Splitting

Also called: split, checking, checks, end splits

Splitting is what happens when wood breaks apart along the length of its grain, the direction its fibres run, rather than being cut cleanly across them. Woodworkers make this happen on purpose to convert logs into usable stock, and spend just as much effort stopping it happening by accident everywhere else.

Why wood gives way along the grain

Wood is far weaker across its grain than along it, so past a certain point a cutting edge working along the fibres needs less force to break the wood ahead of it apart than to actually saw through it. That is why long grain, an exposed surface running with the fibres, has a known tendency to break out during work, though the right technique can avoid it. The direction a split takes follows the wood's own structure: in softwood, the cracks called spiral checks spiral around the wood following the direction of the cellulose inside the tracheids, the long thin cells that make up most of a softwood's structure, running at an angle of 45 degrees or less to the length of the cell.

Splitting on purpose: riving

Deliberately splitting wood along its length is called riving, one of the oldest ways of turning a log into usable stock. A whole green, unseasoned log can be riven lengthwise, starting just above where the roots flare out and running up to the first branch, which lets the grain be checked before the piece is cut to length, where this is practical. A large felled tree is treated according to what that upper length is like: if the timber above the first main branch break is still sound, loggers fell and send the whole trunk to the sawmill; if it is not sound, only the trunk's butt goes to the sawmill and the section above is riven into cordwood, firewood cut to length, instead. For hand-riving, a crotch, a forked section cut from the felled tree, can brace a piece while it is split, and a cleaving brake, a fixed frame built to grip the wood, holds it steady specifically while a froe, a blade set with its handle at right angles, is driven along the grain to open it up; a brake more generally grips wood while it is riven or shaved, often ahead of finishing on a shaving horse, a bench where the piece is clamped by a foot-operated jaw for further shaping. Hardwood wedges called gluts can be driven in to finish opening a piece once the froe has started it, and larger pieces worked open by hand generally call for a froe, hatchet, or chisel. Some traditional chairmakers rive their stock so accurately to the marked line that they skip the hewing hatchet, a broad axe used for rough squaring, entirely, going straight from axe and froe to the drawknife, a two-handled blade pulled toward the worker, and the spokeshave, a small shaving tool with handles either side of a short blade.

Splits and checks as a defect

Not all splitting is wanted. Timber dried too quickly in strong sun develops sun checking, a pattern of small cracks along the grain across its surface. In grading rules for finished lumber, a crack running in from the end of a piece counts the same as an end check, a crack reaching right through the piece's thickness, and pallet and container timber has to meet minimum limits for this kind of cracking, set according to what the timber will be used for. Veneer, wood sliced into thin sheets by knife, shows its own version of the fault: knife checks, small cracks left by the slicing knife on the back of the sheet. Thinner slices show less noticeable knife checking as the cutting thickness comes down, setting the pressure of the nosebar, the bar that presses the log just ahead of the knife, to roughly 15 to 20 percent of the veneer's thickness nearly removes the checks, and using the largest workable rake angle, the angle the knife is set at relative to the wood, also keeps checking to a minimum. A hand plane can cause a related fault: one with an overly wide mouth, the gap in its sole the blade projects through, causes real tear-out of the kind shown in exaggerated form in period diagrams.

Material, protection and repair

Species and moisture state make a real difference to how readily wood splits. One softer, easily worked species stays pliable right after cutting but turns brittle and prone to splitting as it ages, unlike its harder Cuban counterpart. Other timbers behave the opposite way: one species is very hard to work once dry, since it tends to crack apart, and resists most machine operations, while another hard-to-dry species is prone to surface checking and cupping specifically when kiln-dried. Because end grain, the surface left where the fibres are cut across rather than along their length, dries and swells faster than the wood around it, coating it on exterior woodwork reduces paint peeling and helps stop it checking, cracking, or bowing. Fixing wood also carries a risk: driving a nail into hardwood, or into thin or narrow stock generally, risks splitting the board along the grain, and driving a wedge into a tenon to secure it carries the same risk to the tenon. A recently split panel, one where the parts still come together to a good joint, can be repaired by lifting out the mouldings around it, then clamping the panel with blocks and wedges. Surface checks are said not to reduce a timber-frame member's overall structural strength, along with ordinary shrinkage and warping, though the checks themselves remain visible.