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a) Explain briefly why it is necessary to season timber.
b) State a suitable moisture content for joinery.
Seasoned Timber
Timber that is well seasoned is lighter and more stable, this means it is less likely to shrink warp and split in use.
A practical example of the effect of shrinkage of unseasoned timber in use is if we manufacture and fix say architraves when the moisture content is too high and then after installation the timber dries to the equilibrium moisture content (emc) in-situ. As the timber shrinks across the board it will gave an opening effect on the mitre joints. Anyone who did not understand this shrinkage effect would comment that the mitres were poorly fitted. In fact what would be poor in this situation is cutting and fitting the mitres before the timber reached the emc. Consider second seasoning for ways to deal with this shrinkage effect.
Moisture content in use
It is desired to specify a moisture content (or a range of moisture contents) for timber to be used in most parts of buildings. With such constructional timbers as rafters and joists, moisture content can be fairly high when the wood is installed, as it will dry out in-situ without the resultant shrinkage causing any serious trouble. Interior joinery on the other hand, is a case where correct moisture content is important if ugly open joints, splits, or worse troubles are to be avoided. This is particularly true in air conditioned buildings where the moisture content should not exceed 10%. For normal heating in private houses 14% would prove satisfactory for the joinery work. For structural members and carcasing timber 22% is acceptable, provided the timber is capable of reducing its moisture content in-situ. For timber which is to be painted or varnished moisture content should not exceed 16%.
The chart on the left shows the ratio of dimensional change 'shrinkage' to the rate at which the moisture content reduces. Reference to Shrinkage and Warping will show the effect on timber of the differential shrinkage (radial shrinkage Black Line) and (tangential shrinkage Red Line) where as can be seen the tangential shrinkage is twice as much as that of radial shrinkage.
REMEMBER: Fungi require water to grow, so the moisture content of timber is of -prime importance to its survival. Normally dry seasoned wood is attacked by insects but is usually immune to fungal attack. - FUNGI WILL ONLY ATTACK WHEN MOISTURE CONTENT EXCEEDS 20%.
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