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 Tempurature, supra.)

[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.]