Timber being hygroscopic material it can attract or give off water molecules from surrounding environment. These materials will in fact, try to be in equilibrium for humidity with the surrounding environment. Timber resembles a series of small ‘hollow’ tubes while the tree is growing these tubes or cells are filled up with water.
In comparison with dry wood the natural wood content twice or double water. The moisture content is 200% more than dry wood in natural wood. After falling of tree this moisture or water content needs to be removed to make this water usable for articles or solid wood furniture. Care must be taken in removing water content from timber, rapid removal of moisture can damage the timber in its dimensions, strength, and quality.
Here are two basic types of wood seasoning – Air (natural) seasoning and Kiln (natural) seasoning.
Air(Natural) Seasoning –
In Air seasoning, timber is being stacked carefully outdoors in the natural air where we let the timber dry slowly in the natural environment. Though this a cost-efficient way of drying wood naturally, the duration for drying wood is been prolonged to one year per inch thickness of plank. Thicker the plank more is the time required to dry wood in natural seasoning. With the surrounding environment, timber dries over a period, but the moisture content varies with an environment which averages out around 16%. To make the joineries in finished product the moisture content needs to be lower than 16%, this demand the Kiln or artificial seasoning of timber.
The stacking of wood plays a key role in natural seasoning. Care should be taken to protect the stack of wood from adverse weather. Stacking wood above the ground and below the cover is one of the recommended approaches for air or natural seasoning.
Each layer of stack/planks is separated with skids or sticks between two layers of the stack, to allow the air to pass between the planks. These skids or sticks needs to be of the universal thickness and needs to be lined up on top of each other, to keep the planks flat and to prevent bowing or bend in wooden planks. Failing to pay an attention to thickness of the skids and the line up of stacks can force the timber to bow or miss-shape in its intermediate state. This results in increasing waste in manufacturing final product. The misshape of bow wooden planks is very difficult to judge for use in a final product. The natural bend in wood due to the exertion of moisture is different than the bend due to misalignment in its stacking. The bent wood is very difficult to get into its final stage of finished products. An artificial filling used while final finishing of finished products may not be sustainable in case of bent wood.
Kiln (Artificial) seasoning: –
A large building is built with a capacity to increase its temperature in a controlled environment to bring down timber’s moisture in a dedicated chamber. Steam and heat are used to increase a temperature in the dedicated chamber where the stack of wooden planks will be loaded. The steam and thus the humidity will then be slowly reduced of the kiln to allow timber to give up its moisture to bring it in an equilibrium with surrounding atmosphere. Once timber reduces its moisture to that of required, heat in the kiln is reduced to allow the kiln and timber particularly to adjust itself to reduced moisture conditions.
Failure to follow strict guidelines in kiln seasoning may have an adverse impact on the condition of the wood. For example, heating kiln chamber’s quickly using too much steam initially or allowing the wooden stack to cool down immediately will bring up stresses in the timber. These stresses are called as case hardening. This will bring lots of problem in wood which may result in loose joineries and uneven working conditions during precision woodworking and to maintain sustainable quality.
With characteristics of wood, Seasoning brings the dimensional changes in it through shrinkage and wrapping. Below are few of the strategies can be adopted in the preparation of wood for the manufacturing of joinery components