Yesterday user Paul Bunion was explaining that the water in wood is stored basically two ways, in the cells and as non-bound up water in the straws. He explained that the free water in the straws evaporates fast compared to the water bound in the cells.
What is the nature of the cells? Do they hold water because they are porous, absorbent and sponge-like? Or more like water balloons with a skin that holds the water in?
Cells hold water because they contain molecules that like to be bound to water (like carbohydrates, salts etc.). Due to its polar nature water likes to be bound to ions or molecules that also have polar groups. You can test that yourself. Put a drop of water on a flat surface and put the same amount of water on some sugar. The "free" drop will evaporate pretty quickly; the one on the sugar will take a long time and you may even need some additional heat to get rid off all of it. Same with wood.
Quote:
Water and Wood
A commonly mistaken belief about lumber is that once dried it is permanently seasoned in its final dimension. A dry piece of wood will exchange water molecules with the surrounding air according to the level of atmospheric relative humidity. Loss or gain of moisture in wood products may cause such troublesome results as shrinking or swelling, interference with paint adhesion, and increased susceptibility to decay and stain.
Water is found in wood in three forms.
Free water is found in its liquid state in the cell cavities or lumens of wood.
Water vapor may also be present in the air within cell lumens.
Bound water is found as a part of the cell wall materials. As wet wood dries, free water leaves the lumens before bound water. Water can be removed from wood fairly easily up to the point where wood reaches its
fiber saturation point (FSP). The FSP is defined as that MC where the cell wall is completely saturated with (bound) water, but no liquid water is present in the cell lumens.
Wood
does not start to shrink until it has dried below its FSP. FSP for most wood species falls in the range of 25 to 30% MC. It becomes increasingly hard to remove water from wood after reaching the FSP. Remember, it is only after water begins to leave the cell walls that the wood begins to shrink and its strength begins to increase.
How Wood Dries
Wood will seek an
equilibrium moisture content (EMC) in relation to the relative humidity (RH) and temperature of its surroundings. That is, as wood is dried below its FSP, the amount of moisture leaving the wood will be determined by the relative humidity of the atmosphere surrounding the wood. Table 1 shows the EMC over a range of humidities. For wood to air dry, the moisture content of the air must be less than that of the wood.
Table 1. EMCs at Different Humidities for 30 to 90°F
RH 102030 40 50 60 70 80 90
EMC 2-3 4 67-89-10 10-1112-1315-1610-21
Lumber drying is usually accomplished by evaporating the moisture from the surface of the wood. Wood dries “from the outside in”; that is, the surface of the wood must be drier than the interior if moisture is to be removed. Moisture will move from an area of higher moisture content to an area of lower moisture content within the wood. When the surface moisture evaporates from the sides or ends, moisture moves from the interior toward these locations. This process continues until the wood reaches its EMC. At this point the moisture content is equal throughout the piece of wood. Thicker lumber exposed to the same drying conditions will take longer to reach its EMC than thinner lumber.
Wood dries along the grain up to 15 times faster than across the grain. Therefore, a board will dry at a faster rate from its ends. However, because a board is usually many times longer than it is thick, most of the moisture loss occurs across the grain and out the surfaces of the piece. In other words, the moisture travels across the grain at a slower rate, but it has to cross a much shorter distance and, except near the ends of the board, it dries more through the surfaces.
The rate at which lumber dries is controlled both by the rate of evaporation from the surface and by the rate of movement of the water within the piece. As long as the moisture can move from the interior to the surface at a fast enough rate to keep the surface moist, the drying rate will be increased if the surface evaporation rate is increased. This can be accomplished by:
- Increasing the air across the surface of the wood. As long as the RH is low enough, the air will continue to dry all exposed surfaces of the wood.
- Increasing the temperature of the air surrounding the wood. Warmer air holds more moisture; by increasing the temperature, the moisture-carrying ability of the air is increased.
- Reducing the RH of the air. Water evaporates faster into the drier air.
Source:
http://www2.ca.uky.edu/agc/pubs/for/for55/for55.htm
Also why can Maple can go from green to dry in under a year while Oak takes 3 years to let go of its water?
Oak is much denser than maple. Thus, it has more fiber which will hold water for longer.
