_Hardness_--In some woods hardness is considered an advantage, but not in others. If sugar maple were as soft as white pine, it would not be the great floor material it is; and if white pine were as hard as maple, pattern makers would not want it, door and sash manufacturers would get along with less, and it would not be the leading packing box material in so wide a region.
It is generally the summer growth in the annual rings which makes a wood hard. The summerwood is dense. A given bulk of it contains more actual wood substance and less air and water than the springwood. For the same reason, summerwood gives weight, and a relationship between hardness and weight holds generally. It may be added that strength goes with weight and hardness, but it is not a rule without apparent exceptions.
Some woods possess twice or three times the hardness of others. Among some of the hardest in the United States are hickory, sugar maple, mesquite, the Florida ironwoods, Osage orange, locust, persimmon, and the best oak and elm. Among the softest species are buckeye, basswood, cedar, redwood, some of the pines, spruce, hemlock, and chestnut.
The hardness of wood is tested with a machine which records the pressure required to indent the surface. The condition of the specimen, as to dryness, has much to do with its hardness. So many other factors exercise influence that nothing less than an actual test will determine the hardness of a sample. A table of figures can show it only approximately and by averages.
_Cleavability_--Wood users generally demand a material which does not split easily, but the reverse is sometimes required. Rived staves must come from timbers which split easily. Many handles are from billets which are split in rough form and are afterwards dressed to the required size and shape. In these instances, splitting is preferable to sawing, because a rived billet is free from cross grain.
The cleavability of woods differs greatly. Some can scarcely be split. Black gum is in that list, and sycamore to a less extent. Young trees of some species split more readily than old, while with others, the advantage is with the old. Young sycamore may generally be split with ease, but old trunks seem to develop interlocked fibers which defy the wedge. A white oak pole is hard to split, but the old tree yields readily. Few woods are more easily split than chestnut. With most timbers cleavage is easiest along the radial lines, that is, from the heart to the bark. The flat sides of the medullary rays lie in that plane. Cleavage along tangential lines is easy with some woods. The line of cleavage follows the soft springwood. Green timber is generally, but not always, more easily split than dry. As a rule, the more elastic a wood is, the more readily it may be split.