Air-dried vs. Kiln-dried Wood
christopherschwarz.substack.com
christopherschwarz.substack.com
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.
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.
Even then, building a barn with dried pine or hemlock is much more tedious and incurs many more trips to the sharpening wheel. It is in no way 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.
A green wood specialty in my neck of the woods is sauna ladles (used for throwing water). You can buy wooden ones but they are made from seasoned lumber with CNC machines and don't survive more than a year before they crack. The one I made from green wood is still going strong after 7 years in extreme humidity and temperature environment.
I've also had great results using silica gel on smaller items, although it can be hard to scale it to larger vessels. Much faster drying than air alone, with greatly reduced distortion and cracking.
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.
The annual movement of wood depends (basically) on the local RH swing, thickness, absorption/diffusion rates, and swelling coefficients.
So giving any percents here without more data is just incomplete.
This is assuming bare wood too, with no coatings/etc.
A lot of the bare percents you see are making assumptions of various sorts. Usually they ignore the diffusion rates/etc and shoot for EMC at some parameters (the calculator you linked does) because doing it for real require more complex math. The calculator you linked is better than most for sure, but it is still a simplification of reality where it may be off by orders of magnitude depending on thickness.
It will be much closer to reality for thinner pieces than thicker ones.
By the way your relative humidity figures assume constant temperature. Wood cares about absolute humidity (mass of vapor per volume of air), and temperature is the dominant factor in absolute humidity. Rainy day at +1C (100% RH) is less absolute humidity than a sunny day at +30C.
This matters to me a lot because half of my woodworking projects are outdoors or not temperature controlled indoors.
?
It's not - it's exactly the same as anything else. The wood doesn't know it's green.
The calculator you gave is shortcutting it, and has an entire article in how they shortcut it the same way as anyone else, based on the swelling coefficients/etc, but assuming thickness is small enough to not matter.
If your projects are outdoors, you will be affected by more than just humidity - UV will also have a significant effect on the properties of your projects :)
The moisture transport is also not as simple as you are making it out to be, and has a not insignificant effect.
See:
https://gupea.ub.gu.se/handle/2077/54179
https://www.mdpi.com/2076-3263/8/10/378
https://www.sciencedirect.com/science/article/abs/pii/S12962...
I have my woodworking projects in temperatures ranging from -25C to +100C (sauna) and extreme humidity changes from near zero to 100% RH. It is a form of art to make wooden things survive that, and I don't always succeed.
Frankly, the idea that a piece of wood after initial drying was moving even an in/ft (eg "only" 8%) would be pretty shocking. Even good joinery won't deal with much more than a quarter of that ~2%.
Also, most green hardwood starts at about 30%.
No reputable lumber supplier is kiln drying hardwood that is 200% moisture content. That's crazy town.
Even if they dried it super slowly, it would end up as mostly checked/warped garbage that they couldn't sell.
Beyond that, wood is moving a lot, sorry you don't believe it, but its still gonna do it.
Rather than say it's "pretty shocking", and dismiss it, care to present any studies that back up your assertion?
I sent plenty, both in here and other comments. I'm not aware of any sourced, actual scientific research that says anything other than what i did, since I was careful to use cited figures from actual research studies, and not random pages on the internet about "wood movement"
I think you are also assuming a lot about how it moves and what 8% radial swelling/shrinkage really means that isn't necessarily true.
Also your point about joinery doesn't seem to make a lot of sense. While it's true that most joinery can't handle lots of flex, if everything expanded or contracted uniformly, it wouldn't be a problem.
You seem to be assuming the opening will not expand the same as the thing going into the opening. It will. That is why you try not to mix conflicting grain directions in joints, and why you see so many joints that go out of their way to do that (IE 90 degree mated dovetail boards are not made by two conflicting grain directions, the pins and tails are made of the same grain direction that happens to mate at an angle)
https://cad.onshape.com/documents/3e489410fcf65e1f0f82663d/w...
I made two tabs for you, one with a 25% transform and one without.
Notice the opening gets larger when scaled. So would the mate. They would still fit fine. The same is true if you made a dovetailed box. It would just become a bigger/smaller box.
I didn't bother to scale it differently for tangential vs radial but it wouldn't matter as long as the same scaling factors apply equally to the mate, and the mate is made the same way. As is true of most woodworking joints, on purpose.
So the only issue is if (assuming 2% was the limit) the non-uniformness lead to >2% difference somewhere that mattered.
All of this is also why wood glue has such expansion/contraction characteristics. if wood was only changing 0.1%, it wouldn't matter.
So far i've seen a lot of doubt but nobody else actually seems to be bringing any real scientific rigor to that doubt, or saying some silly things.
Please feel more than free, i'd love to see papers with real measurements that suggest something else.
https://extension.oregonstate.edu/catalog/pub/em-8600-wood-m...
Fiber saturation is a thing and it rarely exceeds 30% for usable lumber.
I'm mostly going on what I've read though I went outside to measure a 30" fir just now and it's 100% (REED Pinless). The 100 yr old apple trees were over 80%, but that's not peer reviewed (nor is it pay walled). Maybe people can cut down trees and toss them straight into an Alaskan for exactly this reason? Firewood sitting covered outside is <10%.
As for pretty shocking. Yes it would be, if the 36" door (flat sawn book matched) to my house didn't open, because it was an inch too big (it accommodates <1/4"/ft). It's about a hundred years old, and I'm pretty sure that's never happened.
I don't even know what you are arguing and why - it seems to change with every post.
So I give up - you still haven't actually shown me a study that says it's wrong, and now your argument is "my door would be too big".
This is a silly discussion.
Since you still haven't given me a single scientific study suggesting the movement doesn't actually occur, I guess i'll offer you this and then walk away:
Is your door surrounded by brick or something rigid? Or is it surrounded by wood and blocking, like most doors? What species is it? What are the radial/tangential shrinkage rates? Is it painted or otherwise sealed in a way that would affect rate of absorption, like most doors?
As an aside, did you know that basically no door company will warranty unpainted doors because of exactly the issue you say doesn't happen? Just about every single one will say something on the order of "this door must be painted or stained within x days or the warranty is void", where x is usually <7, and will unequivocally state that unpainted and unstained doors will warp. Because they do! Like potato chips, a lot of the time.
There are some made to be bare unpainted wood, but it's not common and it requires different construction techniques. Most of them are not solid wood either, they are 1/4" or 1/2" veneer pretending to be solid wood. Otherwise, doors left exposed to the elements often totally fall apart in years. All the time. I can show you one that fell apart due to movement in <5 years.
Beyond that -
Doors surrounded by brick or rigid things frequently become too large to open/close at various times.
My home was built in 1929, and the doors are painted, but the jamb is surrounded by limestone or brick on all sides. Not a facade. The jam is up against well-set brick or limestone. This is actually a super-bad construction technique, since in most cases, the brick/limestone is a facade to avoid this issue. I can send you videos if you want to see what happens.
In the winter, it is about 1/2-3/4 inch smaller than it is now overall. I've measured it. It does in fact, become unopenable in the summer. It actually is right now. I plane it until it can be opened again. It will show a very large gap in the winter.
This is on a painted door, so not even one that is totally exposed to the elements.
This is uncommon, again, because most doors are not surrounded by highly rigid materials. If they are, it's a facade instead of structural. Those doors that are structurally unable to move, will in fact, break apart. This is one of many reasons totally solid wood doors are uncommon (besides weight and cost)
Since you seem big on anecdote, and your door is your baseline, there's a door for you.
Most people with historic homes would laugh at what you are saying. Since you say your door is >100 years old, i'm sort of shocked at your view.
For example, my wife's interior office door, is wildly out of square and plumb. By about 2 inches. The concrete foundation and tile is exactly in the same place, and perfectly level and square. No tiles have broken or cracked, and they are original to the home. Only the things made of wood are no longer where they should be. The two exterior doors in her office on opposite sides were built identically ~100 years ago. They don't even close to line up any more, and are easily 1" off. Again, foundation is exactly where it should be. only the wood has moved.
But still, i'm out since we aren't actually having a useful discussion that involves more than vibes about doors.
Please reread the section about fsp and measured MC, because it explains clearly why wood does not expand beyond it's fsp ~30%. Then look at the simplified MC% vs RH% table and read data for shrinkage vs MC for various wood types. No pay walled university papers required, it's not that complicated.
If your wife's door is 2" out of square you probably have a house framing issue not door expansion due to indoor humidity.
I've sawn enough timber and built enough decks out of them that I know wood moves.
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.
They aren't off by that much. You are further off if you assume some standard parameter ranges :)
But in the end, it depends on factors i didn't see listed.
Overall, the percents are usually calculated by swelling coefficient. Swelling coefficient is percent change in radial/tangential for each 1 percent of moisture change. There are well-known sources for these that calculated them in sane ways. The US forest service is one of them, and they publish their methodologies/etc for how they determine them. See, e.g., https://wfs.swst.org/index.php/wfs/article/download/1004/100...
Take standard flat sawn red oak. The swelling coefficient is 0.001-0.002 for radial (0.1% per 1%), and 0.004-0.005 for tangential (0.4% per 1%).
So in initial drying, which is usually 30%->15%, it will move 1.5-3% radial and 6-7% tangential.
Without humidity control, houses swing from 30%<->60%. Sometimes per day, sometimes per month, sometimes per season. So even more than initial drying. But because the swing varies, depending on thickness/etc, how much moisture change you get in the wood, and how fast, will vary a lot.
If you assume it causes a 10% change in moisture content over the year, throughout the wood, we get 1-2% radial movement, and 4-5% tangential movement for red oak. But that is both swelling and shrinking, not solely one or the other.
So the GP would be off by a factor of 2 in one, but not off in the other.
It's obviously trickier in practice to calculate the actual rates because the moisture is going to diffuse through the wood at some rate, and as long as the RH is changing faster than the diffusion rate, the wood will not really have a consistent moisture content all the way through. To be accurate, you'd have to slice it into enough pieces to capture the different moisture levels in the wood, apply the coefficients to each slice, and, etc. Worse, because boards are rarely square, and instead often much wider than they are thick (IE 12"x1") , you'd have to slice and calculate it one way to deal with this for radial, and slice and calculate it the other way to deal with tangential.
I'm too lazy to calculate how coarse/fine of a slice you'd need to get within say 5% of the "real" number.
I'm also assuming you are trying to do it by hand, since this is obviously an integral of some sort that you could also just directly solve. I'm sure it's in a paper somewhere.
This is all for bare wood too, with no topcoats. The topcoat would seriously affect absorption rates, etc, even assuming you applied it to all sides.
Nobody does any of this calculation in practice, we just accept large error bars and build floating tables :)
But all things being equal, yes, they generally can only help keep moisture content more steady over time.
https://www.wagnermeters.com/moisture-meters/wood-info/how-r...
This table shows up to a 4% moisture content seasonal difference in a climate controlled house (20-50% RH).
The 10% number was not meant to be real, i just was giving an example :)
Real is much harder.
4% is not a horrible guess from as best i can calculate (but see below because this page has some crazy claims). Studies suggest that wood RH tracks RH pretty closely, slowing down with depth. Transport also appears to depends on temperature, independent of humidity itself. But if you assume it's going to track RH closely and throw out the rest, you can just assume the wood will always fall within the EMC range for the RH range.
If you look at
https://www.fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapte...
You can see that between 30-60% RH, you really don't get more than like a 7% span (i'm eyeballing it) of EMC that the wood could vary around at any temperatures likely to exist in your house.
So 4% is probably not a horrible guess.
However,the site you link to says some very wrong things, interestingly:
"Temperature Has No Significant Effect on Wood MC"
This is 100% wrong, in more ways than one.
First actually even wrong if you ignore humidity entirely, because studies suggest wood moisture transport changes at high/low temperatures, even ignoring humidity. The exact mechanisms are not pinpointed (AFAICT from skimming), but that's what real data says.
Second, the temperature affects the EMC (and relative humidity).
It's very weird for them to go on and on about how humidity affects would but then say temperature doesn't matter at at all.
You can't actually separate these things, and say humidity level matters but temperature doesn't, because they are linked.
If you want real data/simulations to try to figure out more, here's some references - i didn't read all of them, busy morning, but i did at least look at most of them.
https://www.sciencedirect.com/science/article/abs/pii/S12962...
https://gupea.ub.gu.se/handle/2077/54179
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.
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.
I hadn't heard it used to get dry wood for harvesting but I recall it being useful because it actually kills the tree, so if you cut the tree down (after it's fully dead) the stump will decompose instead of trying to continue to grow.
If you sawed it, it would either pinch or spring apart. I made the sales rep come and see it.
Did you use a moisture meter?
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.)
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.
Can always toss in some charcoal to compensate if it's too wet. Kind of hard to do much if it's too dry.
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.
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!
> 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.