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Wood Movement: Part 2 – Minimise The Effects

Jul 25, 2019 | Wood Info | 0 comments

Things you need to know – How to Minimise Wood Movement

If you’ve read this article, then you’ll know that movement is an unavoidable consequence of using this beautiful natural material. Movement means any or all of: Bending, twisting, splitting and cracking – all potential risks with a table made from solid wood.

 

What Determines How Much a Table will Move?

There are several factors affecting the degree of movement you can expect:

 

Moisture content

The principle of movement is very simple – Wood moves because it is trying to equalise its moisture content with the surrounding air. There are different types of moisture in timber, but to make smart choices you just need to know that moisture content is measured in % and there are 3 general categories:

1 – Green wood – this is essentially fresh-sawn timber, so the wood still has all of the moisture from when the tree was alive. Water in a tree is like blood in a human, it is the system that transports nutrients around the living tree and is essentially everywhere. You would use green wood in moist conditions, or where it doesn’t matter if it moves. A good example is Green Oak garden sleepers.

2 – Air dried wood – this is wood that has been stacked outside for a period of months or years to allow the moisture to escape slowly as it equalises with the outdoor environment. A rule of thumb is that for every inch of thickness of a piece of timber, 1 year is required to equalise with the outdoor environment. A typical air dried timber is structural Oak beams used for buildings (although some use green oak beams too). Only furniture that will live outdoors, or in a room that is essentially open to the outdoor air circulating 12 months of the year, should be made from air-dried-only timber.

3 – Kiln dried wood – this is wood that has been air dried first, then moved into a kiln which is intended to further reduce the moisture content by mimicing the conditions of an indoor environment. If kiln drying is done slowly and gently as it should be, the idea is that the wood acclimatises to the indoor environment as gently as possible, to minimise the movement it experiences as it acclimatises and also to ensure that the wood dries evenly all the way through. If wood is kiln dried too quickly then moisture can remain in the middle, carrying the risk of the wood moving at some point in the future.

french-oak-boules-kiln-drying

French Oak Boules sticked and stacked for kilning after the air-drying process

The philosophy of kiln drying is this – acclimatise the wood to indoor conditions in a kiln, then the furniture maker planes and cuts the pieces flat and straight, cuts out or fills any cracks, then the furniture should hold its shape in someones home. Makes sense right?

The problem, or at least the caveat, is that kiln drying reduces moisture content to around 8-12% in the kiln. In reality, in the period of time between the kiln and the furniture maker (which might be weeks or months or even years), the wood will likely be stored outdoors under cover so it will be acclimatising to the outside air again and gaining moisture, undoing the good work of the kilning process. A good furniture maker will cut his pieces of timber and then bring them indoors for a period of weeks to allow the pieces to lose some of the moisture it may have gained since kilning, before starting the build process.

But there is one other issue with kiln drying, which is that a modern centrally heated home with double glazing and well-sealed doors, will have a much lower moisture content than a commercial kiln. Maybe 5% or lower. So a modern home in the colder months of the year is essentially acting as a kiln for any new timber furniture and the effects will be identical to a commercial kiln, which is to reduce the moisture in the wood, causing shrinkage and potentially bending, twisting, splitting and cracking.

So if someone tells you that kiln dried timber = no movement problems, take that with a heavy pinch of salt and instead consider the next choices you have…

 

Wood Type (Species) & Grain Pattern

Some species of wood move more than others. Movement, or stability, has lots of different measures you might hear such as “movement coefficient” or “movement in action” and they are generally given a low-medium-high-breakdance kind of description, depending on where you look.

I will write a separate article explaining the ins and outs of different species and their properties, but a basic understanding is that the stability of a wood species relates to the density and the cell composition which determines how it holds the different types of water.

From a practical point of view there is an aesthetic trade-off with wood movement, another rule of thumb you can use: timbers with very plain straight grain moves less, timbers with more interesting grain patterns move more.

The reason for this is that the shrinkage that causes movement problems occurs across the wood grain. So as wood acclimatises and loses moisture, a board will generally become a little narrower and a little thinner (although not always noticeably). That means if the grain runs straight, the movement is going to be more even, whereas if the grain twists and swirls and goes all over the place (which for me makes a more beautiful table), then the shrinkage will occur in all directions and the movement effects will be much less predictable as you can imagine.

This idea doesn’t just give you an idea of the risk of movement when you’re looking at different species of timber, it also tells you which pieces might be more susceptible to movement when comparing different pieces of timber of the same species.

One of the major benefits of oak and the reason it is so widely used is that is has a nice balance of beautiful grain pattern and a mid-level risk of movement. For our tables we use a light character grade of French Oak, which is the highest quality of Oak that exhibits the character features that maximise the wood’s unique natural beauty in each table.

abacus-prime-oak-slab-komodo-table

A Komodo table made from 3 wide slabs of Prime Grade QBA French Oak, special ordered to achieve the cleanest, most flawless appearance as specified by the client

Walnut is a timber with similar stability to oak, but Walnut (arguably) has some more striking colouration and grain patterns. This is one reason why Walnut is more expensive than Oak.

Elm is an example of a very characterful and beautiful timber, but it has a high movement coefficient. So, as you might guess, the price is generally closer to oak.

So being aware of how likely different timbers are to move is a big advantage when choosing a table and understanding the trade-off between beauty and uniqueness, versus potential movement, puts you much more in control of your table’s destiny.

 

Character Features

I’ve talked about the grain pattern above and how, when combined with the species of wood, it is an indicator of the movement you can expect.

Other features that occur in the grain can also affect movement. A knot is essentially a mini stump of a branch that came out of the tree trunk. A knot will have different density and grain orientated differently to the main body of the timber, which means it is unlikely that it will move in the same way as the surrounding timber. In my experience, smaller knots over timber will stand a little proud of a table top surface, which you will feel as you run your hand across the surface of a table. Large knots can constrain the movement of the whole piece of timber, meaning the rest of the piece moves as it wants to but the knot essentially anchors the wood around it, which can cause unevenness in the surface and edges of a piece of timber as it acclimatises.

Knots are the most common feature in the grain of timber, but the lesson here is that anywhere the grain is interrupted is an area where thing might not stay looking nice and flat and even. For me, these visual elements of timber are the appeal of the material, so this understand should not necessarily prevent you from choosing a particular piece, but what you will have is much more realistic expectations of what might happen as the timber settles. In this article I explain what you can do once the timber has settled down.

 

Piece Size and Thickness

It’s difficult to dispute that a table made from a huge, thick slab of timber is incredibly striking and naturally beautiful. So I’m not going to argue – I love them!

What I will do is explain 2 trade-offs when deciding what you want your table made from. First choice is thickness – when a tree is chopped down it gets sliced into boards of even thickness. Timber is priced based on volume, so the thicker the timber you choose the more expensive it will be.

Plus, the thicker the timber, the more difficult (and expensive) it is to get all the moisture out of the middle of the piece. The outside of a piece of timber will dry out more quickly than the centre, but at a certain point the outside will start to deteriorate if it is dried too much, which inevitably means leaving some moisture in the middle.

So you have to accept that in the thicker timbers, even those that have been kiln dried extremely slowly, you can never eliminate the inherent risk of movement – yet another timber trade-off between aesthetics and longevity!

The other main choice is in the piece size, or width, of timber boards in your table. Big wide slabs are very impressive, but the only thing to be aware of  from a movement perspective is that the wider the piece, the more visible any movement will be. A slight bend in a piece of timber 100-200mm wide may be unnoticeable visually. But the exact same slight bend in the middle of a 400-500mm wide board may result in the edges of the piece moving up or down by 5-10mm, which will definitely be visible and could crate steps in the table top or, deepening on how the table is jointed together, the whole table to wobble. All of our tables come with an aftercare service specifically designed to address these types of issue.

How the pieces of your table are jointed together has a big effect on whether this movement will materialise and I explain what the right type of joint is for a solid wood table in this article.

 

Your home environment

Regardless of how well your timber has been dried, the species, the level of character, the thickness and piece size…. arguably the greatest level of control you have over visible movement issues is your home environment.

The kindest environment for a solid wood table is a home with fresh outdoor air circulating throughout the year. This is the environment that is most familiar to the timber, very much like air-drying conditions, and any moisture loss or gain will be as slow and gradual as it can be, meaning that any movement will be at the absolute minimum.

The harshest environment, as I outlined above, is a well-sealed room with dry air. Doors and windows closed, air conditioners (which normally have a dehumidifier), underfloor heating… creates a stagnant environment with air containing practically no moisture (which can’t be great for health either?) that is in-effect a super-kiln for your solid wood table. And because this environment is such a change for the timber, it forces moisture loss to happen quickly and the movement effects will therefore be maximised.

Underfloor heating is a particular issue because it causes uneven drying of the tables underside and top surfaces. Essentially the underside of the table can dry out quicker and tighten up in relation to the top surface, causing the table top to bend like a piece of stale bread.

 

But I Live in a Super-Kiln – What can I do?

It’s likely you do live in something more like the harsh environment I’ve described than table Heaven. But there are some important steps you can take to make your home more table friendly, allowing your wood to acclimatise as slowly and gently as possible.

1 – Add a source (or sources) of moisture to the room. You could have a house plant or plants growing in the room, because they naturally put moisture back into the air through their breathing process (photosynthesis). Or an even more basic measure would be a container or bucket of water, allowed to evaporate slowly and keep some moisture in the environment.

2 – Have a humidifier in the room to control the moisture level in the air, which is essentially a technical version of a houseplant but gives you more control of the environment.

3 – Allow fresh air to circulate through the home as much as possible, which means windows open as often as you can and doors between rooms open so you aren’t creating a kiln for your table just in that room. If possible turn off, or at least down, the heating initially and slowly creep it up to where you want it to be over the first 2-3 months.

You now know what the movement risks are, how to look out for signs of them, how to mitigate the risks, the trade-offs in your timber choices and how to set up your home environment to minimise any potential movement issues. Remember, the first 12 months is the toughest time for your table because it is the first time the timber has experienced the 4 seasons indoors. So once you’re through this settling period the table should have acclimatised and experienced the majority of movement you can expect.

At that point you can choose to live with the table as it is (there may be visually no change at all from new!) or you may want to have it restored back to new, then it will stay like new for life. I explain what makes a restorable table, versus a disposable table, along with how we as a company deal with wood movement, in this article.