| 20 | 253 | 83.3 | 84.2 | 91.4 | 86.3 |
| | 5 | 20 | 253 | 85.0 | 78.1 | 84.2 | 82.4 |
| | 6 | 20 | 242 | 95.2 | 89.8 | 76.0 | 87.0 |
| | 10 | 20 | 255 | 73.7 | 82.0 | 76.0 | 77.2 |
| | 20 | 20 | 258 | 67.5 | 65.0 | 99.0 | 77.2 |
|------------------------------------------------------------------------------------------|
| [Footnote a: It will be noted that the temperature was 230 deg.. This is the maximum |
| temperature by the maximum-temperature recording thermometer, and is due to the handling |
| of the exhaust valve. The average temperature was that of exhaust steam.] |
|------------------------------------------------------------------------------------------|

"(3) A high degree of steaming is injurious to wood in strength
and spike-holding power. The degree of steaming at which
pronounced harm results will depend upon the quality of the wood
and its degree of seasoning, and upon the pressure (temperature)
of steam and the duration of its application. For loblolly pine
the limit of safety is certainly 30 pounds for 4 hours, or 20
pounds for 6 hours."[52]

[Footnote 52: _Ibid._, p. 21. See also Cir. 108, p. 19, table
5.]

Experiments made at the Yale Forest School showed that steaming
above 30 pounds' gauge pressure reduces the strength of wood
permanently while wet from 25 to 75 per cent.

PRESERVATIVES

The exact effects of chemical impregnation upon the mechanical
properties of wood have not been fully determined, though they
have been the subject of considerable investigation.[53] More
depends upon the method of treatment than upon the preservatives
used. Thus preliminary steaming at too high pressure or for too
long a period will materially weaken the wood, (See TEMPERATURE,
above.)

[Footnote 53: Hatt, W. K.: Experiments on the strength of
treated timber. Cir. 39, U.S. Forest Service, 1906, p. 31.]

The presence of zinc chloride does not weaken wood under static
loading, although the indications are that the wood becomes
brittle under impact. If the solution is too strong it will
decompose the wood.

Soaking in creosote oil causes wood to swell, and accordingly
decreases the strength to some extent, but not nearly so much so
as soaking in water.[54]

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