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How Timber Works

At the Mill: Processing Timber
buttercup xylem and phloem
© Sun Kim
In this microscopic image of a buttercup, the primary xylem appear as large white cells in the tooth-shaped section toward the right-hand side.

Before we dive into the milling process, it might help to take a closer look at the structure of wood. We mentioned earlier that wood is made up of cells. Let's get a little more specific. Water-conducting cells are the key building blocks of a type of tissue known as xylem. These cells are long and hollow, like pipes, making them ideal transport systems to haul water from the roots to the top of the tree. Sugar-conducting cells are located outside the xylem. They form a second type of tissue known as phloem, which functions to carry sugars and other nutrients from the leaves to the rest of the plant.

In between the xylem and phloem lies cambium, a thin layer of stem cells. Cambium generates new xylem cells to the inside, new phloem cells to the outside. As new xylem and phloem cells are added, the tree grows thicker. That means the xylem closest to the center of a trunk is the oldest part of the tree. This is the heartwood, or the part of a tree that makes it so strong. The newer xylem, which lies just inside the cambium and still functions to carry water, is called sapwood. Outside the cambium is the bark. Bark contains new phloem cells and, on the very periphery, older phloem cells that become crushed as new tissue is laid down to the inside.

The heartwood and sapwood are the most useful parts of a tree and can be turned into paper, lumber, veneers and plywood. Bark can't be used to make paper or lumber, although it can be used for fuel and mulch. So, the first step in processing timber is separating the bark from the main part of the trunk. Logs are passed through a large, open-ended cylinder known as a debarking drum. Inside the drum, logs spin and rub against each other until all of the bark is removed.

A debarked log may be sent straight to the chipper if it's destined to become paper. The chipper reduces the log into small squares about 2 inches (5 cm) on each side and 0.25 inches (0.6 cm) thick. Then the chips are mixed with strong chemicals and heated in big pressure cookers known as digesters. Digesting separates the cellulose fibers from the lignin. The soft, wet fibers, now known as pulp, are blown from the digesters, washed and bleached to the proper shade of whiteness. From there, the fibers are mixed with water again and formed into large mats of pulp. These mats enter a series of rollers and presses that flatten the pulp into sheets and extract water. Finally, a starch solution is added to both sides before the final drying process.

Higher-grade logs don't go to the chipper, but to the sawmill. One log can become many different boards, planks and beams. The primary saw of a mill, the head rig, breaks down the log into all of its rough-cut pieces. Heartwood, because it's much older, often has more knots and is typically used for heavier planks or rectangular beams. Sapwood contains fewer knots and is ideal for a full range of boards and planks. After the head rig reduces the log, each piece of lumber passes through an edger, which removes irregular edges and defects, and then through a trimmer, which squares off the ends at standard lumber lengths. Finally, the lumber is sorted, stacked and dried in kilns.

­Next we'll l­ook at how forests are managed to ensure there's a renewable supply of timber for years to come.