  These stacked timbers will continually absorb large amounts of moisture from the surrounding trees and damp ground.

Once the timber has reached equilibrium with its surroundings, which may well be above 15% and then brought into the shop, mandolin makers will have to wait longer than usual for the material to dry thoroughly or acclimate before it’s ready for use.  It pays to have a high quality moisture meter, a Wagner is a good choice, available at Stewart MacDonald and other good luthier supply shops.

NOTE: Air dried lumber is a hand tool woodworkers best friend, it’s easy to work and is more stable than kiln dried lumber because it’s not forced dried through high temperatures, but rather a natural slower process.  As the timber air dries the cells collapse, slowly causing them to compress and stay put, so when air dried lumber absorbs moisture, it doesn’t swell as much hence it becomes more stable.  Luthiers find kiln dried wood more physically demanding to work with hand tools.

Drying at warm temperatures or hotter has a definite effect on wood properties. Therefore, air drying for a year and then continued drying in a home or office to achieve the correct final moisture content is essential. Air-drying alone will reach only 12% in most of North America; 7% is the typical final moisture content required for interior uses. We also know that rain on the lumber enhances certain other properties. For example, white oak lumber is normally quite acidic in character. But if you air dry it for 2 years, there is a vanilla odor that becomes very obvious and enhances the flavor of wine and whiskey in barrels made of such material! In short, air drying cannot be replaced for musical instruments--even low temperature systems do not do as good a job.

Kiln Dried 

Kiln dried is a forced but controlled process where humidity and temperature is controlled using steam and fans for drying.  The drying process normally takes between 6-8 weeks.  Because of its fast drying due to high temperatures, the cells collapse quickly rather than slowly as it would with air drying, making it unstable.  Because of this when moisture is absorbed, the cells expand rapidly filling up with water quickly than it would with air dried lumber.  However, the positives with Kiln drying due to high temperatures, any laid eggs and bugs are killed off. There is treatment for mold and insects at an extra cost.

If hot dry air is used, then the surface dries too rapidly and develops case hardening, ‘checks’, so kiln drying requires careful control of both air and temperature.  The idea is to prevent stagnant layer of excessively humid air from lingering around the timber, as in the case of air drying, the air is frequently renewed which prevents this from happening.

I've been to mandolin factories in China where they use the kiln-dried process, loading pallets of billets into rooms with a furnace, closing the doors and letting it cook for a few days.  Not good.

Beyond the Dry

Henan Junyu Export & Import Trading Co., Ltd.
Since wood is porous, even if a builder buys wood that is carefully and correctly dried, it must still be kept in a controlled environment or the moisture content will drift. This applies to the mandolin in your home, too. Maintaining a relative humidity of around 35 to 40% will keep your mandolin close to the environment that it was (hopefully) built in. It is important to note that some of the drying parameters of commercial lumberyards do not include considerations of cell damage that may affect resonance despite being structurally acceptable for furniture applications. In those cases, structurally sound has nothing to do with the sound it makes!

In the end, it comes down to having a mandolin that functions properly and won’t shift too much during use. Beyond that, it has to do with resonance. I'll cover that in the next blog post.

DISCLAIMER: I'm not an expert.  I'm just a mandolinist and mandolin teacher, but I have been to the mandolin factories in China, Vietnam and South Korea as well as Weber in Montana and other shops in North America.  I've had many long conversations with luthiers about the tonewood they use and why.  I've been to tonewood suppliers and lumber mills in Kunming, China as well as Oregon, Washington, Vermont and Canada.  Most of the mandolin factories in China buy their tonewood from lumber mills in Kunming.  Much of that wood comes from Burma (Myanmar).  The wood is green when it arrives at the factory and must be dried before using.  I can list on the fingers of one hand the number of musical instrument factories that air-dry their billets for more than 3 years (only 1 that I know of).  All of the others use some sort of kiln-dried method.