Wood as a living material

Our resident expert on the organic properties of wood, Don Pridgen, weighs in.

Using wood as a construction material is a particularly beautiful thing. Unlike modern man-made materials, wood will continue to move and react to natural conditions long after the living tree has fallen. In fact, the natural events that the living tree experienced in its lifespan can often be seen at the sawmill, through the drying process, and in the final timber product. As craftsman with wood as our medium, we are always conscious that wood is a living material and how this affects the lifetime performance of our products. In fact, the living nature of wood should be celebrated.

The following is some PhD level stuff, based on experience, with a very real and applicable perspective...

Wood will always reach approximately equilibrium moisture content with its surroundings. It cares not how it arrives at that point. The kiln is about time and control of the conditions. A kiln speeds things up and can also lower drying losses from cupping and checking. Ambient conditions can be harsher than kiln conditions. Today we were working in low relative humidity with a breeze and in the sun. The green white oak sills we were working with were actively checking badly. A kiln would have been running a higher relative humidity (%RH) at that point to avoid drying the surface so aggressively. I've ruined refractory species in the first hour off the mill on a hot dry breezy day when not paying attention. More drying loss is initiated above 40% than below.

Colonists seasoned tool handle stock by the fireplace, roughing and then final shaping as the wood approached final moisture content. Instrument makers would have a deep supply drying for years prior to use. Furniture makers would also dry for longer periods than we generally find convenient. Part of that was the master/apprentice form of education, the apprentice would inherit the master's stock. I've read that Stradivarius was using wood that had been seasoned for 20 years. Timbers for Timber Framed (TF) buildings of higher importance like churches were dried. Those for buildings of lower importance were erected green. In building a fine home in the early industrial age the sawmill cut the framing and trim. The framing was erected green and the house typically took 2 or 3 years to build, the interior wood was dry by then. The lath and plaster grounds were nailed to the frame, the grounds surrounding the rooms and openings were set straight, level and plumb then the plaster floated out the plane hiding the irregularities in the framing. Since WWII we have fast tracked construction and now work with straighter framing and sheet goods that are less forgiving of warp. Previously, the pace of work was slower and more in tune with those slower natural processes. I tend to think they were more accepting of wood movement and also didn't ask "are we there yet?", they understood better, without instruments, that it was ready when it was ready.

Earlier in my career,  when I worked with thousands of feet per day I developed a feel for the moisture. My hands knew to a pretty tight range what the moisture content was. The sustaining ring of dry wood. We use various crutches now to help assure satisfaction among a less forgiving clientele. The kiln can set pitch in softwoods and kill insects and their eggs.

That moisture meter of mine came about because I knew I was heading for court. I was involved in 2 lawsuits over green materials. One settled. The second the green materials supplier tried every way possible to blame me and then my crew. I have no real desire to work green and only do if the client is fully understanding. Nothing will induce me to attach my name to green interior work. Exterior is really disposable so I can go either way there as long as the client is aware of impending movement and understanding of that. Our notions matter far less than our clients, never lose sight of that. If wood movement excites you make sure it excites them.

I have 2 pieces of wood here who's constant movement I thoroughly enjoy. One is the lightning struck tree out back. It ripped loose about a 4' thin strip of wood. On a wet day the strip hugs the tree, on a dry day the loose tip is a foot away from the tree. The second is a deck board that has juvenile wood on the upper face and more mature wood on the lower. As the juvenile wood shrank lengthwise it sheared 4 screws in the first 2 joists. On a wet day the deck board lays flat as the juvenile wood expands the length of that face. On a dry day the juvenile face shrinks in length while the normal wood on the underside does not shrink lengthwise and the board lifts 6" off the deck. The only thing in the normal range that causes wood to move is moisture change. Temperature and sunlight only cause movement because they are driving moisture change.

As for our forefathers having superior wood, some thoughts. There is no such thing as old growth virgin timber. Trees like any living organism have a lifespan. Some can indeed live a very long time but our oaks and poplars tend to tap out around 400-500 years. Trees that have struggled up through a dense forest are suppressed, slow growth rather than what many people think of as virgin, timeless, healthy, old growth. Nature is always "logging", through fire, flood, slide. Prior to 1927 fire swept through the local forest every 5-10 years causing firescar in many trees. The wood records those events through char and shake. It was not the pristine, perfect wood forest we might imagine. Slow diameter growth in ring porous trees equates to lower strength because the wood contains more thin walled vessel cells, a fast grown oak is considerably stronger than a slow grown one.  My oak frame is stronger than my grandfathers. Diffuse porous woods are less linear but a modern poplar is stronger than a slow grown one. I like the look of tight rings but realize that is more nurture than nature. Grandpa's slow grown oak is more dimensionally stable, wood shrinks not the air within vessels, my oak is denser thus stronger but also moves more as it dries. Define "better"... strength or stability.

In a large furniture slab of ring porous wood, slow growth is a good thing, in a timber maybe not.