Turning Archive 2004
Bob Smalser, Seabeck, WA
>“Hi Bob, Could you possibly create a post explaining why and how tight ring counts
improve lumber qualities? I know that tight ring counts are preferred, but why is it better?”
My pleasure…it’s really quite simple, looking at a close-up of the growth rings:
In early spring the wood grows quickly with large, thin-walled cells…”earlywood”…seen as the lighter colored portion of the growth ring. As the season progresses and growth factors such as light, temperature, nutrients and water become more limiting, the cells get smaller, thicker-walled, denser and more resinous…”latewood”…seem as the darker portion of the growth ring. You can actually see the cells get smaller in the photo as they darken. Smaller, denser cells with more natural resins are stronger…and a higher proportion of them in the wood makes for stronger and more durable timber…significantly stronger and more durable timber.
Even in the tropics there is usually some limiting factor to create latewood…like an annual dry season…and it is the rare tree that doesn’t show some sort of growth ring.
But it also helps to understand something of how the tree grows…many folks think of growth rings in three-dimensional form as cylinders, but they are really cones as shown above. That’s useful to understand when ripping bending stock from wood originally milled from a cant as most sawmills do…there is almost always some grain runout in straight, sawn boards that should be identified before selecting stock for bending. That’s why riven or split boards like the Vikings’ early lapstrake planking are almost always stronger than straight boards, and for most store-bought wood, boards milled from nearer the pith will have less grain runout.
There are some sawmills that can compensate for log taper without making a cant, taking their waste from the pith rather than the slab…the Lucas and Peterson swing-blade mills come immediately to mind. If you can find a local sawyer using one and explain your requirements, you can obtain higher quality bending stock.
Wood is composed of hollow, spindle-shaped cells of cellulose cemented together by lignin and arranged parallel to each other along the trunk of the tree. Trees grow by adding cells in the (again, cone-shaped) cambium layer immediately beneath the bark…the trees’ existing cells do not grow larger. Thus new wood is laid down atop of old and the diameter of the trunk increases…stretching, cracking and sloughing off the bark as the tree grows.
The wood immediately beneath the cambium layer …the sapwood…stores and transports the tree’s water and nutrients. As sapwood contains much water and minimal resins, it is never rot resistant…even in rot resistant species…and is undesirable for use in boats. Generally, the faster the tree species grows, the higher percentage of sapwood…yellow pines from modern plantations take decades to develop heartwood and today’s SYP lumber generally contains no heartwood. Black Locust is an exception…a fast-growing pioneer species that generally only contains a half-inch of sapwood and high rot resistance and strength in its heartwood…hence an excellent boatbuilding timber to propagate.
Heartwood consists of dead, inactive cells that no longer transport or store food and water. The transformation of sapwood to heartwood is accompanied by a general loss of water and a dramatic increase in the “extractive” content…the resins, tannins, gums, oils and minerals that give heartwood its distinctive darker color and in some species resist rot.
Most woods like also have “rays”…which are horizontal cells that transport water radially across the grain. In White Oak these cells are larger and more conspicuous, especially in quartersawn wood. “Tyloses” also occur in some hardwoods like W. Oak, ash and hickory. These are ingrowths that clog the sapwood’s pores as it transforms to heartwood and prevent water transmission. This makes White Oak excellent for boats and cooperage, but also prevents the penetration of preservatives in the wood, so there is a trade off. Softwoods transmit water differently than hardwoods…they transmit water from longitudinal cell (tracheid) to longitudinal cell and don’t have pores…. that’s how you tell a “softwood” from a “hardwood” botanically, [i]not[/i] from the leaves. The Ginkgo or Maidenhair Tree in your local park is certainly a broad-leafed tree…but it also has tracheids and is really a “softwood” botanically.
As a practical example above, here’s a Doug Fir log in my stack yard recently harvested from a neighbor’s farmstead. The tree above is what we call locally a “mini-old growth”. It was a seedling in 1900…well before the area was first selectively logged in 1936…yet it is only 21” in diameter. You can see that as a seedling, it had a tough time competing with its larger forest neighbors, and there are 20 rings to the inch until its larger neighbors were thinned in 1936. Then with more, but not full sun, the tree grew at a rate of 8 rings to the inch…and you can see those rings tighten up some as its neighbors also shot up…until the area was completely cleared for a farmstead in the 1970’s, where it got full sun and grew at an even faster rate of 4 rings to the inch.
In contrast, its close neighbor was also the same size…around 21”…yet this tree didn’t sprout until the around 1975 when the farmstead was cleared, and has spent its entire life in full sun…3 rings to the inch.
There’s little doubt which log will become boat framing and which log will become studs.