Describe the recovery and conversion of logs into timber, including sawing methods, and explain seasoning by air and kiln drying, moisture content, equilibrium moisture content and the defects caused by poor seasoning

What this dot point is asking

This dot point covers everything that happens between the standing tree and the dry board on your bench. NESA wants you to describe how logs are recovered and converted into boards by different sawing methods, and to explain seasoning: why green timber must be dried, how moisture content and equilibrium moisture content work, the difference between air and kiln drying, and the defects that result when timber is dried badly. Because seasoning faults wreck finished furniture, this content directly informs the material decisions in your Major Project.

Recovery and conversion

After felling, logs are recovered to the mill and converted into boards. Conversion is simply the way the log is sawn, and it is a balance between yield (how much usable timber you get) and quality (the grain and stability of each board). The main methods are:

• Plain or back sawing: the log is cut in parallel slices straight across. It is fast, gives high yield and produces attractive flame or cathedral figure, but the boards move and cup more because the rings run roughly tangential to the face.
• Quarter sawing: the log is cut so the growth rings run roughly at right angles to the face. Yield is lower and it is more wasteful, but boards are much more stable, wear evenly and show straight grain or attractive ray figure in species like oak.
• Radial and combination methods: mills often mix methods to balance yield and stability across a log.

Specialty features such as flitches (matched consecutive boards) and burls (decorative growths) are recovered for veneers and feature panels.

Moisture content and why we season

Freshly felled green timber is full of water, often well over its own dry weight. Wood is hygroscopic: it gains and loses moisture until it reaches equilibrium with the surrounding air. As it loses water below the fibre saturation point, the cells shrink and the timber moves. Seasoning is the controlled removal of this moisture before the timber is used, so that the bulk of the movement happens before, not after, you build.

Moisture content is the weight of water in the timber expressed as a percentage of its oven dry weight. Equilibrium moisture content (EMC) is the moisture content at which the timber neither gains nor loses moisture for a given temperature and humidity. Furniture for a heated interior should be seasoned to a low EMC, often around ten to twelve per cent, so it does not shrink further once in the home.

Air seasoning

In air seasoning, boards are stacked in the open or under cover, separated by spacers called stickers that let air circulate. It is cheap and needs little equipment, but it is slow, taking roughly a year per twenty five millimetres of thickness, and it can only reach the EMC of the outdoor air, which is too high for indoor furniture. It is often used as a first stage before kiln drying.

Kiln seasoning

In kiln drying, stacked timber is dried in a large chamber under controlled temperature, humidity and air flow. It is fast, taking days to weeks rather than months, it reaches the low moisture contents needed for indoor work, and it can sterilise the timber against insects and fungi. It costs more in energy and equipment, and timber dried too aggressively develops defects, so kilns follow a carefully managed schedule.

Seasoning defects

When timber dries unevenly or too fast, the differing shrinkage of its parts sets up stresses that distort or crack it:

• Cupping: the board curves across its width.
• Bowing and twisting: the board curves or winds along its length.
• Checking and splitting: surface or end cracks as the outside dries faster than the core.
• Case hardening: a dry, hard shell traps moisture inside, causing problems when the board is later re sawn.
• Collapse: cells flatten under severe drying.

Recognising these defects lets you select sound stock, store it correctly and avoid building movement and failure into your project.