The flammability of wood can be reduced by chemical treatment (see the section Preservation). The thermal conductivity of the wood is set at 0.1 Watts per meter Kelvin (W/m-K) and the metal at 1 Watts per meter Kelvin (W/m-K). Normally, wood absorbs a very small portion of acoustic energy (3–5 percent), but special constructions incorporating empty spaces and porous insulation boards can increase absorption to as high as 90 percent. The velocity of sound in wood is reduced by moisture, which therefore contributes to faster damping of sound. 0000002516 00000 n 0000003056 00000 n Conversely, when subjected to an electric field, wood exhibits mechanical deformation (changes in size). Very high thermal conductivity measurements up to 22,600 w m −1 K −1 were reported by … k= (BTU * inch) / sq.ft. Infested wood can be rendered free of insects at temperatures of 50–60 °C (122–140 °F), by the introduction of insecticides, or by exposure to toxic gases. Black Friday Sale! A thermal conductivity of 0.1213 W/mK, for pine was the product of the HFM measurement. * hour * degF. Furthermore, wood is destroyed by fire. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. In general, softwoods possess a higher heating value than hardwoods, and extractives have an important influence; for example, a kilogram of the oleoresin in pines has a heating value of about 8,500 kilocalories. Practical use of the relationship of wood’s moisture content to its electric resistance is made in electric moisture meters. Wood is also subject to degradation by changing climatic conditions (e.g., by rain and sunlight causing repeated wetting and drying), mechanical stresses (e.g., imposed on railroad ties), and exposure to chemicals (e.g., acids and alkalies). 0000014604 00000 n These properties—dielectric constant and power factor—assume a practical importance in drying wood with electric current (a theoretical possibility, although not presently a reality), gluing wood with high-frequency electric current, or making electric meters (capacity and radio-frequency power-loss type) for measuring its moisture content. For musical instruments, a preference exists for selected spruce wood, but fir, pine, maple, and tropical woods also are used. Marine borers (certain species of mollusks and crustaceans) attack wooden structures in seawater (wharf pilings, boats, and other submerged wood) and cause severe damage. Preservative treatment imparts considerable resistance to these organisms. For example, the thermal conductivity of pine in the direction of the grain is 0.22 W/moC, and … 0000016161 00000 n Insects bore holes and tunnels, and some reduce the interior of wood to dust, leaving only a thin outer layer. Thermal conductivity is highest in the axial direction and increases with density and moisture content; thus, light, dry woods are better insulators. * hour * degF. 0000013331 00000 n C is known only when the k, the thermal conductivity of a material is known. Premium Membership is now 50% off. Other factors, such as species and density, have little effect on the electric resistance of wood; differences among species are attributed to the chemistry of the extractives. Noteworthy is the spectacular decrease of electric resistance as moisture content increases from zero to the fibre saturation point. 0000009347 00000 n 0000017837 00000 n A moisture content below 20 percent inhibits growth of fungi, as do temperatures lower than 10 °C (50 °F) and higher than 30 °C (86 °F). Thermal conductivity for common materials and products: Hardwoods (oak, maple..) Mineral wool insulation materials, wool blankets .. Softwoods (fir, pine ..) Wood is degraded or destroyed not with the passage of time but only under the action of external factors. Important also are the dielectric, or poor-conductor, properties of wood. Bacteria also appear during prolonged storage of wood in water, including seawater (e.g., in the case of old sunken ships). Both walls have the same surface area and the same interior and exterior temperatures. Blue stain (sap stain) of pines is the most common and serious consequence of attack by stain fungi. 0000024289 00000 n 0000024016 00000 n The maximum heating value of one kilogram of oven-dry wood averages about 4,500 kilocalories (with a range of 4,100–6,800 kilocalories). Axial resistance is about half that of the transverse. Within this range, electric resistance decreases more than a billion times, whereas from the fibre saturation point to maximum moisture content, it decreases no more than about 50 times. Presently, wood has become an alternative to brick in the construction of homes. Oven-dry wood is electrically insulating. The sound can be amplified, as in violins, guitars, organ pipes, and other musical instruments, or it can be absorbed, as in wooden partitions. However, the type of underlayment used with a wood floor can increase the overall R-value considerably. Resistance increases with decreasing temperature; in oven-dry wood it doubles over a temperature drop of 12.5 °C (22.5 °F). 0000010909 00000 n 0000005950 00000 n Blue stain may appear very quickly in warm weather, sometimes within hours or days after the tree is felled or the green wood is sawed or otherwise processed. 0000007457 00000 n Degradation can affect wood of living trees, logs, or products, causing changes in appearance, structure, or chemical composition; these changes range from simple discoloration to alterations that render wood completely useless.