Air-dried vs. Kiln-dried Wood
Are they different to saw, chisel and plane? Does it matter?
Note: This is the second article in our series where we do deep research into thorny woodworking topics. The first article was on mahogany, a beautiful material that carries environmental and human rights baggage. This month, we look at wood-drying processes. So buckle up.
For the first seven years I was in the woodworking business, I used kiln-dried lumber exclusively. Not by choice. That’s just what was available at the commercial yards here in Cincinnati.
But my fellow employees talked a lot about air-dried wood. How its color was better. And working with it? It was much more pleasant to saw and plane. And the kiln-dried wood we were building with every day? They told me this:
Kiln-dried wood is brittle.
You can’t bend it with steam.
Its cell walls have been collapsed, so it can’t take on much moisture.
It moves less with the seasons.
It’s harder to work.
It’s filled with stress and honeycombing.
It’s much more likely to warp.
I always wondered if their ideas about air-dried wood were a “grass is always greener” thing. After all, wood is incredibly variable. Not only from species to species, but from tree to tree – even within the same tree. How can you make blanket statements like “it’s harder to work?”
In 2003, when I started building chairs, I got my first taste of working with air-dried ash and red elm. My conclusion?
“Yep. It’s wood alright.”
Since then I’ve worked with wood that has been dried by every means available. And I still struggle to make generalizations about the differences between air-dried and kiln-dried stuff. So we asked our managing editor, Kara Gebhart Uhl, to spend weeks researching the published literature and talking to people who live and breathe the drying process.
As it turns out, these are some differences. They surprised me. Here is her report.
— Christopher Schwarz
Folks have strong opinions about air-dried vs. kiln-dried wood. But lately we’ve wondered if there really is much of a difference when it is being worked on the bench? It turns out that little research has been done to compare the two. So, we talked to some experts to find out.
First, a Good Explanation of Moisture Content (MC)
“The total amount of water in a given piece of wood is called its moisture content (MC),” writes James E. Reep, wood products and utilization specialist at the University of Kentucky’s Cooperative Extension Service, in “Drying Wood”.1 Although we are accustomed to the fact that 100 percent signifies the total amount of something, the MC percent of wood can be greater than 100 percent. This occurs because the water can weigh more than the wood, and the MC of wood is usually based on the ratio of the weight of the water to the weight of the wood after it has been dried.”
What Happens to Wood When It Dries?
In simple terms, wood shrinks when it loses water and swells when it gains water, says Stavros Avramidis, professor head of the Department of Wood Science, Forest Sciences Center at The University of British Columbia and current president of the International Academy of Wood Science.
“A dry piece of wood will exchange water molecules with the surrounding air according to the level of atmospheric relative humidity,” Reep writes in “Drying Wood.” “Loss or gain of moisture in wood products may cause such troublesome results as shrinking or swelling, interference with paint adhesion, and increased susceptibility to decay and stain.”
Avramidis agrees.
“There are all kinds of defects that may happen when you take water away from wood,” he says. “That’s one of the major problems that we have with this material. Water is part of the life of the tree, so it’s part of wood.”
Avramidis likes to think of a piece of wood as a 2-year-old at bedtime. If you abruptly tell the child to stop playing and go to bed, “you’re going to have a reaction, and sometimes, it’s going to be really bad,” Avramidis says. “But if you do it slowly and use the right words, you might not have a negative reaction.”
A 2-year-old having a tantrum may cry, kick, throw, hit, scream and go limp.
Wood can also react strongly. It can bow, crack, crook, check, cup, shake, shrink, stain, twist and warp. Removing water too fast can also cause internal stress.
As such, for centuries, we’ve sought out ways to avoid tantrums when removing moisture from wood.
The History of Drying Wood
In 2019, near a river basin above Kalambo Falls in Zambia, archeologists discovered “two interlocking logs joined transversely by an intentionally cut notch,” according to a 2023 article in Nature. Using luminescence, the archeologists estimated this rare find was 476,000 years old.
Woodworking, it seems, pre-dates Homo sapiens.
We don’t know when humans realized the benefits of drying wood, exactly, but we do know people have been air drying wood for thousands of years – and trying to speed up the process. Avramidis says that 5,000 to 6,000 years ago, Egyptians were putting wood over fire to accelerate the air-drying process.
For years, air-dried wood worked well in houses that lacked efficient heating and cooling methods and insulation.
“You didn’t get really low-equilibrium moisture contents in houses in the winter, so there wasn’t as much need for kiln drying,” says Mike Milota, professor emeritus of Wood Science & Engineering, College of Forestry, Oregon State University. “And then as housing changed and we got more heating in houses, it became more of a necessity.”
According to a 1923 article in The California Lumber Merchant, contributed by the Moore Dry Kiln Co., “The dry kiln came into commercial use, in a limited way, about 1870. … The builders kept pace with the changing and enlarging mill conditions and developed the dry kiln to meet demands so that from 1890 to 1900 it became a valuable part of the manufacturing equipment in some sections. At this time, it was so decidedly apparent to many mills that their dry kiln boards indicated a difference between profit and loss. In these early days when the lumber industry was young the dry kiln through sheer merit proved itself a new source of increased profit.”
William Smith, professor and director of the Wood Utilization Service in the Department of Sustainable Resources Management at SUNY College of Environmental Science and Forestry, says the basic kiln-drying process – controlling temperature and relative humidity – has not changed much.
“Over the years, there have been continual incremental improvements in more precisely controlling these conditions,” Smith says.
For example, we’ve learned how to dry softwoods relatively fast, using bigger fans for better airflow.
Air-dried Lumber
“This is an old and very traditional form of drying,” writes Lost Art Press author Richard Jones in “Cut & Dried.” “First, converted planks are swept clear of sawdust produced by the milling to deter mould growth during subsequent drying. The boards are piled up with stickers between each layer for air to circulate. The ground should be firm; a concrete base is ideal.”
Air-drying is done by home woodworkers and large companies alike. Avramidis recalls visiting huge operations in the southeast U.S. where millions of board feet of wood were air-dried on thousands of acres.
Different types of air drying include forced air drying, accelerated air drying, low-temperature warehouse pre-drying and climate chambers.
“Air drying outdoors is the least-controllable method to dry wood,” Jones writes. “The speed at which the material dries and the final moisture content of the end product is rather dependent on the weather and the climate.”
For example, if it rains nonstop for days, there will not be much air drying, Avramidis says.
“Of course, the wood needs to be protected, not exposed to rain or sunlight,” Avramidis says. “You cover the wood, but you don’t have air circulation. It can be windy one day, but when there is no wind, the drying slows down significantly. You can’t control air temperature, humidity and velocity. This is why humans decided to build kilns.”
Kiln-dried Lumber
Kiln drying relies on three things: heat, humidity and air circulation.
“The higher the temperature, the lower the relative humidity, the faster the water comes out of the material – it’s as simple as that,” Avramidis says. “And a dry kiln is nothing but a box where you can control the humidity, the temperature and the air circulation inside the box.”
The fundamentals of kiln drying have remained the same for more than a century.
“What’s changed is the technology,” Avramidis says. “Some remote places still use old-technology kilns from the ’50s, ’60s and ’70s. They have analog systems. Then, the CPU appeared in the ’80s, and now everything is digital. Some kilns now use AI for better control of the conditions – the temperature, the humidity and the air velocity inside the kiln. Some systems are actually very sophisticated where the velocity on one side of the kiln is different than the air velocity from the other side of the kiln because of different moisture contents.”
Today, Smith says most hardwood companies utilize kiln samples, which are boards about 30" long, to monitor moisture content levels and adjust temperature and humidity schedules appropriately.
“A substantial proportion of mills use steam, mostly generated by burning wood waste such as sawdust and chips, to heat kilns, with vents to exhaust excess humidity,” Smith says. “Boilers are also heated by some with natural gas, oil or propane when economic conditions make sense and attractive alternative markets exist for mill residues. Dehumidification drying is also utilized, where excess humidity is removed via heat pump condensation.”
According to “An Overview of Drying Hardwood Lumber” published by The Ohio State University Extension, the final moisture content for kiln-dried hardwood is typically 6 to 8 percent in North America, with residual stress relieved (more on that below). In Europe, the target moisture content for furniture-grade wood is 12 percent, plus or minus 3 percent. (You can read more about this in section 8.10.1 of “Cut & Dried.”)
“Hardwoods tend to be pretty small sawmills whereas softwoods tend to be large,” Milota says. “I was in a softwood mill last week and they dry 800,000 board feet a day. That’s more than most hardwoods do in a month. That mill was running 14 kilns, and they were changing them out all the time. They were running a 30- to 45-hour drying process.”
In addition to technology, there have been some other kiln-drying discoveries along the way.
“In the Appalachian Mountains, there were and still are some really small family operations that dry hardwoods,” Avramidis says. “These are small sawmills, and they have small kilns, and at 6 p.m., they go home. Nobody stays back so they have to shut off the kiln. They cannot let the kiln run overnight because what happens if something goes crazy? Nobody’s going to be there to fix it or change things that need to be changed. So they turn it off and then they come in the morning and they turn it on again. Over time, they discovered that this produces a very relaxed and very high-quality wood. Why? Because during the night, the wood relaxes.”
This is called oscillating drying.
In the ’70s, Avramidis says, Sweden developed continuous or progressive kiln drying.
With batch drying, also called compartment or traditional, drying, “You have a box, you open the door, you push the wood inside, you close the door, you do your drying, for one, two, three, four, five days,” Avramidis says. “When the drying is done, you open the door, take the wood out, put fresh wood in, close the door.”
Progressive drying is basically a long tunnel.
“The wood moves slowly through the tunnel, goes in wet, comes out from the other side dry,” Avramidis says. “The tunnel might be the length of a football field. And it might take three or four or five days for the wood to move from one side to the other side. It’s an old idea, but the Scandinavians commercialized it back in the early ’80s.”
If you’re only drying one kind of species, progressive drying is great.
“But here we have softwoods, we have hardwoods, there’s lots of variability in North America,” Avramidis says. “In Europe, when you talk about commodity lumber, 2x4s and whatnot, for construction purposes, it’s Norway spruce.”
For this reason, North America shied away from progressive drying for a while. But some kiln manufacturers couldn’t stop thinking about the ease of the process.
“Why bother opening doors, closing doors, doing this, doing that, and then not knowing how much you produce every day?” Avramidis says. “Progressive drying is like sausage production – in and out, you know exactly how much kiln-dry wood you have every day.”
Eventually, a North American went to Europe and bought a progressive kiln.
“And you know how it is,” Avramidis says. “You have to go to the dealership and kick the tires before you can buy something. So once this company installed a progressive kiln, others came looking. ‘Oh, look at this! Oh, you don’t have to open and close doors! You don’t have to have people typing drying schedules! And oh boy, we know exactly how many thousands of board feet of KD lumber we produce every day. I want one of those.’ Then it was, ‘I want one of those,’ and ‘I want one of those.’ Now we have, I don’t know how many, I’ve lost count.”
For some of the largest lumber companies, such as Interfor and Western, progressive kilns have been a boon as they’ve shifted from 2x4s to cross-laminated timber (CLT).
“But if you want softwoods to manufacture windows, doors, furniture and things like that, the continuous process is not the one to go with,” Avramidis says. “You have to go back to the batch process. Why? Because you cannot have stress relief in the continuous process. And, of course, hardwoods should be dried in boxes only.”
Other specialized types of kiln drying exist, such as vacuum drying, electrical seasoning and superheated steam vacuum (SSV) drying, but not many companies use them.
“We develop the Ferraris and the Lamborghinis, but very people use those cars,” Avramidis says. “Most of the population, they use the Hondas, the Chevys.”
Stress
Another important development in kiln drying has been the introduction of stress relief, also called conditioning or case-hardening relief.
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