The surface qualities by which wood is so highly prized as a decorative material arise from variations in the number and arrangement of different cells and can be classified under the headings of grain, texture and figure. Properly speaking, the term grain refers to the longitudinal alignment of wood cells. That is, the direction of the fibres relative to the axis of the tree or the longitudinal edges of individual pieces of timber. This term is extensively misused to describe a number of other characteristics, including texture and figure. Texture refers to the relative size, and the amount of variation in size, of the cells. Figure refers to the pattern produced on longitudinal surfaces of wood as a result of the arrangement of the different tissues and the nature of the grain.
Six types of grain may be distinguished. These are straight, irregular, diagonal, spiral, interlocked and wavy. In straight-grained timber the fibres and other elements are more or less parallel to the vertical axis of the tree. This is a contributory factor in strength and ease of working but does not give rise to ornamental figure. Where strength is the primary consideration it is usual to specify that timber shall be straight grained. A slope of 1 in 25 reduces bending strength by 4% while a slope of 1 in 5 reduces it by up to 45%. Stiffness is also reduced by sloping of the grain and it will be readily apparent that this is therefore an import ant consideration in the selection of material.
Timber in which the fibres are inclined at varying and irregular angles to the vertical axis in the log is said to have irregular grain. Although this is frequently restricted to knots and other similarly limited areas it is a very common defect and, when excessive, seriously reduces strength as well as making working difficult. It may produce attractive figure, however. Pronounced irregularities in the direction of the fibres, resulting from knot-like elevations or conical depressions in the annual rings, produce blister figure and bird's eye figure respectively.
Diagonal grain is a defect arising during conversion and results from otherwise straight grained timber being cut so that the fibres do not run parallel to the long edges of the board or plank.
Spiral grain results when the fibres follow a left- or right-handed spiral course in the living tree. It is not always readily apparent but may often be detected from the direction of surface seasoning checks. Spiral grain is a serious defect in timber intended for important structural work.
Interlocked grain, or interlocked fibre, results from the fibres of successive growth layers being inclined in opposite directions. This produces a ribbon or stripe figure on quarter-sawn surfaces. Interlocked grain is relatively uncommon in temperate woods but is a characteristic of many tropical hardwoods. It may not appreciably affect the strength of timber but can cause serious twisting during seasoning and severe difficulties in working, especially in planing when the timber picks up leaving a very rough finish. In timber with heavily interlocked grain sawing difficulties may be very great and special machinery and modified techniques are needed to overcome these difficulties. It may be an advantage in some cases that such wood is virtually impossible to split.
When the direction of the fibres is constantly changing, so that a line drawn parallel with them appears as a wavy line on a longitudinal surface, the grain is said to be wavy. This type of grain gives rise to a series of diagonal, more or less horizontal, darker or lighter stripes on longitudinal surfaces. This is due to variations in the angle at which light is reflected from the surfaces of the fibres and is called fiddle-back figure. This type of grain is exploited entirely for its decorative possibilities, any reductions in strength being regarded as of secondary importance. Wavy grain and interlocked grain may occur together in the same piece of timber giving rise to a broken ripple called roe figure on quarter-sawn surfaces.
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