National Hardwood Magazine - February 2024

NWFA REVIEW Moisture

NWFA REVIEW Moisture Content And Wood Many who work in the wood products industry know that trees are comprised mostly of water, more than 50 percent in some species. Because of this, it is important to understand moisture and how it affects the products our industry produces. Wood is a hygroscopic material, which means it can absorb or lose moisture. When wood absorbs moisture, it expands; when wood loses moisture, it shrinks. Moisture can exist in wood as liquid water or water vapor in the cell cavities of the wood. This is called free water. Moisture also can exist in wood as water chemically held within the cell walls. This is called bound water. The moisture content at which the cell walls are saturated completely, but no water exists in the cell cavities, is called the fiber saturation point (FSP). FSP usually is between 25 percent to 30 percent, depending on the species. Wood fibers are dimensionally stable when the moisture content is above the FSP. When below the FSP, all moisture gained or lost is bound water. As the wood gains or loses bound water, the dimension of the wood begins to change. Wood also is an anisotropic material, meaning it shrinks and swells differently in each direction. Wood shrinks and swells the most along the circumference of the growth rings (tangentially), about half as much perpendicular to the growth rings (radially), and only in miniscule amounts along the grain (longitudinally). The average range for shrinkage for most wood species from FSP to oven-dry is 5 percent to 15 percent tangentially, 2 percent to 8 percent radially, and .1 percent to .2 percent longitudinally. In some species, juvenile wood from near the center of the tree, called pith, can shrink up to 2 percent or more from FSP to oven-dry. The moisture content (MC) of wood is measured as the weight of the water in the wood expressed as a percentage of the weight of the wood itself. The weight of the wood is determined by drying the wood to a point where all of the moisture is removed. This is referred to as oven-dried. This MC affects other properties of the wood, such as its weight, shrinkage, and strength. In living trees, for example, the MC may be as high as 200 percent of the weight of the wood. When wood is harvested and milled, however, it is dried to a MC for its end use. This typically ranges from 6 percent to 9 percent. Please turn to page 76 TRUSTED NATIONAL LEADER FULL BIOMASS WOOD BOILER SYSTEM Design, Fabrication and Installation – Hot Water and Steam • Lifetime Support of Equipment with Full-Service Team and Parts Division • Experts in All Aspects of Boiler Room Equipment – Combustion System, Boiler, Emissions Control Equipment, Controls, and Material Conveying Equipment • Installing throughout the US and Internationally BY BRETT MILLER, VICE PRESIDENT OF TECHNICAL STANDARDS, TRAINING & CERTIFICATION, NATIONAL WOOD FLOORING ASSOCIATION, CHESTERFIELD, MO 800-422-4556 WWW.NWFA.ORG 906-466-9010 www.BurnChips.com sales@burnchips.com 18 FEBRUARY 2024 n NATIONAL HARDWOOD MAGAZINE www.RealAmericanHardwood.com/industry www.RealAmericanHardwood.com/industry FEBRUARY 2024 n NATIONAL HARDWOOD MAGAZINE 19