Seasonal Movement
Overview
Moisture content in timber has a profound effect on the material, causing swelling in high humidity, and shrinkage in low humidity environments. These changes can be seen seasonally as it's typically drier in summer than in winter in Melbourne. Even timber that has been dressed down, assembled, glued, and finished with continue to move over its lifespan. Understanding and accounting for movement caused by moisture in timber is critical to creating lasting furniture.
Hygroscopic and Anisotropic
Timber is hygroscopic, meaning that it will gain or lose moisture from the air based upon conditions of the surrounding environment, as the wood gains or looses moisture, the cells expand or contract resulting in shrinking or swelling, which ultimately affects the overall dimensions of a piece of timber, often referred to as the wood’s movement.
Timber is also anisotropic meaning that this movement is dependant on the direction or orientation of the grain and is it’s not the same in all directions, often referred to as dimensional shrinkage.
Free water
The cells that make up a tree contain a central hollow area called a lumen. When the tree is living, and just after it is cut down, these lumens contain liquid water (free water) and water vapour. The cells themselves also contain water bound to the molecules that make up the cell walls (bound water). Once all the free water is removed from the timber during the drying process it is said to be at the fibre saturation point and up to this point there will be very little to no shrinkage. As the drying process continues past the fibre saturation point the timber will experience shrinkage.
Dimensional Shrinkage
A basic measurement of shrinkage—expressed as a percentage—is the amount that the wood shrinks when going from its green to oven dry/kiln dried state. In other words, since wood in its green state is at its largest dimension, and ovendry represents its driest (and therefore smallest) volume, green to ovendry is a measurement of the maximum possible percentage of shrinkage; this is referred to as the wood’s volumetric shrinkage.
Volumetric shrinkage tells how much a wood species will shrink, but it doesn’t indicate the direction of the shrinkage. The two primary planes or surfaces of wood where shrinkage takes place are across the radial plane, and across the tangential plane, corresponding to radial shrinkage, and tangential shrinkage; these two values, when combined, should roughly add up to the volumetric shrinkage.
Longitudinal Shrinkage
The longitudinal shrinkage (down the length of timber) is very minimal and it not usually regarded.
Radial Shrinkage
Radial shrinkage is perpendicular to the growth rings, it is shrinking in the direction towards the centre (pith) of the tree. Radial shrinkage is up to 50 times the amount of the longitudinal shrinkage.
Tangential Shrinkage
Tangential shrinkage is tangential to the growth rings, it is shrinking the direction around the growth growth rings. This is where we see the greatest amount of movement that can be up to double the radial shrinkage.
EMC
EMC or Equilibrium Moisture Content is the percentage of moisture content a piece timber will stabilise at when it is kept in the same conditions throughout the year. For example, a piece of timber in a 30°C room with 65% relative humidity at will equalise at 12%. In an ideal world you would store your timber in a climate controlled room and let it reach EMC before starting to build with it.
Shrinkage and Swelling
The percentage of shrinkage and swelling varies between species as timbers that have a thicker walls swell and shrink more than those with thinner walls.
Calculating Movement
In a regular domestic environment a piece of furniture can be expected to endure changes in moisture content up to 4%. The table below shows how much movement this 4% variation can cause in 1mm of timber. The radial movement is what you would see in a quarter sawn board and the tangential movement is what you would see in a backsawn board. So, for example, a 200mm wide backsawn American Ash board would have up to 2.6mm of movement (Calculation... 200 x 0.013 = 2.6mm).
Species | Tangential Movement (mm) | Radial Movement (mm) |
American Ash | 0.013 | 0.0082 |
American Oak | 0.0131 | 0.007 |
American Walnut | 0.0104 | 0.0073 |
American Maple | 0.0116 | 0.0067 |
Vic Ash/Tas. Oak | 0.0144 | 0.0092 |
Blackbutt | 0.0148 | 0.0104 |
Tassie Blackwood | 0.0108 | 0.0064 |
Jarrah | 0.012 | 0.0096 |
Spotted Gum | 0.0152 | 0.0128 |
Myrtle Beech | 0.0128 | 0.0072 |
Douglas Fir | 0.0104 | 0.0064 |
Radiata Pine | 0.0108 | 0.008 |
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