Forest Products Notes on...

Drying Defects in Sawn Timber (causes and prevention)

Seasoning or drying defects are second only to knots in reducing the quality of processed timber.  Once timber has dried below the Fibre Saturation Point (FSP) which is about 28% moisture content, the wood begins to shrink.

Shrinkage of wood is anisoptopic.

Shrinkage tangential to the growth rings is greater than in the radial direction, which in turn is much greater than in the longitudinal direction.  Furthermore, shrinkage in the outer parts of the wood will occur sooner than in the inner regions.  Due to this difference in rates of shrinkage, stresses build up in the wood.  Such stresses can lead to distortion, or rupture of the wood, or may   remain unnoticed until the wood product is further processed.  With high temperature kilning there is potential for these stresses to be relieved as the wood is able to flow to some extent.  The amount of shrinkage depends upon the anisotropic nature of the material and upon the way in which the drying was controlled.

Defects Arising from Shrinkage Anisotropy (“warping”)

Twisting

This drying defct is caused by anisotropic shrinkage acting on spiral grain or areas where there are density variations in the wood.

Twist in a board

Weighting of the stack can help reduce the occurrence of twisting.

 

Cupping

This defect arises due to the effect of growth ring orientation: - one face of a board is more “tangential” (and therefore shrinks more) than the other face. This is a problem if a log is “flat-sawn”, not if it is “quarter-sawn”. Case- Hardened boards will also cup if re-sawn.

Cupping of a board: this defect can be reduced by ensuring correct piling of the stack and weighting the stack.

Diamonding

In the diagram below AD (being more “tangential” than BC) shrinks more than BC (being more “radial”) and therefore the cross-section distorts:-

Diamonding: AD has shrunk more than BC

Bowing

This is a warping “end to end”. It is caused by stickers being too far apart (too few stickers are used), or if reaction wood is present in the board:

Bow in a board

Spring (Crook)

Here the board remains flat, but bends. It is caused by the release on sawing of growth stresses which occur in large trees.  Spring can be reduced by careful stacking.

Spring

Dead knots

Knots which are not integral with the surrounding tissue (often surrounded by bark). These knots fall out or protrude during the drying process and may snag in machines on further processing.

Good practice to reduce the defect of warping of timber

· Ensure only boards of uniform dimensions are incorporated unto the stack.

· Ensure the stack has good foundations and close spacing of bearers.

· Ensure stickers are spaced correctly, of the correct size and vertically aligned.

· Apply weights to the top of the stack if necessary.

All the defects in dried timber given above are related to the anisotropic shrinkage of wood. Other drying defects can arise however....

Drying Defects in Timber not Primarily Associated with Anisotropy

Stresses which develop during the drying process can lead to “creep” (or “flow” of the material), case hardening or rupture of the wood.

ONE OF ARTS OF SUCCESSFUL DRYING IS KNOWING WHAT DEGREE OF “BAD TREATMENT” THE SPECIES CAN SUSTAIN WITHOUT DEFECTS DEVELOPING.

Ruptures

In the timber trade ruptures of the material have specific names:

Checks

These are superficial ruptures which do not go right through the board. The incidence of checks can be reduced by end-coating (using flexible, impermeable paint) or shading the end of the board from the sun using a sticker.

Splits

Here the defects are ruptures that do go right through board from one face to the other.

Shakes

These are ruptures in large pieces of timber (e.g. logs). Shakes can also be caused by bad felling techniques, although it is believed that trees grown on certain sites are especially prone to this drying defect.

Ruptures: Top: Left, a check in a board. Right, a split. Bottom: Left, ring shake in a log. Right, star shake.

The NEXT PAGE of these notes describes another, often misunderstood, yet very serious drying defect in sawn timber is Case Hardening of Timber...

Twist in a board
Cupping of a board.
Diamonding: diagonal AD has shrunk more than diagonal BC
Bow in a board
Spring defect in dried timber board
Ruptures: Top: Left, a check in a board. Right, a split. Bottom: Left, ring shake in a log. Right, star shake.
Image of Solar Kilns for Drying Timber Page
Image of Case Hardening of Timber Page

Read about the“low tech” approach of using Solar Kilns for Drying Timber on the PREVIOUS PAGE...

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