Wood Shrinkage

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Wood Shrinkage

Fibre Saturation Point In Wood

Water is present in wood in 2 forms. These forms are bound water and free water. Water which is absorbed by the wood and held within the cells is called bound water. Any water that is in excess of the fibre saturation point is called free water.

Fibre saturation point (FSP) is the moisture content of wood where the cell walls of the wood are completely saturated with bound water, but the cavities of the cells are devoid of any free water. This fibre saturation point is usually expressed numerically of moisture content in wood. The numerical range is usually between 25 - 30%.

Sap within living trees is mostly water with just a few small amounts of dissolved nutrients and minerals. Wood within a living tree is always in a condition above the fibre saturation point even though the moisture content level in that wood can vary.

When the tree is harvested and then the wood seasoned for use all the free water and some of the bound water is dried away. Bound water is never lost until all the free water is taken away.

Wood moisture meter

Concrete moisture meter

When working with wood an understanding of the reason it shrinks, moves and the direction the shrinkage is likely to take is essential. One has to bear in mind as well that in these modern times there is an increased usage of artificial heating which contributes to drier atmospheres and increased shrinkage within wood. It doesnt matter on the age of the wood as all wood is affected. It is not the heating which is the problem, but the drier air which is created by the heating.

When you think of wood shrinkage and the direction it takes, you should visualize a tree as a compact cylinder which has many smaller cylinders inside it. When a tree does dry out shrinkage will occur in every direction except for the length. The circumference of the tree will shrink and all the inner cylinders will shrink and tighten up, becoming more tightly packed.

Shrikage factors along the length of wood can be ignored in most case as the shrinkage is only fractional. You need to concentrate on tangential and radial shrinkages where the shrinkage can be substantial. A huge problem here is that the most shrinkage in wood takes place between critical moisture content of dried wood and fully conditioned wood. Just looking for oak as an example can highlight this issue. When oak is dried down from a 20% moisture content level to 12% content level it will shrink by approximately 2.5%. A low number you may think but a substantial change in size!

All woods shrink when dried which causes internal strains and stresses. The results can be artificial defects to the wood. Usually a wood structure has natural elasticity and any stresses are distributed evenly. But if the wood structure lacks elasticity or incorrect seasoning is employed, the wood will distort in various forms.

Cupping - Here a rounding on the underface takes place.

Bowing - The wood curves and takes on the shape of a bow.

Springing - Also known as edge bend. The wood here stays flat but bends edgewise.

Twisting - Here the long egdes are straight and the diagonals are curved.

Splitting - wood will begin to split if subjected to rapid drying.

Flaking - The surface of the wood lifts in many small flakes creating a rough surface.

Case hardening - If wood is dried too quickly in a kiln then the surfaces will dry out quicker than the body and create a hard casing with a soft inner.

Honeycombing - If you dry case hardened wood in a kiln then the interior will dry out but any moisture will be locked in the outer case causing a honeycomb effect.

Moisture Meter Guide 2010 Contact details:garnett65@hotmail.com

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