Moisture content of the wood
The moisture content of the wood is very important thing in the woodworking industry. Wood is sourced from the tree which is contain wood as it main ingredient when it alive. The wood has to be dried until most of its water is released to get its maximum strength. The wood drying is considered the most important thing in the woodworking.
Freshly cut wood has very high moisture content, after it is left for some time the moisture content in the timber will decrease until it reaches equilibrium with humidity of his environment. This process can happen naturally, but it takes long time to get the wood dry. In the process of furniture production, to speed up the drying time of wood we use oven or kiln dry. The wet wood are cut and placed in wood oven or kiln to faster the drying process. In the kiln dry the hot and dry air is pump and circulated to attract the water to release from the wood. Out from the oven the moisture content wood usually ranges between 5%-7%. But when the wood pieces is released out form the oven, then the moisture of the wood will change according to its environment. The wood is hygroscopic substrate, when it is placed in the humid room, he will absorb water from his surrounding until get equilibrium with the environment. Meanwhile during its moisture adjustment the wood will shrinkage or swelling. The wood movement is creating many problems such as crack, split, open joint or change in the dimension of the wood product.
The wood movement can be minimized by layering finishing materials (coating sealer and lacquer) to close the wood surface. But it must be done when the wood MC is already meet the standard should be. The application on both side is needed to keep give protection for the wood panel. The sealer or top coat will cover the pores and grain and prevent the change of the moisture content of the wood. However the movement of wood still could happen in finish wood. The finishing layer most time is not enough to block the wood from his environment. The best way to make wood furniture is to control the moisture content in the wood. The moisture of the wood should be set at the range at which he is in the equilibrium with the humidity environment where the wood product will be used.
Environment humidity vs moisture content of the wood
The relationship of wood moisture content with air humidity environment can be seen on the graph above. So to get the ideal moisture content we need to get data about the humidity at the area where the wood product will be used. Then after we get the humidity range we can go to the graph to find the moisture content to the wood that is considered safe for the product. Generally the moisture content of the wood for the products that is marketed to "dry" country such: as US and Europe is in the range of: 6% to 12%. While for high humidity area such as in Indonesia we can go with the higher moisture content (10% to 15%).
For the woodworking products that are used in the same area with the woodworking process, usually the problems of moisture content is less happened. After being dried, the moisture content of the wood will get fit with its environment. But if the wood product are made to be marketed to other region, especially when there is a different between the average humidity of both places, then difference will make the production process to be more complicated. Some special facility (dehumidifier room) may be needed to control the wood products to fulfill the moisture content standard in the destination of the wood product.