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Softwood trees also have larger, less dense cells in the earlywood than hardwoods. This helps explain why a nail can be driven into a wide-celled pine board more easily than a tight-grained oak board; the cell structure is less dense, allowing easier penetration.
Another property worth noting is that hardwood trees allow their branches time and space to grow in almost any direction, in order to maximize leaf exposure to sunlight. The internal stresses present in the wood, resulting from the weight of these outspread branches, create interesting figure and grain patterns in the wood (See below). However, there is a price to pay for that beauty: highly figured wood tends to distort more readily than straight-grained boards as the stresses are released.
HARDWOOD VS. SOFTWOOD
Despite what the categories imply, the distintion between hardwoods and softwoods has to do with leaf type and is not a measure of wood hardness. Oak, a common hardwood, has broad leaves that shed in the fall, while pine, a coniferous softwood, retains its needles all winter.
COLOR, FIGURE & GRAIN PATTERN
Part of the attraction of woodworking comes from the opportunity to work with wood displaying dramatic differences in color, figure and grain pattern. Wood color is a product of of how its tannins, gums and resins react to exposure to the air. Often, wood will continue to darken and change color over time, developing a rich patina. Figure—the surface pattern on a board—can be the result of numerous natural causes ranging from drought or freezing to prevailing winds, disease, age or insect damage. Grain display is dependent on the direction and regularity of the wood fibers relative to the center of the trunk as well as how the lumber is cut from the tree.
Three centuries ago, colonial woodworkers cut their lumber from vast tracts of virgin coniferous forest. It wasn’t uncommon for them to glean white pine boards measuring 2, 3 and even 4 ft. wide, with no knots or other disfigurement. It’s no surprise that much of their early furniture was built from softwood. Boards culled from today’s replanted pine forests, on the other hand, have knots every 12 to 18 in. along their length (one year’s growth). Because of their minimal girth at harvest, boards often contain considerable sapwood as well.
The best uses for today’s softwoods are in applications where straight, abundant, less expensive lumber is needed. Mills cut softwoods largely into construction lumber for framing walls, floors and roofs or process it into plywood, chipboard and oriented strandboard to sheathe buildings. Of course, a percentage of this lumber also is headed for woodworkers, but premium-grade softwoods can command prices that compete with hardwoods.
Hardwoods, on the other hand, are most often sturdier, heavier, more figured and show a great variety of colors. So, it’s no surprise that the more attractive, yet less available hardwoods are more costly and are the natural choice for furnituremaking, cabinetry and trim work. It also explains why hardwoods are not available in the same nominal dimensions as softwoods intended for construction purposes.
Massive sawmill blades make quick work of slicing a log into green lumber. Once cut, these boards will be graded, stacked, dried and possibly planed smooth on faces and edges before they’re ready for sale.
TYPICAL LUMBER CUTS
Lumber is cut from logs in a number of different ways, to maximize yield or to control wood grain direction and avoid log defects. Plain-sawn boards are produced by rotating a log in quarter-turn increments and cutting around the center pith area. Quartersawing, a less efficient way to maximize board yield, nevertheless produces more dimensionally stable lumber. Quartersawn oak also displays prominent medullary rays that would not otherwise show if the boards were plainsawn. A third milling method, through-and-through cutting (not shown), involves simply slicing the log completely across, which produces a mix of plain-sawn and quartersawn lumber.
Cuts of Lumber
Mills saw lumber in a variety of ways, depending on the intended use of the boards and the species and quality of the logs. The most common cuts are plain-sawing and quartersawing. Plain-sawing (also called flat-sawing) involves cutting the log to maximize lumber without including the center pith area. The log is rotated to make successive series of cuts around the pith. Plain-sawing produces lumber most economically for both the mill. It is suitable for most construction and woodworking purposes, but since the cuts are made tangentially to the growth rings—the direction of greatest wood movement—the lumber is more prone to distortion than quartersawn lumber. (For more on wood movement, see here).
Plain-sawn lumber is cut so the growth rings run tangentially to the board faces, producing a wider, wavy grain pattern. The growth rings on quartersawn lumber, on the other hand, run radially to the board faces, resulting in a tigher, parallel grain pattern.
LUMBER DRYING METHODS
Air-drying: Short lengths of scrap wood, called stickers, are inserted between each board in a stack to allow air to circulate all around the boards. The stacks are left to dry fully exposed to the elements, are covered up or stored in open sheds for months or even years at a time. Without stickers, green lumber will dry unevenly or attract mold and slowly decompose.
Kiln-drying: Once stickered lumber is loaded into a kiln, the kiln is closed up and heated evenly for several weeks until the moisture inside is reduced to acceptable levels. Drying time will vary depending on wood species and the grade of lumber. Kiln-drying is a faster method of producing general-purpose lumber, and kiln-dried boards are what you’ll find at all discount lumber outlets and home centers.
Quartersawn lumber is made by first sawing the log along its length to create four wedges. These are then ripped so the growth rings run more or less perpendicular (radially) to the board faces. Quartersawing yields boards with close, tight, straight grain. In some hardwoods, like oak, it also exposes beautiful, translucent medullary rays that have been coveted by woodworkers for generations. The downside to quartersawing is that it produces less lumber per log than plain-sawing, making the lumber more expensive to buy. Generally, quartersawn hardwoods are more common to find than quartersawn softwoods.
Methods of Drying Lumber
When boards are first cut from a log, they are considered “green”, which means they contain a high percentage of water weight and must be dried before they are suitable for most uses. Lumber is dried commercially in two ways—by air or by kiln. Air drying is simply that: the stock is stacked in such a way that the air can circulate through and around it. Small pieces of lumber, called stickers, are inserted between the boards at regular intervals. The stack is then left to dry for a long time, sometimes several years, until the moisture evaporates to acceptable levels. Variations on the method involve covering the top layer with plastic or canvas, turning the sides to the prevailing wind, periodically dismantling and rebuilding the stack in reverse, all in an attempt to control the drying process. If lumber is improperly dried, it may begin to mold, which leads to a sometimes desirable defect called spalling (See here).