Know your wood By Neil Summers, *AHEC Technical Consultant – Welcome to Technical Matters
The first in a number of technical articles that are designed to inform the reader about various aspects and properties of timber as a material before discussing different processing methods such as kilning and finishing and describing North American hardwoods in more detail along the way.
This first piece will explain how wood is formed, the fundamental differences between hardwoods and softwoods and their different growing regions around the world. ‘Wood’ is an incredibly variable material and because of this diversity, it is a slightly misleading term, as there is not one individual species of wood that can perform every task that you might want it to do for you.
Wood grows on trees, or it is the product of the growth of that tree over many years, before it is sawn and converted into timber or lumber, as it is known in America. There are thousands of diverse types of trees that grow on the planet and each one produces wood with distinct characteristics and properties that will be suitable for different end uses.
These properties may differ slightly within a single species, but the differences can be huge between wood species – you might say the difference between chalk and cheese! There are at least 60,000 distinct species of wood in the world which have been discovered so far.
Only a small proportion of these species are traded commercially around the world, but the important fact to understand is that they are traded with a specific purpose or use in mind. You might require a durable timber that is not going to rot or be attacked by insects; appearance could be a factor when choosing a timber for making furniture; or a lower cost timber for simple structural applications. In each case you would choose a different timber species for these diverse types of application.
So, it is important to understand exactly what sort of wood it is that you are looking for. Wood grows naturally in billions of trees around the planet, and in all these trees the main constituent is an organic substance called cellulose.
The tree is a natural chemical factory and makes cellulose from carbohydrates that are formed by the combination of water drawn up from the root system and carbon dioxide that is sucked out of the air.
A particularly useful by-product of this ingenious chemical process is oxygen, which the tree releases back into the atmosphere. Cellulose is in effect a natural form of carbon fibre, which is what gives wood extremely high strength properties in relation to its weight. In fact, it has been proven that wood is stronger, weight for weight in tension, than steel. If you are involved with the timber industry in any way then you will be familiar with the terms ‘hardwood’ and ‘softwood,’ two basic sub-divisions that are used to categorise timber species.
These descriptive terms do not necessarily mean that a ‘hardwood’ is hard or that a ‘softwood’ is soft. For example, the classic misconception between softwoods and hardwoods is the fact that the balsa wood tree, which produces one of the softest timbers in the world is a hardwood. The two terms relate to the tree itself and not to the wood that it produces.
Softwood trees are described as conifers, trees that have needle like leaves and bear cones with open unprotected seeds. An example of a softwood tree would be Southern Yellow Pine (SYP). Hardwoods are broadleaf trees with leaves that are properly defined in a variety of shapes and sizes, and which produce some form of fruit that protects the seeds within it.
Examples are American white oak or teak. Softwood trees have a much more primitive cell structure than hardwoods, which have evolved much later in time. Due to this evolutionary trait, there are thousands more hardwood species in the world than there are softwoods.
As a rule, softwoods prefer to inhabit the colder parts of the world, whereas hardwoods tend to grow better in warmer climatic conditions, including hot tropical conditions. Therefore, you will find an abundance of conifer trees in cold countries such as Sweden, Finland, and Canada. There are exceptions such as Parana pine from South America and even SYP mentioned earlier, which favours the hotter regions of North America. The greater evolutionary development of the broadleaf trees has produced two distinct categories of hardwoods.
Temperate hardwoods frequent both the temperate zones of Europe and North America, where the temperature is warm for most of the year, while tropical hardwoods grow in the tropical regions, where the climate is much hotter.
Again, there are exceptions with species such as American hard maple, which prefers the colder temperatures of the American Canadian border. In the next Technical Matters, we will explore the relationship of water and wood. In the words of one of America’s pre-eminent architects, Frank Lloyd Wright: “Wood is a friend of mine. When we use the tree respectively and economically, we have one of the greatest resources of the earth. It is a beautiful material, friendly to man, the supreme material for his dwelling purposes. If a man is going to live, he should live with wood.”