Humidity and wood movement - the basics

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profchris

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I've seen quite a few posts recently where the answer was 'humidity has made it move'. Humidity can be a mystery, especially when you're starting out.

Because I make guitars and ukuleles, which quickly fall apart if you don't understand humidity, I've had to read up on it lots. So I thought I'd write up the basics in case they help someone avoid ruining a project.

1. Humidity is the moisture in the air, and it changes all the time. The BBC weather app (and others probably) will give you hourly humidity forecasts throughout the day.

2. Wood constantly absorbs and loses moisture from the air, as the atmospheric humidity rises and falls. Even if it's 'well-dried'. Wood is constantly changing its moisture content.

3. Wood moves as it become more humid, mainly across the grain and hardly at all along it. Different species move different amounts - mahogany moves maybe 2%, pine more like 4%. So if your board is 100mm wide, it might grow as much as 4 or 5 mm across.

4. If the end grain shows curves in the grain, it will cup in that direction as it swells. The longest lines in the curves swell most.

5. You can't stop it moving (short of bolting it to a very sturdy framework, say heavy angle iron). It's going to move, so design and build to cope with that.

6. If you build in one level of humidity (say your workshop) and move the piece to a different humidity (say indoors, where the heating dries the air), your piece will change shape. If the humidity changes in your workshop, it will move too.

This is all you need to know, if you're prepared to think about how that translates into design and build. It explains the rules which experienced woodworkers tell you, such as:

Alternate the cupped grain on table tops (they'll still go a bit wavy, but if all the cupping is in the same direction they will imitate a banana)

Don't glue in panels (they expand and shrink cross grain, but are surrounded by long grain which hardly moves at all)

For guitar makers, build a dome in your tops and backs (when they move they will simply rise or fall a little rather than tearing themselves apart if they shrink or expanding into the body)

Glue up in low humidity to avoid panels which must be fixed (like guitar tops) cracking in lower humidity

And so on ...
 

Stigmorgan

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That all makes sense to me but one thing that has always baffled me is how humidity affects a finished piece, we tend to finish our (woodturning) projects with a wax coating of some form, wax is a waterproof substance yet water/humidity still manages to pass through it into/out of the wood, ive been had it when using a varnish coat to finish the piece.
 

Droogs

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One of the best ways to learn about the bundle of straws we play around with for both beginners and those who think they know about wood is "Cut & Dried" Richard Jones (our very own Sgian Dubh of this parish). It is the result of many decades of research and for me should be regarded as the starting point of any woodworkers journey into understanding what happens when we have at it with gay abandon in our man caves.

1643747383488.png
 

profchris

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That all makes sense to me but one thing that has always baffled me is how humidity affects a finished piece, we tend to finish our (woodturning) projects with a wax coating of some form, wax is a waterproof substance yet water/humidity still manages to pass through it into/out of the wood, ive been had it when using a varnish coat to finish the piece.
I think I know the answer (sort of).

In liquid form, H20 molecules hang tight together. Thus surface tension which holds water droplets together.

As a gas, H2O molecules cavort about solo, or maybe as couples, and so can get through tiny apertures.

Most finishes will let the gas through, but the holes in it are too tiny for the liquid form.

Are any finishes gas-proof? Maybe 6mm of epoxy?
 

Droogs

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wax is liquid water repellant but still permeable to water vapour. there is a full study here if you want to really delve into it. This is the summary:

The water vapor (WVP) and oxygen (O2P) permeabilities of beeswax (BW), candelilla wax (CnW), carnauba wax (CrW) and microcrystalline wax (MW), formed as freestanding films, were determined. CnW and CrW both had small values for O2P (0.29 and 0.26 g·m−1·sec−1·Pa−1 × 10−14, respectively), which are less than half the value for high-density polyethylene and about a decade greater than the value for polyethylene terephthalate. O2P values for BW and MW were about 6−9× greater than those of CnW and CrW. WVP of CnW was 0.18 g·m−1·sec−1·Pa−1 × 10−12, which is about one-half the value for CrW and MW and about one-third the value for BW. The WVP of CnW was somewhat less than that of polypropylene and somewhat greater than that of high-density polyethylene. Differences in permeabilities among the wax films are attributed mainly to differences in chemical composition and crystal type as determined by X-ray diffraction.



clear as mud lol
 

Sachakins

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wax is liquid water repellant but still permeable to water vapour. there is a full study here if you want to really delve into it. This is the summary:

The water vapor (WVP) and oxygen (O2P) permeabilities of beeswax (BW), candelilla wax (CnW), carnauba wax (CrW) and microcrystalline wax (MW), formed as freestanding films, were determined. CnW and CrW both had small values for O2P (0.29 and 0.26 g·m−1·sec−1·Pa−1 × 10−14, respectively), which are less than half the value for high-density polyethylene and about a decade greater than the value for polyethylene terephthalate. O2P values for BW and MW were about 6−9× greater than those of CnW and CrW. WVP of CnW was 0.18 g·m−1·sec−1·Pa−1 × 10−12, which is about one-half the value for CrW and MW and about one-third the value for BW. The WVP of CnW was somewhat less than that of polypropylene and somewhat greater than that of high-density polyethylene. Differences in permeabilities among the wax films are attributed mainly to differences in chemical composition and crystal type as determined by X-ray diffraction.



clear as mud lol
Ow, my head hurts now 🤯🤯🤯🤯🤯🤕🤕🤕🤕🤕
 

mr rusty

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@profchris - have you ever considered using accoya in a musical instrument (because it almost doesn't change dimension with humidity)? This isn't a recommendation - I'm genuinely just interested if it would have any application. I wonder what it's sound qualities would be? Is humidity movement even a big problem with instruments? just curious...
 

profchris

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I don't think I'd use accoya. For an acoustic instrument there are three main drawbacks:
  • increased density. For soundboards, dense wood is a way of achieving a very quiet instrument.
  • can't use traditional glues which are reversible/repairable. PU is a nightmare on instruments, epoxy is messy (in my hands anyway).
  • appearance. Radiata pine is dull enough to start with, people like their instruments to look pretty as well as sound good.
Also, will it bend with heat?

Humidity movement is definitely a problem with instruments. It changes their playability, even if well-built, as soundboards rise and fall by a mm or two. Badly-built instruments destroy themselves. Even well-made instruments need to be protected - I've linked an image of typical humidity-caused cracking (this one's not too bad). If anyone wants to replicate this crack, just store the guitar next to a radiator for a few days!

gallery_51218_105_146281.jpg
 

Droogs

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I forgot to mention that Sgian Dubh's book is also available from Classic Hand Tools for those who would rather buy in the UK
 

D_W

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That all makes sense to me but one thing that has always baffled me is how humidity affects a finished piece, we tend to finish our (woodturning) projects with a wax coating of some form, wax is a waterproof substance yet water/humidity still manages to pass through it into/out of the wood, ive been had it when using a varnish coat to finish the piece.

I have a short answer for you, both from making guitars and from making planes.

No matter what you use to finish wood, it will eventually shrink. The better the barrier, the slower it will do it. The slower it does it, the less likely you'll have catastrophic failures.
 

chris13

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Is not the direction of the annular rings important when making sound boards ? When loosing moisture annular rings will straighten and contraction is in the direction of the ring. Should not timber be quarter sawn for sound boards with the annular rings set perpendicular to the plane of the board? Perhaps this is only possible in the very best quality boards.
 

profchris

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Is not the direction of the annular rings important when making sound boards ? When loosing moisture annular rings will straighten and contraction is in the direction of the ring. Should not timber be quarter sawn for sound boards with the annular rings set perpendicular to the plane of the board? Perhaps this is only possible in the very best quality boards.

Yes, vertical grain is the ideal. Same for the backs. But trees tend to twist as they grow, so a bookmatched top tends to have vertical grain at one edge and some slant at the other. It's usual to join the two along the vertical grain edges, so that expansion in the centre (where the top has least support) is less than at the edges of the lower bout (the wings).

It's not unknown to make a 4 piece top, where the wings are glued on using the offcuts from the waist. But that requires some thought, as the runout direction needs to match or the wings glare out as different boards.

On instruments from the 20s and 30s you sometimes see one piece tops and backs - that's a huge tree!

Small instruments are more forgiving, because the overall movement is less. This one I made for a friend is tiny - around 11 inches total length, a little over 3 inches wide at the lower bout. The board (koa) had been rejected by a professional maker because it was too far off vertical grain to hold up at full size (around 6.5 inches wide lower bout), but for this baby it's been fine. It is actually playable, though only a dozen or so people in the UK can manage to play it well :)

IMG_20131110_091346.jpg
IMG_20131110_091415.jpg
 
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chris13

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That is a lovely instrument. I am a pianist besides an occasional cabinet maker so I spend a lot of time thinking about how my piano soundboard and its bridges are adjusting to humidity changes. I would imagine that the board would have been kiln dried to a pretty low moisture content before the ribs were glued on .
 

chris13

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I came across this graph in another forum which shows dimension changes and moisture content in poplar as humidity is cycled from about 45% to 75% every six hours. Moisture content (MC) follows humidity with a phase lag but the dimensional changes are in phase with moisture content. T is the dimensional changes tangential to annular rings and R the dimensional change on the radius of the rings. The experiment was conducted at an elevated temperature of 40 deg C just to accelerated the take up and release of moisture.
IMG_20220209_082957_resized_20220209_083953721.jpg
 

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