Imperfections in the wood used in furniture reduces the overall quality of the piece, and affects not only the appearance of furniture but also the structural integrity of the piece. Softwoods, hardwoods, plywood and veneers are all graded according to appearance and amount of defects, or imperfections. The grading system for softwoods is based on evenness of grain and percentage of allowable defects, with appearance grade being the best. Hardwoods are graded mainly by the percentage of defect-free wood, with F for firsts, or FAS for firsts and seconds, with firsts being the best grade. Plywoods are graded on a letter system from A through D, with A being the best. A grade of AA indicates a good face and back, while an AC indicates a good face but an average back. Veneers are graded for appearance only.
Some of the most common imperfections in wood are caused either by defects in the tree itself or by improper seasoning and drying. Wood shrinks as it dries, and the outer rings usually shrink more than the inner rings. The way a log is cut can affect the amount of bending or warping as much as the way it is stored and dried. Wood for furniture should always be quartersawn wood. Quartersawn wood is cut into quarters, and then the quarters are cut into boards. This keeps the rings approximately the same length and promotes consistent drying.
Some typical imperfections are knots, holes, splits, discoloration and unevenness of grain. Surface and end splits degrade the structural integrity of the wood, and are usually brought about by drying the wood too fast, and not allowing the moisture to be gradually reduced. Bowing and warping is usually caused by not stacking and storing wood properly during the seasoning process. When wood is dried unevenly the inside tends to shrink more than the outside and produces a honeycomb effect due to torn fibers.
Common imperfections caused by defects in the tree itself are ingrown bark and knots. Ingrown bark occurs when poor cell structure of the tree prevents old bark from falling off before new bark can grow, and the old bark becomes lodged between new wood, or sapwood and the core of the tree or the heartwood. Knots are the remains of branches, and can eventually fall out leaving a hole in the wood. The grain around a knot is usually distorted.
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