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It is now time to look at the timber of the oak as a material, and to examine its technical properties from the various points of view of those who employ such material. Oak timber may be described as follows:

(1) Appearance and Structure.—Pith pentangular, 1 to 4 mm. diameter, whitish at first, and then browner, formed of small, thick-walled cells.

Sap-wood narrow and yellowish-white; heart-wood varies in shades of grayish or yellow brown (fawn color) to reddish or very dark brown. It darkens on exposure, and works to a glossy surface if healthy.

Annual rings well marked by the one to four lines of large vessels in the spring wood, whence radiate outward tongue-like and branched groups of smaller and smaller vessels, tracheids, and cells, in a groundwork of darker fibers. Indistinct peripheral lines of parenchyma are also visible, especially in the broader annual rings. The annual rings are slightly undulating, bending outward between the large medullary rays (Fig. 38).