The object of seasoning timber is to get the sap out, according to carpentry expert P.J. Smith in his book, "Notes on Building Construction." By using seasoning methods, the sap can be dried up or extracted. Unseasoned timber can rot and decay, making it unusable for builders and carpenters. Seasoning makes timber lighter, so seasoning success can be judged by the weight of the wood. Wood is seasoned with air drying or kiln drying. An older method, less often used, is seasoning with water. The wood to be seasoned is exposed to the air, heat or water to remove the sap still in the timber.
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To air season timber, it is stacked in the open air. The wood must not be packed tightly, but have some room for air to circulate. Wooden or plastic piling sticks are built vertically into the stack to space wood, stabilise it and assure air circulation. The exposed ends of the boards are sealed with a sealer or cover so that they do not dry out too rapidly. The stack of wood is raised off the ground so that it does not rot on the bottom. The entire pile is top-covered to keep off moisture or particularly hot sun that might impede seasoning.
Kiln seasoning is done by forcing air flow over the timber using heat, blowers and fans. How much heat for how long depends upon factors such as the size and type of the timber, how large the stack of timber to be seasoned and the kind of heating system used. The two kinds of kiln drying are compartmental seasoning and progressive kiln seasoning. The first involves an enclosed container for the wood. The heat can be controlled and is applied to the wood until the desired reduction in moisture content has occurred. A progressive kiln brings the wood through heating chambers on trolleys. During this passage, the heat is adjusted, creating various atmospheres to create drying conditions.
Water seasoning is done by chaining a pile of just-cut timber underwater for a period of weeks until most of the sap washes out of it. It must remain completely submersed. When it is taken out of the water, it is laid out, dried off thoroughly and left in freely circulating air to dry. It is turned each day during the drying process. Timber seasoned this way warps and cracks less often than air- or kiln-seasoned lumber but can be brittle.
Deciding when wood is seasoned is the tricky part of the process. A rule of thumb is that it is finished when it has lost one-fifth of its weight if the lumber is to be used for carpentry. It should have lost one-third of its weight if it is to be used for building. Another way to look at that is to say that well seasoned timber should have a moisture content of below 20 per cent.
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