Author Topic: Lesson in Wood Movement  (Read 2061 times)

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Offline HarveyWildes

  • Posts: 968
Lesson in Wood Movement
« on: August 05, 2019, 04:58 AM »

A while back, my son got a block of cottonwood that was about 2' x 1' x 1'.  Cottonwood looks really nice, but it is notorious for shrinking and expanding with moisture changes, so most people around here treat it as junk wood.  It had some cracks in it that he wanted to stabilize, so he inlayed in some butterflies made from straight-grained cocobolo.  The narrow point in the center of the butterflies was about 1" wide, and the butterflies were 3/4" thick.

After about six months, late at night, he heard a sharp pop that he described as something between fireworks and a gunshot.  The next morning he was looking around and noticed that the cottonwood had moved with such force that it snapped one of the cocobolo butterflies across the grain where it was most narrow.  When it went, it went with a bang.

« Last Edit: August 05, 2019, 05:01 AM by HarveyWildes »

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Offline Birdhunter

  • Posts: 3451
  • Woodworker, Sportsman, Retired
Re: Lesson in Wood Movement
« Reply #1 on: August 05, 2019, 05:43 AM »
In my earlier woodworking days, I build a very large TV credenza out of black walnut. Back then large screen TVs were about the size of a Volkswagen. I built it in sections so I could get it up from the basement shop into the family room. I assembled it and beamed with pride. It was beautiful.

Six months later we were watching a crime show and the credenza let out a shot louder than anything on the crime show. I had built the sides without any provision for movement. The crack ran down the side from top to bottom. Fortunately, that side wasn’t easily visible from where we usually sat so I just ignored it.

This was a powerful lesson and, thereafter, I allowed for movement.

Offline Rob-GB

  • Posts: 1101
Re: Lesson in Wood Movement
« Reply #2 on: August 09, 2019, 02:24 PM »
One of the most expensive bits of kit I purchased almost 30 years ago was a digital moisture meter. At the time it was a huge leap of faith but within 6 months it saved  me having to replace a house full of doors when it proved they had been too full of moisture before installation in a central heated house.(Long Story)
Internal joinery and furniture needs to be around 8% moisture content or less for modern heated homes, it often pays to leave the wood in the room it is to be placed for a while before working on it, a tough call when it is for a client who wants it now!
Well worth investing in a quality moisture meter to prevent future issues if you can.

Offline Oldwood

  • Posts: 470
  • Alberta, Canada
Re: Lesson in Wood Movement
« Reply #3 on: August 10, 2019, 11:14 AM »
I have not used this product but reviews indicate it works well.

Lots of other places to buy that is just the first one that came up.
Real knowledge is to know the extent of one's ignorance.

Offline Woozal

  • Posts: 67
  • Lovin' the Festools!
Re: Lesson in Wood Movement
« Reply #4 on: May 21, 2020, 10:12 AM »
I have not used this product but reviews indicate it works well.

Lots of other places to buy that is just the first one that came up.

That’s hilarious. Nothing but nothing is gonna keep wood from changing moisture content. I wonder how those ‘lil domino things pop when edge to edge wood starts it’s mighty pull on them in the summer.
Woodworker and software developer.

Offline HarveyWildes

  • Posts: 968
Re: Lesson in Wood Movement
« Reply #5 on: May 21, 2020, 01:43 PM »
I have not used this product but reviews indicate it works well.

Lots of other places to buy that is just the first one that came up.

There are some treatments that will pressure treat wood with catalyzed plastic that will virtually stop moisture shrinkage/expansion, but it's a pretty expensive deal, both the equipment and the artifacts.  Plus to me, it almost seems like it's not really wood any more.  My understanding is that these don't turn the wood into a solid block of plastic, but instead the plastic coats all of the internal surfaces so that moisture may get into the wood pores, but can't absorb into the wood tissue.  It may be that the stabilizer that Rockler sells works like that, but without pressure treating it, it would only absorb into a thin surface layer of the wood - I've heard of products like that.  If I were going to use it, I'd do more research into how it works.  If you have to make a solution and soak your wood in the solution (per the Rockler Description), it can really only be practical for small parts - sounds like things that would fit in a 5-10 gallon container unless you wanted to mix up a horse trough worth of it.  Furthermore, it would take a good long soak to get anything other than superficial penetration without pressure, so perhaps the intent is only to protect a thin layer of wood close to the surface.  Another factor to consider is how the product affects any finishes or glues that you might be using.

What you are trying to stabilize is also a factor.  If you are trying to stabilize something really porous (like "punky" wood that was in the process of rotting, but not quite there yet), it might soak through really quickly, but in that case you probably want something that will add strength, not just moisture resistance.  (I've heard of people using CA glue for that, but I've never tried it.)  If you are trying to stabilize an oily wood like teak or cocobolo, it may absorb the product differently than cherry or walnut - after all, oily woods already have oil in the wood tissue.  Open grain woods like oak may absorb better than closed grain woods like hard maple.

A custom woodworker or hobbyist can treat wood to slow down how fast the moisture content changes in other ways.  The best known is coating end grain in something like wax or acrylic so that the moisture exchange happens more slowly through the side grain.  Turners sometimes store wet-turned blanks in plastic bags which they open every month or so to slow down the drying process.  Some finishes will slow down moisture exchange enough to minimize seasonal expansion/contraction.  Thick surface finishes that are (nearly) impervious to moisture and many (20-25) wet-sanded coats of oil that eventually hardens come to mind.  Nothing I know of aside from the pressure injected plastics will prevent issues when moving from a dry to wet climate, or vice-versa.  However, slowing down the moisture content exchange will mitigate some, but not all, issues.

There is no substitute for designing for expansion and contraction.  Knowing the properties of the wood you are using, including both radial and tangential shrinkage, can affect the design.  Lining up grain, floating panels, breadboad ends, manufactured panels, etc. are all techniques that contribute to a design that isolates a piece from moisture problems.

When talking about the properties of wood that woodworkers generally care about, I heartily recommend "Understanding Wood" by Bruce Hoad.  It's not an easy read, but the info is invaluable.