[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

Preliminary to making a series of timber tests it is very important that a working plan be prepared as a guide to the investigation.  This should embrace:  (1) the purpose of the tests; (2) kind, size, condition, and amount of material needed; (3) full description of the system of marking the pieces; (4) details of any special apparatus and methods employed; (5) proposed method of analyzing the data obtained and the nature of the final report.  Great care should be taken in the preparation of this plan in order that all problems arising may be anticipated so far as possible and delays and unnecessary work avoided.  A comprehensive study of previous investigations along the same or related lines should prove very helpful in outlining the work and preparing the report. (For sample working plan see Appendix.)

FORMS OF MATERIAL TESTED