Thursday, 29 April 2010

Reaction Wood

Have you ever been working or turning away on a piece of wood and thought “Something’s wrong here, perhaps I’m not in the right mood or maybe the tools need a touch up”?  The tools are sharp when you check them but you’re struggling  to get a decent finish.

If so then the chances are that you are trying to work a piece of REACTION WOOD and quite often you will only realize this after the piece has already proven troublesome.

Reaction wood is wood altered as a response to a lean and is present in every branch. It differs greatly from normal wood.  In hardwoods, cell walls thicken on the upside of the lean or top of a branch.  An off-centre pith is a dead giveaway.

This reaction wood is called tension wood. Tension wood forms above the affected part of the tree, pulling it up. It has a high cellulose content, which means that the fibres will lift quite easily and the wood will be more difficult to turn or work.  When fibres lift the wood takes on a wooly texture and tends to produce a fuzzy surface especially when worked green.

To make matters interesting reaction wood in conifers (softwoods) forms thicker growth rings with thicker cell walls on the down side of the lean or underside of a branch.  In conifers it is called compression wood and is rich in lignin.

In an ideal world we would be working nice straight-grained, knot-free timber which had been harvested from a forest where the trees are evenly spaced, drawing each other up, forming a closed canopy and shading out lower branches.  When I was a tree surgeon and starting out green woodworking I used mainly wood from gardens and often struggled with bits of branchwood and leaning trunkwood until I realised what was going on.  I remember well one particular batch of Horse Chestnut (Aesculus) which was like trying to turn a marshmallow!  The bottom part of a hardwood branch (called, I think, opposite wood) actually works quite well but there is a natural tendency to go for the bigger ‘half’ - chuck it on the firewood heap.

Of course branches, forks and crotches with natural bends are appealing to use for spoon carving - you just have to work out which bit is best and struggle and sharpen the tools.

I am eternally grateful to the late, great Dr Alex Shigo for this knowledge.


Sean Hellman said...

I have also found that reaction wood and branch wood goes banana shaped when dried out. Have you found that a straight spindle will curve when dried?


Yes Sean - it can move in most mysterious ways!
Things can also go banana shaped if you use a high proportion of sapwood to heartwood in your billet. Nice for 3-legged stools if you can predict it but you have to shave the tenons when it's dry rather than turning.

Luke Townsley said...

Interesting explanation. Thanks!