Processing of wood materials

STAGE ONE:HARVESTING

Growing plants for food is called agriculture; growing trees for human use is silviculture—and the two things have a great deal in common. Wood is a plant crop that must be harvested just like any other, but the difference is how long trees take to grow, often many years or even decades. How wood is harvested depends on whether trees are growing in plantations (where there are hundreds or thousands of the same species, generally of similar age) or in mature forests (where there's a mixture of different species and trees of widely differing ages).

 

Planted trees may be grown according to a precise plan and clear-cut (the entire forest is felled) when they reach maturity. A drastic approach like that makes sense if the trees are a fast-growing species planted specifically for use as biomass fuel, for example. Individual trees can also be selectively felled from mixed forests and either dragged away by machine or animal or even (if it makes economic and environmental sense) hauled upward by helicopter, which avoids damaging other nearby trees. Sometimes trees have their bark and small branches removed in the forest before being hauled away to a lumber yard for further processing, though they can also be removed intact, with the entire processing done offsite. It all depends on the value of the tree, the growing conditions, how far away the lumber yard is, and how easy the tree is to transport. Another interesting form of forestry is called coppicing, which involves removing long, thin, low-growing branches from trees such as hazel and willow in a careful and respectful way that does no long-term damage.

Stage Two: Storing/ Transporting

Next, the logs are stored in a clearing or in the forest until they are needed at the sawmill. This also allows some of the ‘free’ water content to evaporate, reducing the weight of the tree/log, which will result in lowering the cost of transporting and handling.

 

The trees are usually cut into smaller lengths on-site and then picked up by a timber lorry, which transports the timber to a processing site, such as a sawmill, paper mill, pallet, fencing or construction producer.

Stage Three: On Site

At the chosen site, the logs are debarked and bucked, or cut to the required length. Then they are cut into boards, using equipment such as circular saws and bandsaws. This is called ‘conversion’. The first stage of conversion is a process called ‘breaking down’ - which means rough sawing. The second stage is called ‘re-sawing’ and refers to more precise cutting and finishing, such as planing and further machining.

 

Two types of rough sawing can be used in the breaking down process - through sawn and quarter sawn.

 

The ends of each log is trimmed to ensure they are straight and cut into boards. Large circular saws are then used to further-process the boards, removing the curved edges. Each processed piece of wood now looks like a board.

Stage Four: Seasoning

Seasoning of natural wood is the process of removing excess water/moisture content. When a tree is felled, it still contains a large proportion of water/moisture – usually between forty to fifty per cent water content.

Water is held inside a tree in two ways:

1. Free Water: Water that is held in the vessels and cells in order to distribute nutrients inside the tree.

2. Cell water: Also known as ‘bound’ water, is an essential part of the tree’s cell walls.

During the seasoning process, a tree loses its free water and a high proportion of its cell water and as a result, is less likely to warp or deform.

Wood that has not been seasoned and still has a high water content is called ‘green wood’ and can be more difficult to work with because it has a tendency to change shape.