EFFECT OF MOISTURE ON WOOD The comparative properties of the various species of wood as indicated in the foregoing tables (Tables 2-8) are based upon tests of green timber, which give decidedly different results from tests upon dry timber.
Water occurs in wood in two forms: First, the water which fills the spaces between the cells in green wood; and second, that which saturates the walls of the cells. Often half the Table 8 Hardness of Various Woods (Pressure in pounds required to indent specimen to depth of one-half diameter of a .444-inch diameter steel ball) Softwoods Fir, Alpine 219 Pine, Norway 342 Spruce, Englemann.... 243 Spruce, Red 346 Cedar, Western Red.... 246 Cypress 354 Cedar, Northern White. 266 Tamarack 375 Pine, White 296 Fir, Grand 375 Pine, Lodgepole 315 Hemlock, Eastern 406 Pine, Western Yellow.. 320 Douglas Fir 408 Pine, Sugar 324 Hemlock, Black . 464 Fir, White 328 Pine, Longleaf 512 Pine, Table Mountain.. 333 .Average Hardness 340 Hardwoods Basswood 242 Beech 824 Buckeye, Yellow 286 Maple, Hard 882 Willow, Black 334 Elm, Rock 888 Aspen, Largetooth .... 366 Birch, Sweet 894 Butternut 386 Oak, Yellow 926 Cherry, Red 386 Ash, White 941 Elm, White 511 Witch Hazel 977 Cucumber 515 Oak, Red 982 Ash, Black 548 Ash, Green 1,007 Sycamore 580 Ash, Blue 1,028 Sumac 590 Oak, White 1,063 Maple, Silver 592 Oak, Post 1,074 Maple, Red 612 Oak, Bur 1 108 Elm, Slippery 653 Oak, Swamp White . .. .1,158 Cherry, Black 664 Laurel, Mountain . 1 299 Hackb 677 Dogwood 1,408 Tupelo 700 Locust, Black 1,568 Birch, Yellow 745 Locust, Honey 1 846 Ash, Pumpkin 752 Osage Orange 2,037 Average Hardness 844 weight of green wood, and sometimes more, consists of water. The amount of water required to saturate the walls of the cells is from 25 to 30 per cent of the weight of the wood when absolutely dry. This is called the "fiber saturation point." The amount, of water in wood above this point has no effect upon the strength of wood; but, of course, it makes the wood heavier. When wood is dried below the fiber saturation point, its mechanical properties change rapidly, and the extent to which they change depends upon the degree to which the water is removed from the cell walls. Seasoned wood is stronger, stiffer, and harder than green wood. On the other hand, it may not be so tough as green wood, since dry wood is more likely to break than to bend and subsequently regain its form.. Small pieces of thoroughly seasoned wood may be twice as strong as pieces of the same wood in green condition. Owing to the checks which frequently develop in the seasoning of large timbers, it is not safe to count upon any such great increase in strength in them as occurs in the seasoning of small timbers. This question is further discussed in the chapter on Structural Timbers.
Tests of small, clear pieces of wood dried to a moisture content of 12 per cent give the results shown in Table 9.
A comparison of the specific gravity of these woods at 12 per cent moisture, with the specific gravity of "oven-dry" woods given in Table 1 (page 15), shows that the latter are much lighter. On the other hand, the strength at 12 per cent moisture is much greater than for green timber as given in Table 2 (page 17).
Table 9 Weight and Strength of Wood with Moisture Content of 12 per Cent