andpaper or by means of a sand
blast.

[Footnote 16: See articles by Gabriel Janka listed in
bibliography, pages 151-152.]

CLEAVABILITY

_Cleavability_ is the term used to denote the facility with
which wood is split. A splitting stress is one in which the
forces act normally like a wedge. (See Fig. 21.) The plane of
cleavage is parallel to the grain, either radially or
tangentially.

[Illustration: FIG. 21.--Cleavage of highly elastic wood. The
cleft runs far ahead of the wedge.]

This property of wood is very important in certain uses such as
firewood, fence rails, billets, and squares. Resistance to
splitting or low cleavability is desirable where wood must hold
nails or screws, as in box-making. Wood usually splits more
readily along the radius than parallel to the growth rings
though exceptions occur, as in the case of cross grain.

Splitting involves transverse tension, but only a portion of the
fibres are under stress at a time. A wood of little stiffness
and strong cohesion across the grain is difficult to split,
while one with great stiffness, such as longleaf pine, is easily
split. The form of the grain and the presence of knots greatly
affect this quality.

|---------------------------------------------|
| TABLE XIII |
|---------------------------------------------|
| CLEAVAGE STRENGTH OF SMALL CLEAR PIECES OF |
| 32 WOODS IN GREEN CONDITION |
| (Forest Service Cir. 213) |
|---------------------------------------------|
| | When | When |
| COMMON NAME | surface of | surface of |
| OF SPECIES | failure is | failure is |
| | radial | tangential |
|-------------------+------------+------------|
| | Lbs. per | Lbs. per |
| | sq. inch | sq. inch |
| | | |
| Hardwoods | | |
| | | |
| Ash, black | 275 | 260 |
| white | 333 | 346 |
| Bashwood | 130 | 168 |
| Beech | 339 | 527 |
| Birch, yellow | 294 | 287 |
| Elm, slippery | 401 | 424 |
| white | 210 | 270 |
| Hackberry | 422 | 436 |
| Locust, honey | 552 | 610 |
|