Wood Movement   

© John Bullar 2004

Wood moves in mysterious ways. The last thing any of us wants to find out is that our latest and greatest woodworking project has self-destructive tendencies, but wood movement or warping could cause this.

 Wood consists of layers of fibres, formed in rings just under the bark each year. These fibres naturally swell with water, but they shrivel up if they dry out, reducing considerably in width, as well as becoming slightly shorter. This causes the wooden boards themselves to become narrower, thinner and shorter (in that order).

  All wood contains water. Live wood in trees or freshly felled timber is brimming with water but this has usually dried out by the time you buy it. In the long term, the amount of water that stays in dead wood reaches a natural balance with the amount of water in the air around it. This means wood is continuously changing its dimensions!

A second set of fibres at right angles to the main vertical ones, grow in thin bands between the outside and the centre of the trunk. This second set produces the patterning known as Medullary rays, most noticeable in oak and visible to a variable extent in many other hardwoods. The cracks that form in felled logs, left to dry, usually follow the line of one of the rays.

The annual growth rings of tree trunks appear as a series of arcs, visible on the end grain of sawn boards. If wood has been sawn by the routine through and through method, the arcs are quite long, often stretching from one side of a board to the other. As the wood dries out the annual growth rings try to straighten themselves. In a log, this causes cracks to form but in sawn boards, it causes cupping. Seen end on, the board curves in the opposite direction to the rings.

The real problems arise when we come along and join pieces together. Usually the grain direction of the joined pieces will be different and so they do not move by the same amount. Once a piece of wood decides that it wants to shrink, no amount of brute force will persuade it not to. It is a bit like when an irresistible force meets an immovable object, something is bound to snap - either one of the pieces of wood or the carefully crafted joint - and quite likely the temper of the craftsman too!

In the atmosphere, water vapour is mixed up with Nitrogen, Oxygen and all the other gases we breathe. When it is warm, air can hold a lot more water than when it is cold. The amount of water in air is measured in ‘Relative Humidity’, which is the percentage of water relative to the most the air can possibly carry without it dropping out as dew. When cold air is heated, such as when it enters a warm building in winter, it becomes capable of holding more water vapour - if it cannot find any the relative humidity falls so the air becomes drier indoors.

When dry air meets the surface of a piece of wood, it takes water from it to restore the natural balance. With the outside of the wood now drier than the inside, the fibres shrink and  there is tension created that can cause cracks and possibly a change in shape. Expensive veneers are stored in sealed bags to prevent moisture entering or leaving.

Air-drying timber is the traditional method and is still widely used by small-scale operations and specialists. The trunk is sawn into flat boards, which are then stacked up and separated by ‘stickers’ to let the air flow between them. Usually this is done outdoors, but under cover so as to keep off rain and direct sunlight. The rule of thumb is one year per inch of thickness, which should take the moisture content down to about sixteen percent, which is fine for outdoor woodwork. However, it is too high for use in a central heated home so after roughly cutting air-dried timber to size it is usually stored in a warm dry indoor location for some more months.

Kiln drying is a much quicker method, often reducing the time to a couple of weeks, and it is more controllable. Most commercial wood converters who can afford the investment use only kiln drying. The process is one of heating the wood up in a controlled atmosphere so moisture trapped in it is released as water vapour, which is blown off the wood surfaces by fans.

It would be a mistake to think that using kiln dried timber solves all the woodworker’s problems. Because the drying is so rapid, unless it is controlled correctly it can cause serious defects. One common fault from poor kiln drying is where large splits are produced inside the timber, sometimes not visible on the outside at all. This is known as case hardening. The outside of the wood becomes rigid while the inside is still soft and swollen. Then, as the inside dries and shrinks it is pulled apart by the outside.  

One simple way of guarding against movement is to choose the right wood. When oak has been quarter-sawn or cut at right angles to the growth rings the ray patterns are large and clear. As well as making the wood beautiful, this is an indication that it will be stable and not prone to cupping on wide panels. In past centuries, oak cut this way had the special name of ‘wainscot’ and featured extensively in high quality buildings and furniture. 

Woodwork in a central-heated house needs a moisture content of around ten percent.  Measure the moisture content in a number of places to be sure it is evenly dry. Simple meters measure water content of wood by electrical resistance with  a pair of pins that you stick into the surface of the wood while the meter passes a small electric current through them. These only measure the water content near the surface of the wood which may be very different from the inside. They also leave pinholes in the wood, looking like two woodworms have been for dinner.

Some of the better modern moisture meters are pinless. They work by sending an electromagnetic field deep into the wood. These measure the moisture content throughout without leaving marks.

 To maintain the moisture content of wood around the ten percent mark, the air needs to have a relative humidity of about fifty percent. Measuring the water content in the air is straightforward. There are some very fancy electronic meters to do this accurately but a simple mechanical meter or hygrometer will do the job more than adequately. These are available from craft suppliers for a few pounds.

One way to improve the dryness of your workshop is to install a de-humidifier.  This works like an air conditioner and heater combined. A fan sucks air in from the room where it meets a cold set of pipes and forms condensation, which drips out into a collecting bucket. The air then passes over a hot set of pipes, gets re-heated, and comes out again drier.  I installed a drainpipe off my de-humidifier into a water barrel outside the workshop so I do not need to keep emptying it.

In an attempt to reduce end slitting, wood-yards commonly paint the timber ends with oil paint before placing in the kiln. This reduces the loss of moisture out of the exposed end-grain, making its shrinkage rate similar to the rest of the board. 

Small pieces of timber for turning or carving are often sealed with wax over its ends, or sometimes over the whole piece, to prevent moisture entering or leaving the wood during storage. This is extremely effective, while the wax is present, but there is a risk that the wood will experience a very sudden change in moisture content when the wax is removed in the workshop. If you plan to work on expensive wood that has been protected in this way, make sure the workshop is suitably dry before you start.

Cupping of boards that are not cut on a line passing through the centre of the trunk, is almost inevitable. Having accepted this, you can minimise the effect on wide panels such as tabletops by building up the width from narrow pieces joined edge to edge. If the ring directions alternate (so the end-grain pattern looks like a corrugated roof) the cupping effects will cancel out.

Frame and panel is a long established way of building timber in large stable areas. The idea is the long-grain of the framework holds a consistent shape while the panel, trapped in a groove inside the frame, is free to expand and contract. The technique has commonly been used for building internal walls, as well as furniture and doors.

John Bullar featured on the cover of New Woodworking magazine, demonstrating handmade dovetail techniques.

Cover Photo by Ben Daniels

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