[Footnote 50:  See Schlich’s Manual of Forestry, Vol.  V. (rev. ed.), p. 75.]

Under ordinary conditions wood contains more or less moisture, so that the application of heat has a drying effect which is accompanied by shrinkage.  This shrinkage completely obscures the expansion due to the heating.

Experiments made at the Yale Forest School revealed the effect of temperature on the crushing strength of wet wood.  In the case of wet chestnut wood the strength decreases 0.42 per cent for each degree the water is heated above 60 deg.  F.; in the case of spruce the decrease is 0.32 per cent.

The effects of high temperature on wet wood are very marked.  Boiling produces a condition of great pliability, especially in the case of hardwoods.  If wood in this condition is bent and allowed to dry, it rigidly retains the shape of the bend, though its strength may be somewhat reduced.  Except in the case of very dry wood the effect of cold is to increase the strength and stiffness of wood.  The freezing of any free water in the pores of the wood will augment these conditions.

The effect of steaming upon the strength of cross-ties was investigated by the U.S.  Forest Service in 1904.  The conclusions were summarized as follows: 

“(1) The steam at pressure up to 40 pounds applied for 4 hours, or at a pressure of 20 pounds up to 20 hours, increases the weight of ties.  At 40 pounds’ pressure applied for 4 hours and at 20 pounds for 5 hours the wood began to be scorched.

“(2) The steamed and saturated wood, when tested immediately after treatment, exhibited weaknesses in proportion to the pressure and duration of steaming. (See Table XVI.) If allowed to air-dry subsequently the specimens regained the greater part of their strength, provided the pressure and duration had not exceeded those cited under (1).  Subsequent immersion in water of the steamed wood and dried specimens showed that they were weaker than natural wood similarly dried and resoaked."[51]

[Footnote 51:  Cir. 39.  Experiments on the strength of treated timber, p. 18.]

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|                                        TABLE XVI                                         |
|-----------------------------------------------------------

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|                EFFECT OF STEAMING ON THE STRENGTH OF GREEN LOBLOLLY PINE                 |
|                                (Forest Service, Cir. 39)                                 |
|-----------------------------------------------------------

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|           |       Cylinder conditions       |                  Strength                  |
|           |---------------------------------+-------------

-------------------------------|
|           |            Steaming             |       Static        |  Impact  |           |
|           |---------------------------------+-------------

--------+----------|  Average  |