| | 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°. 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] [Footnote 54: Teesdale, Clyde II.: The absorption of creosote by the cell walls of wood. Cir. 200, U. S. Forest Service, 1912, p. 7.] Soaking in kerosene seems to have no significant weakening effect.[55] [Footnote 55: Tiemann, H.D.: Effect of moisture upon the strength and stiffness of wood. Bul. 70, U. S. Forest Service, 1907, pp. 122-123, tables 43-44.] PART III TIMBER TESTING[56] [Footnote 56: The methods of timber testing described here are for the most part those employed by the U. S. Forest Service. See Cir. 38 (rev. ed.), 1909.] WORKING PLAN