Drying Faults in Lumber

The following is excerpted from “Cut & Dried,” by Richard Jones.

Richard has spent his entire life as a professional woodworker and has dedicated himself to researching the technical details of wood in great depth, this material being the woodworker’s most important resource. The result is “Cut & Dried: A Woodworker’s Guide to Timber Technology.” In this book, Richard explores every aspect of the tree and its wood, from how it grows to how it is then cut, dried and delivered to your workshop.

Richard explores many of the things that can go right or wrong in the delicate process of felling trees, converting them into boards, and drying those boards ready to make fine furniture and other wooden structures. He helps you identify problems you might be having with your lumber and – when possible – the ways to fix the problem or avoid it in the future.

“Cut & Dried” is a massive text that covers the big picture (is forestry good?) and the tiniest details (what is that fungus attacking my stock?). And Richard offers precise descriptions throughout that demanding woodworkers need to know in order to do demanding work.

The main drying faults in planks or boards are: distortion or warping that are the result of shrinkage in the grain; plus the internal checking, surface checking and end splitting caused by shrinkage where all these faults may be exacerbated by drying processes. The following faults are entirely drying faults: collapse (aka core collapse in North America), shell set in oversize condition, honeycombing, case-hardening and the very rare reverse case-hardening.

Another drying fault sometimes apparent is discolouration of the wood. One discolouration, sticker stain, has already been discussed in section 8.3. Additional drying-induced discolouration of wood is discussed in section 9.2.

The causes of distortion or warping are discussed in section 7.4, but the natural warping of wood due to moisture loss and aggressive drying, whether in a kiln or air dried, may magnify the distortion.

With reference to figure 9.1, at the beginning of the drying process wet wood is not under undue stress. It is only as it dries that stresses begin to develop. At the beginning of the drying process all the cell lumen are full of liquid, or at least partially filled and, most importantly, the cell walls show no significant sign of stress-inducing shrinkage. It’s not until free water in any cell in the wood has gone and the bound water in the cell walls and the cavities begins to leave that shrinkage starts. It’s counterintuitive but drying faults such as surface checking and honeycombing develop at high wood moisture content, but the following discussion explains this phenomenon.

At the beginning of the drying process water is first lost through the ends of a board where the end grain is exposed, and from the fibres near the board’s surface. The 12″ to 16″ (300 mm to 400 mm) at each end of a board exchange water vapour faster through the relatively porous end grain than the board edges and faces. As wood dries, a moisture gradient develops. If the wood is dried quickly with high heat and fast-moving air, a steep moisture gradient forms. If we take as an example wet wood, e.g., at an average 50 percent MC, and subject it to high heat, this causes moisture at the surface to rapidly evaporate out of the cavities and the cellular structure. The tissue below the surface or shell is still at an average 50 percent MC and also still cool. But the situation changes quickly as the now drier and warm shell transmits heat toward the centre of the wood through the intermediate zone. The additional warmth affecting the intermediate zone encourages moisture movement toward the now drier shell. In turn, the intermediate zone transmits heat toward the core of the wood and moisture starts moving from the core to the intermediate zone, and on outward toward the shell and out of the wood. It’s not difficult to see, having just described the mechanics of drying how, for example, surface checking develops whilst wood still has a high average moisture content.

All these different zones at different moisture contents create the moisture gradient within the wood. A steep moisture gradient means the wood is drying very quickly. For instance, extremely rapid drying occurs in the oven-drying test used to determine moisture content. In this case the samples are small and there is a large surface area (particularly end grain exposure) to volume ratio, letting the moisture out relatively easily. But you could put a piece of green wood 20″ long x 8″ wide x 4″ thick (500 mm x 210 mm x 105 mm) in a large-enough oven and start drying it in the same way. Now the surface-area-to-volume ratio is small compared to the small samples used in oven drying to determine moisture content. The rapid drying of a large piece of wood causes a steep moisture gradient that puts large stresses on it. The surface dries quickly, but the moisture in the cells in the intermediate zone and the core can’t escape fast enough to prevent tension and compression stresses developing in the board.

On the other hand, if you put the same piece of green 20″ x 8″ x 4″ wood in a sealed plastic bag it will barely dry at all. Even keeping the bagged piece of wood in a warm room where heat transfers to the wood and causes the moisture in the shell to evaporate, there’s nowhere for the moisture to go once the air in the bag reaches 100 percent RH. In all likelihood leaving a piece of wood encased in a plastic bag like this for a couple of weeks in warm conditions would result in a fuzz of mould developing. But, importantly, from our point of view of discussing moisture gradients, this piece of wood would exhibit a shallow moisture gradient. Shallow moisture gradients don’t put much stress on the wood, but the problem from a timber or lumber dryer’s point of view with shallow moisture gradients and slow drying is twofold: firstly, stock turning over too slowly to make any profit; secondly, serious disfiguring mould development, which is less likely when wood is dried faster.

Tension stresses are “ripping apart” forces. Compression stresses are “crushing forces.” To dry wood quickly in a kiln requires getting the balance right between tension and compression forces induced by the movement of moisture out of the wood. Get the balance right and the wood comes out of the kiln stress free, or near-enough stress free. Get them wrong and the faults depicted in figure 9.1 reveal themselves.

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Source: lostartpress.com