[ soundtrack : Jussi Björling - Tonerna (1957) ]
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Spoon-carvers say “Wood is a living material.“
Woodworkers say “Wood is a hygroscopic, anisotropic material.”
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A living tree is full of water. It is in the business of drawing it up from the earth and, powered by sunlight, combining it with carbon dioxide to grow itself. Photosynthesis — the basis for all other life.
Woodworkers are mainly interested in the wood that makes up the trunk or bole of the tree. The sawmills keep the timber wet because it requires less energy to saw and plane ‘green wood’ as it’s called. After the log has been cut into beams, planks and boards it is either dried in kilns; large climate controlled chambers that gradually reduce the wood’s moisture content, or the stickered stacks of lumber are allowed to air-dry. Drying lumber is truly one of the dark arts and doing so well is a difficult and valuable skill.
Wood is mostly made of carbon (50%), hydrogen (6%) and oxygen (44%). These three elements combine to make the two basic components of wood: cellulose and lignin. Wood is composed of several different types of cells. The cellulose molecules make up the cell walls — the wood fibres — and the lignin is the glue that adds stiffness and sticks everything together. It’s convenient to think of the most common wood cells as long, hollow vessels, like drinking straws, with needlelike, closed ends. See where wood gets its splinteriness from?
Wood will always contain a certain amount of water — unless we force it all out by heating and drying. In a living tree, or in green wood, this water is absorbed into the cell walls (bound water) and it makes them swell in size. Water is also contained inside the hollow cell (free water) like in a tiny bottle. This can be schematically illustrated like this:
When we dry wood the moisture will evaporate from the outer surfaces, setting in train a movement by diffusion from the wetter interior to the perspiring surface. The water molecules move from cell to cell, passing through pores in the cell walls until they can combine with the outside air and float away. At first, it is the free water inside the cell cavities that is most easily moved out and away. When all the cell cavities are emptied of free water, the cell walls are still saturated:
This condition is called the fiber saturation point (FSP). The amount of water left in the wood at the FSP varies from species to species, but as a rule of thumb we can reckon with a FSP at 30% moisture content (MC). (There are lists and tables of the precise average values for specific wood species in the technical literature.)
It is only when we continue drying below the fibre saturation point that the wood will start to shrink in size. As the moisture in the cell walls (the bound water) is released, the wood will shrink and harden.
So … wood is a hygroscopic material: it will both absorb or give off water as it strives to balance its moisture content against the surrounding air (as explained in my previous post). What’s more; it will change dimensions as it does so. Here’s the kicker: Wood does not expand and contract equally in its three dimensions. It shrinks most around its circumference, quite a bit from the bark to the centre and hardly at all along its length. Wood is an anisotropic material. This is why it cracks as it dries.
The numbers above are rule of thumb averages and the percentages express maximum dimensional shrinkage as wood dries, i.e. from ‘fibre saturation point’ (ca. 30% MC) to ‘bone dry’ (0% MC). T = tangential, R = radial, L = length.
This uneven shrinkage causes the various lengths of lumber to deform in cross section as they dry, which can be illustrated like this:
This is how wood is a hygroscopic, anisotropic material.
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The above descriptions and explanations are basically correct, but also ridiculous over-simplifications. They are just intended to convey the fundamental properties of wood in relation to water, and perhaps whet your appetite … this particular rabbit hole is deep and leads to all kinds of interesting stuff!
Next lesson will show how to use these insights in practical woodworking.
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This is very helpful in bridging the gap between tree > log > drying > milled wood.
I'm having great fun watching two boards of 2 inch thick oak dry in my living room. When I cut the tree down they came from a large limb full of tension and compression wood. When I milled them they were about 40% as they'd been on the ground for a year, now they're at about 25-30% and cracking and spliting. It is fascinating to watch and I hope they'll hold up as boards, once they're down to to 20% I'm going to add some bowties so I can retain them as a show surface for nice log store for the house.
I look forward to your next installment!