Green, air dried and kiln dried timber.

Timber can be bought in 3 states of dryness; green, air dried and kilned dried.

Green wood is unseasoned and has the highest water content; this is the state of the wood when a tree has been felled. Green wood is then taken to the saw mill to be processed into planks. These planks are then very carefully stacked up with stickers between each plank so there is a ¾ inch gap for the air to get in between the planks.

If you were to buy green wood off the sawmill it is highly likely to split, warp and twist as it dries out. Wood needs to be stacked or weighted down as it dries to stop this movement. Green woodworkers such as pole lathe turners and even chair makers can make items from green wood that do dry out in use but do not fall apart or split in use. This is specialised knowledge and you do need to know exactly what you are doing.

Air dried wood is green wood that has be stickered up in large stacks in an airy place but protected from the rain falling on the top of the stack. The stickers are usually placed from 16 inches 20 inches apart across the length of the planks so the air can circulate between the planks and the moisture in the wood can evaporate. The stickers are placed directly above each other in an orderly way. If the wood did not have these air gaps in between them, they would take far longer to dry out. Fungus and mould would grow because the moisture can not escape and your timber would soon be ruined.

It is said that it takes 1 year for every inch thickness of the plank for it to be aired dried, but this does not take into account the variation between species or atmospheric conditions. In general low density woods, such as lime, are easier to dry that high density woods, such as oak. This wood is now dry enough to be used in any outside project. If you want to use air dried wood inside your home you will have to bring it indoors and let it continue to dry until it reaches equilibrium. There can be no hard and fast rules here as different woods dry at very different rates.

Kiln dried wood is stacked in the same way as air dried wood but it is then put in a large air tight container to be dried by the use of heat and dehumidifiers. This process can be as short as 6 weeks depending on the type of kiln. Kilning wood takes the moisture content down to about 8% and this wood is used for interior projects such as fine furniture, doors and windows. It is important that if you buy kilned wood that you do not leave it outside or in a damp place as wood is hydroscopic and will absorb moisture from the air. It is also pointless making garden furniture from kilned wood as it will cost more than air dried wood because of the capital costs of the kiln and the energy needed to dry the wood.

Sean Hellman shows at woodfairs in the South of England, click here to find a list of woodfairs

Copyright Sean Hellman 2007