Wood expands and contracts with moisture content. More moisture makes the fibers "fatten up".
The interesting thing is that this is anisotropic: the expansion/contraction occurs across the grain, NOT along the grain. The rate of expansion also depends on the local characteristics of the grain itself (hence the effects of warping due to uneven expansion) ... Also there's a big difference between the direction "across the growth rings" (i.e. radially when it was still a tree) and tangentially to the growth rings. And these surfaces are curved, of course. But one thing we can always say is: the wood doesn't significantly change size along the grain.
Design and construction methods can make wooden artifacts more or less susceptible to cracking and distortion from this. For example dovetail joints can be pretty good as all the wood expands/contacts together the same way. Especially if the pieces are joined together from the same piece of wood. Stuff like that. Or at the other extreme, metal fixings like nails don't move with moisture at all, which can cause problems with relative movement and stress can accumulate.
Edit: and the repeated cycling of moisture content induced stress can eventually lead to cracking, in a similar way to metal fatigue. Old wood just cracks sometimes, this is probably why.
A good mental model for wood is that trees are a bunch of stacked cones (growth rings) on top of each other.
In the spring it fills with water and the diameter grows but the tree does not get longer because it needs to support a large mass on top and the lengthwise fibers are not able to grow and shrink (they need to be stiff to carry the weight).
Because of this, the circumference of the outermost growth rings need to grow more than the inner ones.
Now cut a board out of it and look at the end grain. Think what happens when the rings closer to the outside need to shrink more than the inner ones for the same humidity change. For a flat sawn board, you will always see it cup so that the concave side is on the outside.
This doesn't explain why boards twist or bow but cupping is the most prevalent wood movement in typical flat sawn boards.
Both are actually explained the same way because in bowing, it dries slower in the middle of the board creating the bow, and twisting is just different type of uneven drying typically due to some open grain drying faster.
One nit: there are times when you do want to use a metal nail or screw in a joint, particularly if there is some sort of cross-grain joinery going on and you want to allow for wood movement. Chris Schwarz (publisher of this article) makes the point himself: https://blog.lostartpress.com/2015/07/11/the-bare-bones-basi...
The numbers here match my experience, a 600mm wide spruce table top shrunk and expanded by about 12mm during a year of being outdoors but under a roof at temperatures from -25C to +30C. The structure had sliding dovetails to allow growth but keep it flat.
I think you’re off by an order of magnitude. With those numbers, a 12” board would expand and contract 1.2”, and an 8’ long board would vary by almost an inch.
Much more reasonable would be 1% across the grain and 0.1% along it. You can confirm this in some of the wood movement calculators found online.
To those learning about wood movement, these ratios are decent but approximate; if you end up caring about these things you’ll want to check the species of the lumber you plan to work with.
A panel door is basically designed to minimize warping as the wood expands and contracts. There is leeway for the panels to move inside the edge pieces (sorry, not sure about the terminology here) and the edge pieces have the grain along the sides of the door. Stuck doors or doors that will not close are no fun.
Small addendum: some traditional wooden joinery is deliberately prepared to account for the varying rates and effects of drying across the timber.
This is particularly relevant in timberframing, where you want to work with the wood when it is as green as possible. Green pine, though heavier to lug around, is significantly more receptive to a chisel than drier lumber. In a classic mortise and tenon joint [0], it's common to leave the outer edge of the shoulder slightly raised from the inner edge to account for the natural warping as the exterior of the beam dries more aggressively.
Although it's more outside my area of experience, I believe fine carpentry also has a few techniques that see a higher frequency of use in areas that enjoy seasonal swings in humidity. The split-tenon is the only one that comes to mind, but, now that I think of it, I realize my mental model isn't great. More surface area to account for seasonal swelling / shrinkage? Maybe someone else can chime with a better explanation of this one.
Timber framing uses dry wood as well, slightly different techniques but in the softwoods and some of the hardwoods its is not all that harder to work dry than green and in some ways easier. It depends on the tradition and location as to the exact process and technique, some preferred dry timbers, some green, some something between.
In US farm country it was common to fell the trees in late fall/early winter after the harvest was all taken care of and then leave the trees where they dropped until the ground froze. After the ground froze you haul them to the build site, much easier to drag logs on hard frozen ground than on soft wet ground. Then you would forget about them until after the spring planting is taken care of and build in the summer. Those big timbers would be far from dry but they will have lost a fair amount of weight and will be more stable which makes everything easier.
Green woodworking is an entire field of its own. Not very common in industrial scale but it was a common method a few centuries ago.
Examples of things where green woodworking is common: spoon carving, bowl turning, chair making, etc.
The idea is that wood is worked while green to make 80% finished blanks, which are dried slowly for some months or years before finishing the rest of it. This gives less distortion to the shape as it dries. And the drying times are faster because it's all small pieces at that point. The time from tree to product is shorter.
It is an almost extinct craft but it is a lot of fun for woodworkers not under schedule pressure.
In particular - the section on drying - air drying would have been too rapid/harmful to the wood - so they put it into a purpose-built dehumidifying kiln for 9 months.
I saw a thing once where a guy would make 3 large cuts at the bottom of a tree, in a particular pattern. This would kill the tree, and it would essentially air-dry over the course of a year or two. I wonder how that compares.
I should note he was a homesteader doing this to provide dry wood with easy access during cold months.
That’s pretty clever! I’ll have to keep this in mind if ever get my dream homestead.
I’m no arborist, but I’d guess the cuts sever all of the tree’s microtubules without felling it? I think 3 would be the minimum amount of cuts you’d need.
I used to be a carpenter and joiner. I once had a batch that was badly kiln dried. We called it 'case hardening', I guess it was done too fast. If you planed a flat face on it and it instantly warped again!
If you sawed it, it would either pinch or spring apart. I made the sales rep come and see it.
Had some poplar milled from some large trees we had to take down here. Air dried in my shop for 4 years before having it made into a table. All it took was 1 winter and it split and bent severely inside the house. I will only kiln dry from now on.
4 years is more than enough time for drying. Rule of thumb is year per inch of thickness.
I'm just speculating here but probably the support structure didn't allow for wood movement. You need something to keep the table top flat while allowing it to move. Screwing it to a stiff frame (steel or cross grain wodo) is certain to crack when the wood moves.
Breadboard ends, sliding dovetails or steel support with elongated holes (going to a threaded insert and bolt) are good ways to support a table top.
The wood was probably stabilized to your shop atmosphere but indoors in the dry winter, maybe with air conditioning or a fire place, and there's going to be movement.
Kiln drying does not stop seasonal wood movement.
If you share a picture we can take an educated guess what caused the table to warp and crack.
Breadboard ends were always confusing to me because they are trotted out as a solution for movement yet they solve two quite different problems.
On flush, jointed boards, they are a permanent jig to hold the ends in vertical alignment. Imagine taping your fingers together to keep your fingers flat. Lateral movement is impossible because the boards are glued tightly together.
If you’re concerned about lateral movement then the more important concern is to have gaps between the boards. The bread board end is now a rail in which your boards can slide like wobbly carriages on a train track: aligned in one direction (up/down for a table) but with the ability to move independently in another (across the width of the table.)
Four years of air drying may not have been enough, depending on the thickness of the boards and the moisture level in the air. Also the issues of wood movement and grain direction must be considered during the design and manufacturing of furniture with that wood. Home sawn wood will often have knots, randomly curved grain, etc, so it can be more difficult to get predictable results.
I’ve never dried anything that long indoors, but from what people told me when I was researching the best way to dry some red oak I had milled there are issues drying indoors doors. Wind does most of the drying outside.
Worse, I've been told that attempting to dry indoors would result in rot (given not enough air flow, which might not be the case in all indoor environments), and consequently have never tried it. I've exclusively air dried pine, oak, poplar, pecan, cedar, all under open walled structures and not had too many problems over the last 20 years (and my dad was doing it for another 30 before that).
I dried three red oak trees using a dehumidifier kiln. ( 4'x4'x16' 1" pink insulation foam box assembled with packing tape with a household dehumidifier and fan inside. Very low tech. Knock it down when not using it.)
The process is mostly: measure moisture content of wood, pick a humidity to maintain, check wood periodically to see if it is drying too fast or too slow. Weigh water coming out to monitor process.
Very low effort if you have space to allocate while in use. The wood came out well, no complaints.
One downside is you won't kill insects with heat, so you could have trouble if it is buggy wood.
What a shame this is a paywalled article. The first part was interesting, but I'm definitely not going to subscribe. If I could pay for this one article, I would.
I don't subscribe to his paid blogs personally, but Chris Schwarz is one of the best known writers in the woodworking world. I own most of his books, and I wouldn't be a woodworker today if it weren't for his writing. If you like this one article, odds are good you'll enjoy more from him, at least enough to pay for a month subscription or something.
Edit: though now I see this particular article was actually written by their editor/researcher, not Chris, so uh nevermind, maybe.
> “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.”
Aha, yes, of course.
(I have no idea what stress relief means here, or why hardwoods are different :/)
The article talks about stress relief when it went down the rabbit hole of tiny mom & pop mills who were turning their kilns off every night because they didn't have a third shift and later discovering that turning them off periodically produces better lumber because it allows the wood to relieve stress and suffer less distortion.
However, the continuous process is basically just a slow moving conveyor belt where you are constantly feeding green wood in one end and dried lumber is constantly being spit out of the far end. I don't see why you couldn't incorporate ambient air chambers in strategic places on the belt to destress the lumber, at the cost of making the entire production line somewhat longer.
If you want to smoke food (BBQ, etc) avoid kiln dried wood. It's too dry. You want dry wood but you generally want some level of moisture (15%-20% is often good, more in some other styles) in most of your wood.
Use wood chucks. After thinking about it, I’ve never thought my wood chunks were too dry but I used to think that a lot about wood chips. Wood chips burn much faster so it makes sense.
> 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.
Holy shit.
It is too bad that the post cuts off in the middle with a paywall notice. We really should ban such links. They aren't conducive to high-quality discussion.
Thanks! I'm sorry I didn't include that link. They say of the dating method:
> Younger samples are dated using single-grain quartz optically stimulated luminescence (OSL) and older samples by postinfrared infrared stimulated luminescence (pIR IRSL) from potassium-rich feldspars (Methods and Supplementary Information Section 2). The pIR IRSL approach used extensively in recent years²⁵,²⁶ does not suffer the problems that can generate large uncertainties associated with thermally transferred OSL (TT-OSL), as seen at Site C North (Fig. 1b)²⁰.
Not having read the full paper, I don't understand why they think the date at which the sand around the wood cooled from magma temperatures is relevant to when the carpenter cut the logs? Or maybe they're assuming the sand was exposed to the sun and optically bleached around the time of the carpenter, so any trapped charge is from after that? https://insu.hal.science/insu-03418831/file/MurrayEtAl-2021-... looks potentially relevant.
Not really—the topic is about wood drying, and this Nature article is the expanded footnote about how long humans have been working with wood.
EDIT: Sorry, you were referring to the GP's interest in the historic part. However, to me it sounded like you were offering this Nature article as a way to go deeper on wood drying, not the athropological footnote. Mea culpa, I was reading too fast. Thank you for the link!
Just an educated guess but I am assuming the age of the wood is a good enough proxy to construction. Making the assumption that wood out in nature will decompose in short order (when thinking of the stated age). Being off a few thousand years is probably ok.
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The interesting thing is that this is anisotropic: the expansion/contraction occurs across the grain, NOT along the grain. The rate of expansion also depends on the local characteristics of the grain itself (hence the effects of warping due to uneven expansion) ... Also there's a big difference between the direction "across the growth rings" (i.e. radially when it was still a tree) and tangentially to the growth rings. And these surfaces are curved, of course. But one thing we can always say is: the wood doesn't significantly change size along the grain.
Design and construction methods can make wooden artifacts more or less susceptible to cracking and distortion from this. For example dovetail joints can be pretty good as all the wood expands/contacts together the same way. Especially if the pieces are joined together from the same piece of wood. Stuff like that. Or at the other extreme, metal fixings like nails don't move with moisture at all, which can cause problems with relative movement and stress can accumulate.
Edit: and the repeated cycling of moisture content induced stress can eventually lead to cracking, in a similar way to metal fatigue. Old wood just cracks sometimes, this is probably why.
In the spring it fills with water and the diameter grows but the tree does not get longer because it needs to support a large mass on top and the lengthwise fibers are not able to grow and shrink (they need to be stiff to carry the weight).
Because of this, the circumference of the outermost growth rings need to grow more than the inner ones.
Now cut a board out of it and look at the end grain. Think what happens when the rings closer to the outside need to shrink more than the inner ones for the same humidity change. For a flat sawn board, you will always see it cup so that the concave side is on the outside.
This doesn't explain why boards twist or bow but cupping is the most prevalent wood movement in typical flat sawn boards.
This is also why properly designed tabletops are attached to the frame with a “floating” construction that can handle those changes.
This is for wood that is dried and stabilized, the shrinking is a bit more from green wood to seasoned lumber (but not an order of magnitude more).
You can use online calculators such as this one for estimates based on the species of wood and your location: https://kmtools.com/pages/wood-movement-calculator
The numbers here match my experience, a 600mm wide spruce table top shrunk and expanded by about 12mm during a year of being outdoors but under a roof at temperatures from -25C to +30C. The structure had sliding dovetails to allow growth but keep it flat.
Much more reasonable would be 1% across the grain and 0.1% along it. You can confirm this in some of the wood movement calculators found online.
To those learning about wood movement, these ratios are decent but approximate; if you end up caring about these things you’ll want to check the species of the lumber you plan to work with.
This is particularly relevant in timberframing, where you want to work with the wood when it is as green as possible. Green pine, though heavier to lug around, is significantly more receptive to a chisel than drier lumber. In a classic mortise and tenon joint [0], it's common to leave the outer edge of the shoulder slightly raised from the inner edge to account for the natural warping as the exterior of the beam dries more aggressively.
Although it's more outside my area of experience, I believe fine carpentry also has a few techniques that see a higher frequency of use in areas that enjoy seasonal swings in humidity. The split-tenon is the only one that comes to mind, but, now that I think of it, I realize my mental model isn't great. More surface area to account for seasonal swelling / shrinkage? Maybe someone else can chime with a better explanation of this one.
[0] https://www.barnyard.com/sites/default/files/styles/full_pag...
In US farm country it was common to fell the trees in late fall/early winter after the harvest was all taken care of and then leave the trees where they dropped until the ground froze. After the ground froze you haul them to the build site, much easier to drag logs on hard frozen ground than on soft wet ground. Then you would forget about them until after the spring planting is taken care of and build in the summer. Those big timbers would be far from dry but they will have lost a fair amount of weight and will be more stable which makes everything easier.
Examples of things where green woodworking is common: spoon carving, bowl turning, chair making, etc.
The idea is that wood is worked while green to make 80% finished blanks, which are dried slowly for some months or years before finishing the rest of it. This gives less distortion to the shape as it dries. And the drying times are faster because it's all small pieces at that point. The time from tree to product is shorter.
It is an almost extinct craft but it is a lot of fun for woodworkers not under schedule pressure.
https://www.thefenlandblackoakproject.co.uk/our-story
In particular - the section on drying - air drying would have been too rapid/harmful to the wood - so they put it into a purpose-built dehumidifying kiln for 9 months.
(It was briefly discussed here a few years ago: https://news.ycombinator.com/item?id=36912861 )
I should note he was a homesteader doing this to provide dry wood with easy access during cold months.
I’m no arborist, but I’d guess the cuts sever all of the tree’s microtubules without felling it? I think 3 would be the minimum amount of cuts you’d need.
If you sawed it, it would either pinch or spring apart. I made the sales rep come and see it.
I'm just speculating here but probably the support structure didn't allow for wood movement. You need something to keep the table top flat while allowing it to move. Screwing it to a stiff frame (steel or cross grain wodo) is certain to crack when the wood moves.
Breadboard ends, sliding dovetails or steel support with elongated holes (going to a threaded insert and bolt) are good ways to support a table top.
The wood was probably stabilized to your shop atmosphere but indoors in the dry winter, maybe with air conditioning or a fire place, and there's going to be movement.
Kiln drying does not stop seasonal wood movement.
If you share a picture we can take an educated guess what caused the table to warp and crack.
On flush, jointed boards, they are a permanent jig to hold the ends in vertical alignment. Imagine taping your fingers together to keep your fingers flat. Lateral movement is impossible because the boards are glued tightly together.
If you’re concerned about lateral movement then the more important concern is to have gaps between the boards. The bread board end is now a rail in which your boards can slide like wobbly carriages on a train track: aligned in one direction (up/down for a table) but with the ability to move independently in another (across the width of the table.)
Did you use a moisture meter?
The process is mostly: measure moisture content of wood, pick a humidity to maintain, check wood periodically to see if it is drying too fast or too slow. Weigh water coming out to monitor process.
Very low effort if you have space to allocate while in use. The wood came out well, no complaints.
One downside is you won't kill insects with heat, so you could have trouble if it is buggy wood.
Edit: though now I see this particular article was actually written by their editor/researcher, not Chris, so uh nevermind, maybe.
Aha, yes, of course.
(I have no idea what stress relief means here, or why hardwoods are different :/)
However, the continuous process is basically just a slow moving conveyor belt where you are constantly feeding green wood in one end and dried lumber is constantly being spit out of the far end. I don't see why you couldn't incorporate ambient air chambers in strategic places on the belt to destress the lumber, at the cost of making the entire production line somewhat longer.
Holy shit.
It is too bad that the post cuts off in the middle with a paywall notice. We really should ban such links. They aren't conducive to high-quality discussion.
[1]: https://www.nature.com/articles/s41586-023-06557-9
> Younger samples are dated using single-grain quartz optically stimulated luminescence (OSL) and older samples by postinfrared infrared stimulated luminescence (pIR IRSL) from potassium-rich feldspars (Methods and Supplementary Information Section 2). The pIR IRSL approach used extensively in recent years²⁵,²⁶ does not suffer the problems that can generate large uncertainties associated with thermally transferred OSL (TT-OSL), as seen at Site C North (Fig. 1b)²⁰.
I had never heard of this archæological dating method before, but Wikipedia comes through as usual: https://en.wikipedia.org/wiki/Optically_stimulated_luminesce...
Not having read the full paper, I don't understand why they think the date at which the sand around the wood cooled from magma temperatures is relevant to when the carpenter cut the logs? Or maybe they're assuming the sand was exposed to the sun and optically bleached around the time of the carpenter, so any trapped charge is from after that? https://insu.hal.science/insu-03418831/file/MurrayEtAl-2021-... looks potentially relevant.
EDIT: Sorry, you were referring to the GP's interest in the historic part. However, to me it sounded like you were offering this Nature article as a way to go deeper on wood drying, not the athropological footnote. Mea culpa, I was reading too fast. Thank you for the link!