Grain

Also called: interlocked grain, straight grain, figured grain, quarter sawn, flat sawn, rift sawn

Grain is the direction the long fibres of a tree run through a piece of wood, and the pattern that direction leaves visible on a cut surface. In practice the word gets used loosely for several related things: the run of the fibres, the way a surface feels or tears depending on which way it is worked, and the decorative figure that appears when those fibres are cut at different angles.

Long grain and end grain

The most basic distinction is between long grain and end grain. Long grain is a surface running parallel to the length of the fibres, such as the face or edge of an ordinary board; end grain is the surface left where a board is cut across its width, exposing the cut ends of the fibres rather like the ends of a bundle of straws. The two behave quite differently under a cutting edge and under glue. A plane set for smoothing a long-grain surface needs a comparatively high cutting angle to stop it lifting and tearing the fibres as it passes over them. Glue takes to long grain far better than to end grain, because end grain simply drinks the glue in along the open fibres rather than forming a solid film between two surfaces, so joints are generally designed to bring as much long grain into contact with long grain as possible. A mortise and tenon joint, for instance, is often built as several smaller tenons rather than one large one, to increase that long-grain gluing area while keeping shrinkage in check. Hoadley (1980) found that glued side-grain to side-grain joints can be as strong as the solid wood itself, even where the fibre direction of the two pieces does not run parallel, whereas a joint loaded in compression across the grain is limited by the wood's much weaker crushing strength at right angles to the fibres.

Reading the end grain

A board's end grain shows the pattern of its growth rings, the concentric bands a tree lays down year by year, and that pattern is the clue to how the board will move as it gains or loses moisture. Wood shrinks and swells far more around the rings than along the length of the fibres, so the curve of the rings visible in the end grain tells a woodworker which way a board is likely to cup or twist. This is also why joints between two pieces set at right angles to each other, an L, a T or a crossed halving, need special thought: unlike a plain long-grain to long-grain glue line, the two pieces will not always move together as moisture changes.

When the grain runs crooked

Grain does not always run straight along a board. Interlocked grain is a condition in which the fibre direction reverses, spiralling one way and then the other, in alternating layers as the tree grows. The USDA (2021) defines it as "a regular reversal of right and left spiraling of grain in a tree stem," common in many tropical hardwoods, and notes that it "alters the relationship between bending strength and compressive strength of lumber cut from tropical hardwoods." Cut on the quarter, meaning sawn so the growth rings run across the face of the board, interlocked grain produces a ribbon or stripe figure as the alternating layers catch the light differently, seen in timbers such as sapele and African mahogany. Diagonal grain is a different fault, arising, in the USDA's (2021) words, "when the long edge of the board is not parallel with the grain," usually because of how the log was sawn up. Sloping grain describes much the same idea seen from the face or edge of a piece, where the bulk of the fibres run at an angle across the surface rather than along it. Straight, evenly running grain along the full length of a board is what turners and framers look for in any part that has to carry load along its length, since each of these crooked-grain conditions weakens a piece by comparison.

Cutting with the grain

Working across end grain with a saw, chisel or router bit risks tearing chunks away as the tool reaches the corner and the fibres run out from under it rather than parting cleanly. Working along an edge, a cutter can catch grain running up toward the surface and lift a splinter out ahead of the cut instead of shaving it away cleanly, which is why a piece is generally worked in the direction that lays the grain down rather than lifting it, wherever the run of the fibres allows a choice.