Physical Properties of Wood

Introduction

Wood is one of the most versatile and widely used materials in the world. It can be used in anything from wood carving and creating small wooden items, such as chairs and rocking horses, to building timber structures and sheds.

It also has a lot of scientific properties associated with it, and every type of wood that is available can exhibit similar, and also different, properties. As such, it is important to understand how different types of wood can be used, so you can make sure that you don’t use the wrong type of wood in your project.

Types of wood

There are two distinct types of wood:

• Hardwoods come from deciduous trees – trees whose leaves fall each autumn – such as oak, mahogany, beech, and birch.
• Softwoods come from coniferous trees – trees whose cones/needles remain all year round – such as cedar, spruce, fir, cypress, and pine.

Specific gravity

The specific gravity (SG) of a substance, is the ratio between the density of the specific substance and the density of a reference substance provided which occupies the same volume. In most cases, the reference substance is water, as its density, in terms of grams per cubic centimetre, is one.

If moisture content is neglected or minimised, the more rings that a tree has within a given space, the denser it is, meaning its specific gravity is higher. Woods that have a higher specific gravity tend to have more lignin in their cells – this increases the rigidity of the wood and its resistance to rotting – and fewer air spaces, meaning that it can’t intake water easily compared to lower specific gravity woods.

Moisture content

One of the most noticeable properties of wood is that it is a hygroscopic material, meaning that it tends to absorb moisture from the air depending on the humidity of its surroundings. If the humidity is relatively high, then the wood will absorb more moisture.

Moisture content is very important when using wood in building, crafting, and manufacturing as it can affect the strength and stiffness of wood. This is because the presence of water affects the bonds between the fibres, effectively weakening the wood and making it more pliable.

Temperature

It is important to understand the thermal properties that wood exhibits.

Thermal conductivity is the ability of a material to conduct heat. For wood, it can range between 0.01 and 0.2 W m-1 K-1, which is relatively low, considering that for copper, it is over 400. The reason for this is because wood is quite porous. The values have a wide range because thermal conductivity decreases as the density decreases.

Thermal expansion is also relatively low in wood, meaning that it can retain its original structure in hot climates.

In freezing temperatures, the water within the wood can expand, causing the it to crack. However, since it has a low thermal conductivity, it means that, in cold weather, it can retain its warm interior and keep the cold weather out, which is why wood cabins are relatively common in arctic conditions.

--G&S Specialist Timber 09:10, 17 Jan 2017 (BST)

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