Chemical and technological properties of wood; drying and storage
Wood consists mainly of organic matter: dry wood is on average 49 % carbon, 44 % oxygen, 6 % hydrogen and 0.1–0.3 % nitrogen. When burnt, its inorganic part remains as ash containing calcium, potassium, sodium and magnesium. The chemical basis of wood is cellulose and lignin. Mechanical properties show how wood resists outside forces: strength (resistance to compression and bending), hardness, elasticity, brittleness, splitting and nail-holding ability. By hardness, woods fall into three groups: soft (pine, spruce, poplar), hard (birch, oak, maple) and very hard (pear, walnut, acacia, boxwood). The drier the wood, the more elastic it is. Moisture content is the mass of water in wood compared with the mass of absolutely dry wood, expressed in per cent; it is found by weighing or with a moisture meter. When wood dries, free water evaporates first, then hygroscopic water. Natural drying is easy and cheap but takes from 7 to 70 days (sometimes longer); in a kiln wood dries quickly but uses a lot of fuel. For drying, boards are stacked on a base at least 50 cm high with spacers between them, the top is covered with a slope, and no rubbish or rotting debris lies around. In the workshop, drying kilns, power tools and heavy boards are handled only by the teacher or a master.
Wood sample check (with the teacher): the class looks at 4–5 different pieces of wood on a table and records colour, weight and grain. Hardness is compared by pressing lightly with a fingernail; pieces are handled with gloves because edges can give splinters.