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Moisture Content

Apr 20, 2013
Moisture Content
  • Comments about water content of wood- Specifically, Firewood.

    Ideal seasoned wood is approx. 20% moisture. Green wood (freshly cut) contains as much as 50% water.

    Q: When will water boil out the ends of the pieces? 26 percent? 30 percent?

    A: When MC (moisture content) is greater than right around 28%.

    Q: Why does wood that has a moisture content between 20-25% not boil water out of the ends?

    A: Because there are two places water is stored in wood. Free moisture is water within the wood cell cavities (inside the wood cells). Bound moisture is water bound up in the cell membrane or the cell wall itself. Removal of bound moisture BTW is what causes wood to shrink.

    Free moisture exits a wood splits ends (during drying and burning) via normal phloem networks utilizing the perforated ends of the wood cells stacked end-to-end and forming something like straws, if you will. These are the same passages responsible for normal water movement from roots to leaves in a living tree. Its actually more complex than I describe, but hopefully that will suffice for the purposes of this discussion. :) Water exiting in this manner when a split is burning is seen boiling out of the ends.

    Once wood dries below around 28% MC all the free moisture is gone. At that point, only the cell walls contain water and they dont release it through the phloem channels (at least not as readily) so it isnt seen boiling out the ends. There can still be quite of bit of moisture (as high as 25 - 30%) left in the wood though, and it still must be evaporated from each cell wall before that cell can exceed 212*F and actually burn. The cells dont actually burn [until after the volitiles] gasify into various combustible chemical components when heated [leaving combustible carbon]. These components in turn undergo additional chemical reactions [during combustion,] into simpler components like C and CO utilizing the heat produced by previous reactions while producing heat themselves and so on until only CO2 and H2O remain if combustion is absolute IIRC.