Water is just as essential to the life of a tree as it is for all living matter. For that reason, green or unseasoned timber contains a large amount of water. Seasoning of timber is the process of drying out most of the water.

Timber slabs stacked with spacer for air circulation.
Image credit – Richard Young / Timber Yard at Langley

There are many reasons as to why this process needs to be completed.

  • It reduces shrinkage, warping and checking.
  • Seasoning will reduce the weight of the timber making handling and transportation easier and cheaper.
  • It increases the strength of the timber
  • Reduces effects of mold and fungi and
  • It improves gluing operations and painting.

Seasoning does not dry out all the moisture from timber. Generally, depending on the location of the timber, the amount of moisture (known as moisture content) which remains in the wood is 12% to 15%.

There is a variety of ways that timber can be seasoned. The easiest way, but not the most effective method of seasoning timber, is air seasoning.
Air seasoning

Air seasoning or natural drying requires the timber to be stacked with spacers between each board in the open, as can be seen in the image on this page. It allows the timber to dry slowly in natural weather conditions.

The timber is laid out and is normally placed in a shed or building, with two sides open to allow air to circulate. The roof and two other sides keep the timber relatively dry and out of the sun. To achieve the best results, the timber should be laid on bearers to keep the timber off the ground. Strips of timber about 20mm x 20mm are placed one above each other and at 500mm spacing. This keeps warping to a minimum. The ends of the boards are sealed with paint so that the end grain does not dry too quickly. The circulation of the air slowly dries the timber. However, this techniques does not give a precise moisture content. This is because air circulates freely and carries moisture, depending on the weather and the time of year.

The seasoning time varies depending on the thickness of the wood boards and the time of year. Usually 6 months per 25mm in thickness for softwoods and at least a year per 25mm for hardwood boards. This can vary depending on the species, the climate, board size and the way the boards are stacked.
Kiln Seasoning

Air drying is a lengthy process and can be unpredictable as it is exposed to varying weather conditions and seasonal changes. In Kiln drying, the process of drying can be cut down to one to three weeks depending on the species. The timber is stacked in a similar way to air drying, placed on a truck or trolly system and placed into an oven looking room.

Advantages of Kiln Drying

Kiln drying has many advantages such as being able to control environmental factors such as humidity, heat and air flow as well as destroying insects and fungi.
– Kiln drying permits drying to the exact moisture content required, which may be much less than that of air dried timber.
– Reduces the drying time considerably.
– The carefully controlled conditions can minimise seasoning defects which occur while drying.
– The heating of the timber while in the kiln kills eggs, larvae and adult borers which may be present in the wood.
– Whilst initial capital costs may be high, greater turnover is possible by reductions in drying time, and lesser need for timber yard storage space.

Many types of softwood, such as Radiata Pine and other European timbers, can be kiln dried easily and fairly cheaply from the green condition. However, many hardwoods such such Australian, Brazilian and African hardwood species are so slow in drying that their kiln drying is uneconomical. It is, however, satisfactory and economic to season them by the combined process of air and kiln drying mentioned below.

Combined seasoning

Combined seasoning combines the advantages of both air seasoning and kiln seasoning. It helps to overcome the disadvantage that sometimes occurs in kiln drying when moisture is removed too rapidly area of the timber. With combined seasoning, timber can be air seasoned for two to three months. The actual time depending on the species of timber, thickness and climate, before being kiln seasoned for a further three to seven days. This process can dry timber to be suitable for either indoor use or outdoor use depending on the need.

“Production and Conversion” Woodwork Set 5

This page and associated worksheet is available in the Tasstudent T.P.T store in “Production and Conversion – Set 5″ Link here to preview