20.--Effect of torsion on different grades
of hickory. _Photo by U. S. Forest Service._]

HARDNESS

The term _hardness_ is used in two senses, namely: (1)
resistance to indentation, and (2) resistance to abrasion or
scratching. In the latter sense hardness combined with toughness
is a measure of the wearing ability of wood and is an important
consideration in the use of wood for floors, paving blocks,
bearings, and rollers. While resistance to indentation is
dependent mostly upon the density of the wood, the wearing
qualities may be governed by other factors such as toughness,
and the size, cohesion, and arrangement of the fibres. In use
for floors, some woods tend to compact and wear smooth, while
others become splintery and rough. This feature is affected to
some extent by the manner in which the wood is sawed; thus
edge-grain pine flooring is much better than flat-sawn for
uniformity of wear.

|-------------------------------------------------------------------|
| TABLE XII |
|-------------------------------------------------------------------|
| HARDNESS OF 32 WOODS IN GREEN CONDITION, |
| AS INDICATED BY THE LOAD REQUIRED TO IMBED |
| A 0.444-INCH STEEL BALL TO ONE-HALF ITS DIAMETER |
| (Forest Service Cir. 213) |
|-------------------------------------------------------------------|
| COMMON NAME OF SPECIES | Average | End | Radial | Tangential |
| | | surface | surface | surface |
|------------------------+---------+---------+---------+------------|
| | Pounds | Pounds | Pounds | Pounds |
| | | | | |
| Hardwoods | | | | |
| | | | | |
| 1 Osage orange | 1,971 | 1,838 | 2,312 | 1,762 |
| 2 Honey locust | 1,851 | 1,862 | 1,860 | 1,832 |
| 3 Swamp white oak | 1,174 | 1,205 | 1,217 | 1,099 |
| 4 White oak | 1,164 | 1,183 | 1,163 | 1,147 |
| 5 Post oak | 1,099 | 1,139 | 1,068 | 1,081 |
| 6 Black oak | 1,069 | 1,093 | 1,083 | 1,031 |
| 7 Red oak | 1,043 | 1,107 | 1,020 | 1,002 |
|