Tuesday, July 24, 2012

Beware the Falling Shake.

A majestic hardwood tree stands in a forest. For well over a hundred and fifty years it has been growing here, but now it bears a forester's mark. This phase of its life will soon come to an end.

The faller approaches, using his skillful eye and experience to determine the best direction for it to fall, based on wind direction, balance of the tree, the lay of the land, position of stumps rocks and other material on the ground. The silence of the forest is shattered by the cough and roar of the chainsaw as it comes to life. It's pitch intensifies as a big wedge shaped piece is first cut from one side of the trunk, about waist height to the faller. With the saw's engine idling irregularly, the faller moves around the bole of the tree to the opposite side from where the wedge was removed. He checks again the intended direction of fall, and lines up the saw on the back of the tree. The whine of the saw increases as the chain bites into the bark and sapwood. Chips fly as the bar gradually disappears into the tree. The skillful faller's arm muscles strain as he holds the saw, moving it in a rocking action and shifting the pressure. It is a skillful technique honed over years of practice. He has felled many hundreds of trees in his time.

Fibres at the base of the tree begin to pop and crack. The faller looks up, to check the direction of the slight movement of the canopy against the sky. He knows how long he can keep cutting - it's a fine line. The long bar is removed from the slowly widening kerf. He kills the engine as he quickly steps back away from the tree. As the popping and cracking of the tree's fibres increases, the faller continues to move away, to be clear from the huge butt kicking back and to be clear of falling branches. These unwelcome surprises, in the form of dead branches falling from above, are known in the trade as "widow makers". He lost a good friend to one of these a few years ago, and fleetingly remembers this as he quickly and carefully moves back through the undergrowth, looking up periodically as he goes. The movement of the tree speeds up as the remaining fibres around the base continue to give way under the huge forces at work. Gravity.

As the tree fell, it rotated slightly as a big branch in its crown hooked into the crown of a neighbouring tree. This slightly altered the path of the falling tree, swinging it just a little to the left as a loud tearing sound sent several huge limbs crashing to the ground. The faller cursed under his breath as the massive trunk fell across the stump from a previously felled tree. The enormous trunk kicked back from it's bole several metres as a shower of leaves, bark, and branches rained down on the forest floor over a wide area. The thud and crash eventually came to an end in a cloud of dust and debris. It was all over. The faller and his offsider approached the huge trunk and fired up their big chainsaws, cutting the big trunk into sections suitable for snigging to the landing and transporting to the saw mill. The crown and its branches would remain on the ground, to begin decomposition and be burnt in future fires - the resulting ash-bed providing nutrients for the seeds which would germinate and join the race to fill the void in the forest's canopy above.

However, the heavy impact of the tree crashing onto an uneven surface would have as-yet unseen consequences... 

What is a Shake?
A "Shake" is the term used for a split or fracture, the separation of fibres which has been caused by reasons other than shrinkage. Differential shrinkage causes "Checks" - the shallow separation of fibres along the grain. "Splits" are the term given to the gaps or cracks which go right through from one surface to the other due to shakes or checking.
Shakes can be caused by a number of factors, but here we are looking specifically at "Falling Shakes", also called "Felling Shakes". These are caused buy the rupturing of fibres through the massive shock forces which can occur in the falling of the tree. In the case of our forest giant above, the tree's trunk fell across the stump of a previously felled tree. The resulting forces can create internal fractures within the log, which at times are almost invisible.   

A falling shake in a piece of New Guinea Rosewood
The picture above shows a Falling Shake which extends right across the piece of wood as well as diagonally. It would easily snap on the fracture lines.

This Falling Shake is like a set of stairs across this piece of Jarrah.
 The problem with most Falling Shakes is that they are not always obvious nor are they always visible to the inexperienced eye. A perfectly good looking beautiful stick of timber can hold surprises.

I built a lot of Campaign Chairs over about a decade, each of which had 4 turned rails at 30mm (1 1/4") diameter. The tenons were tapered and housed into tapered holes in the legs. During that period, I had two chairs which collapsed under the weight of a person sitting heavily into them. Investigations revealed that there had been a Falling Shake in each case - one near the point where the tenon entered the leg, and another  midway along the rail. The former was a Jarrah rail, and the latter was a Sheoak rail, in two different chairs made in different batches, a few years apart. Of course I replaced the rails for free, but was intrigued that I had missed these Shakes in all the handling, machining, turning, sanding, polishing, and assembly.  Of course, that had been a couple of other Falling Shakes that I had found. These rails ha been discarded or recycled.

Another example in Jarrah, with the Falling Shake running through a tiny rot pocket.
The two photos of jarrah above were found by my recently when I was machining up recycled jarrah to build a big cabinet. These larger sticks had formerly been part of the construction of a building. Interestingly, Falling Shakes can hold together pretty well in many situations, as they do not always go right the way through, and the fracture often weaves an interesting path through and across  the fibres. However, they can also be a nasty surprise just waiting to happen. Shakes create a significant weakness in a piece of timber.

The moral of the story? ... Beware the Falling Shake. So keep a close eye out for them.

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