Thursday, 9 December 2010

Growth rings

Hardwoods and softwoods, how growth rate determines strength.

I find my wood in all sorts of places. I am no purist and have used a lot of recycled wood in my time. Whilst getting some large boxes from a white-goods outlet I also helped myself to some wood in their "free wood" bin, it was 1 inch square by up to 2 foot long and was most probably packing material from wood grown in a cold climate. This is almost perfect; a wee bit small for fan birds but is a dream to use. The feathers can be rived out really thinly, and I am sure that very long feathers would not be a problem either. The right material does make all the difference. Ash and hazel are great but they do give problems with riving feathers as it does take more physical effort and can be tiring. The fan birds I have from Russia are made from a very slow grown softwood, and the craftsmanship is amazing.



 Recycled pine at about 50 rings per inch



The picture above is taken with my USB microscope, the graduations are 1milimetre apart. We are looking at about 50 annual growth rings to the inch, so if this tree was 10 inch diameter then it would be around 250 years old. At its slowest there are 4 to 5 rings to the millimetre! In England we are lucky to get any softwood with more than 7 rings to the inch. This is because we have a long growing season, we do not get that cold and have reasonably good soils. In this part of the world softwoods are not for fine furniture or other crafts or artefacts; and to do a fine detailed carving from Western Red Cedar or Douglas fir is unheard of, we use them for chainsaw carving. In the states and Canada these woods are used extensively, and they have very different qualities to our wood as they are slow grown. I used to make Native American style flutes and would never use our native woods, it is just not possible, but in the Americas they are superb, their nice tight grain takes the detail very well.




Recycled pine fan bird, from the above wood

Rived feathers before they are fanned out, from ash. All feathers need to be the same thickness.


So, if there is anyone out there that can supply me or point me in the right direction of very slow grown softwoods, as near the Arctic circle or the Baltic regions or even from Canada, I would be a happy person. That is: tangential face planks that are straight-grained and knot free. Other names for the tangential face are flat-sawn, flat-grain, plain-sawn or even slash-sawn.

So in the first picture the darker rings are the latewood and are harder than the early wood growth which is the bit that varies in width depending on how good a growing season was. So the more growth rings to the inch, the higher proportion of late wood rings, and the stronger the wood is. The earlywood is soft and will absorb stain more readily, it also sands away quicker, so you can be left with an undulating surface with the latewood protruding.

In ring-porous hardwood, that is woods such as oak and ash which have large pores concentrated in the early wood and smaller pores in the latewood, the opposite is true. The wider the rings, the stronger the wood. So, do not make chair legs or handles from ash with 15 or more rings to the inch, they can easily be broken. At the APF this year we had for the competition, a load of ash; some of which was very slow grown, more than 15 rings per inch, and making fan birds out of this was easy. The wood was not hard but the more growth ring to the inch the more likely the feathers would snap off when bent. My ideal ash wood is 3 to 6 rings per inch for chair and handle making.

Fast grown English ash at about 3 rings per inch, the earlywood has the large pores


Slow grown English ash at about 20 rings per inch from the APF

I have not been able to find any relationship in diffuse porous woods between rings per inch and strength; that is, in woods that have pores more or less the same size throughout each growth ring. Can anyone enlighten me on this; how do you tell if you have a strong or weak plank of diffuse porous wood?