for Students of Woodwork
Shrinkage is a defect, and generally a natural defect, occurring during the seasoning process.
When timber is seasoning and it's moisture content (MC) is reduced below the Fibre Saturated Point (FSP) continued drying will cause dramatic change such as increase in strength but also distortion and shrinkage.
Shrinkage is the greatest tangentially over the radial direction with little loss along the length of the board, etc.
Because of this varying shrinkage rates tangential boards tend to cup because of the geometry of the annual rings shown on the end grain. It can be seen that some rings are much longer than the others close to the heart. Therefore they will be more shrinkage at these parts than the others ~ cupping is the result.
Tangential cut boards swell out from the heart and the opposite side cups towards the older wood.
Radial boards shrink more or less across the thickness of the board with a bias on the older wood on the outside. Such boards are more stable.
In square section timber cut from the same place, 'diamonding' is the result.
Knots are the result of the trees attempt to make branches in the early growth of the tree. They are the residue of a small twig, shoot, etc. that died or was broken off by man or an animal in the wood or forest. The tree subsequently continued its growth over this wood.
The knot may be live, sound, or tight or if it has become separated and is contained in residue of bark, dead.
Dead knots become loose and downgrade the appearance and stability of the board. Most grading systems uses the amount of knot area as an indication of its quality. The more knots the less the quality.
Dead loose knots are extremely dangerous to machinists. The cutters may pick these up and eject them rapidly towards the operator.
A separation of the wood fibres along the grain forming a fissure that extends through the board from one side to the other.
It is usual in end grain and is remedied by cutting away the defected area. All boards should have an allowance so that some end grain may be cut away because of possible shakes or splits.
Checks and end checking
A separation of the fibres along the grain forming a fissure which shows up on one face or at the end grain but does not continue through to the other side.
Wind or twisting
Spiral or corkscrew distortion in a longitudal direction of the board.
Due to the board being cut close to the centre of the tree which has spiral grain.
The board is of not much use but small cuttings may be obtained from it with careful selection.
Bowing is concave/convex distortion along the length of the board.
It is a seasoning and or storage defect caused by the failure to support the board with stickers at sufficient intervals. The boards own weight and probably those above it bears down and the resultant bow is inevitable.
This defect can and should be avoided by careful use of stickers supporting the board at the correct width.
Spring is concave/convex distortion along the length of the board again but this time the distortion is in the flat plane of the board.
Boards with this defect may have been cut from near the heart of the board and is the result of growth stresses being released on conversion.
Useable timber may be recovered from these boards by cutting a straight edge and re sawing. The grain direction however may not be satisfactory for aesthetics and care should be taken for placing the possible short grain figure where stability is required.
Shakes are separation of the fibres along the grain developed in the standing tree, in felling or in seasoning. They are caused by the development of high internal stresses probably caused by the maturity of the tree.
The shake is the result of stress relief and in the first place results in a single longitudal crack from the heart and through the diameter of the tree.
As the stress increases a second relief crack takes form at right angles to the first and is shown as a double heart shake.
Further cracks are known as star shakes and show the familiar pattern shown.
Ring or partial cup shakes in the form of longitudal tangential cracks occur as a result of high radial tension. It is often said that it is caused by an early frost freezing the rising sap or perhaps a heavy felling on hard ground.
External radial cracks are caused by the tree laying too long before it is converted and seasoned.
reference sources: H.E. Desch & J.M. Dinwoodie ~ Timber text and graphics © G.Malthouse ~ all rights reserved
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Last uploaded 30th September 2009