Up Wood & Water Specific Gravity Mechanical Properties Enemies of Wood Tree Growth & Wood Quality

Wood & Water

Most problems encountered with wood or wood products, involve water or moisture content.

Wood is HYGROSCOPIC-- Meaning it will take on or give off moisture to equalize with its environment.


Wood has basically 3 types of water in it:

A. water of constitution -- to remove it is to destroy the wood

B. free water -- water just taking up space in cell lumen or other voids

C. bound water -- in cell wall, physiochemical bonds with cellulose chains





EQUILIBRIUM MOISTURE CONTENT (EMC) -- for a given temperature and relative humidity -- a given piece of wood will always take on the same moisture content. See Table A.1. on Page 508 of Textbook.


What happens when you remove water from wood or let the wood increase in moisture content? 

If we start with green wood and slowly dry it, we find that all that we are removing is free water which is just occupying space and with its removal nothing happens to the wood. When we reach the FSP, we start to remove bound water. Now things start to happen with the wood. As we remove more and more water, the wood get stronger at an exponential rate and shrinks at a lineal rate.

If we start with dry wood and add moisture to it, we find that the wood will lose strength and swell until the FSP is reached then nothing more happens. 



Chapter Three revisited:

How are microfibrils constructed?

Cellulose chains grouped together --

Crystalline region -- tightly bonded and encased in hemicellulose -- no access to water

Amorphous region-- free hydrogen bonding sites with hydroxyl groups -- where bound water is located

The percentage of crystalline to amorphous helps determine the FSP.


As the water molecules enter or leave the amorphous region, the cellulose chains either move apart or draw closer together. This causes the microfibril to increase or decrease in size which causes the cell to change size which in turn causes the wood to swell or shrink in size. Also, as the cellulose chains come closer, they bond on each other causing the wood to get stronger. As water molecules are added, these bonds are broken and the wood becomes weaker.

As Chapter three stated: cell walls are made of 4 layers -- primary, S1, S2 and S3. The S2 layer is the thickest and most influential.

How are the microfibrils arranged in the S2 layer?
                    10 to 30 degrees to the longitudinal

Therefore, what are the shrinkage and swelling rates along the principle axes of wood?

Longitudinal        0.1- 0.3 %

Radial                    3 to 4 %

Tangential            6 - 8% sometimes up to 14%

See Table A.2. on Page 509-511 in Textbook for shrinkage values for various domestic woods.

When the principle axes of the wood does not align with the geometric axes of the wood, warpage occurs.




Drying of Wood Products

Wood starts drying from the surface first.  As the surface dries, moisture moves from the interior outward.

Moisture moves in two ways: mass movement of liquid water in cell lumens; and diffusion of individual molecules both bound water or water vapor in cell lumens.

Diffusion does not take place in a cell until it is below the FSP.




The surface of the wood can dry below the FSP while the interior may still be well above the FSP.  A moisture gradient is set up across the wood.  The steepness of the gradient is determined by the speed at which the surface dries.



A good drying program will compromise between speed of drying and flatness of moisture gradient.


The steeper the gradient the more likelihood of drying defects.  Sometimes the degrade caused by fast drying will exceed (moneywise) the savings in drying time.


As the surface dries below the FSP, it wants to shrink but the interior of the wood is still above the FSP and does not want to shrink.  This causes a stress build up: the surface portion is placed in tension and the core is in compression.


As the stress builds up, two bad things can happen.  If the tension exceeds the strength of the surface, you can have splits or cracks in the surface called checks.  If the compression exceeds the strength of the core, you get collapse.


If the wood survives the initial stress build up, the surface will "SET" and as the core continues to dry, the stresses are reversed.  Now the surface is put into compression and the core is in tension as the core tries to shrink.  As the stress builds up, you can have splits in the core called honeycombing.         

           The higher the temperature, the faster the drying rate.

          The higher the specific gravity, the slower the drying rate.

         The higher the air speed, the faster the drying rate.

          The lower the relative humidity, the faster the drying rate.

          The lower the atmospheric pressure, the faster the drying rate.



 Air Drying

Whether you are air drying lumber, fence posts, or other wood products, there are some generalities to keep in mind.

They should be stacked on solid, level supports that are 18-24 inches above level, well-drained ground with the weeds kept mowed.  This way the air will keep the ground moisture from influencing the relative humidity around the wood.

The stacks should be covered to prevent rewetting by rain and exposure to direct sun.  Excessive alternate wetting and drying will cause extreme checking and splitting.  Direct sunlight will cause too rapid a moisture loss.


 If you are drying one stack or more and you are placing the stacks in a straight line, then the stacks should be placed facing the prevailing wind. This way the maximum air flow through the stack(s) will be realized.

 If you are designing the air drying yard for multiple rows, then the stack should be placed parallel the prevailing wind.

If the wind is hitting the stack face, the air flow is slowed going through the stack. The wind blowing over the stack will stay at the same rate and tumble into the area behind the stack mixing with the air coming through the stack causing mild turbulence. Thus, there will be less air flowing through the second stack and consequently even less through the third stack.

If the wind is blowing parallel the stacks, the flow of the air will cause a Venturi Effect and draw the moist air out of the stack and it will be taken away with the wind.


Kiln Drying

 A dry kiln is basically a closed chamber where you can control the temperature, relative humidity, wind speed, volume, and direction.

 By controlling the conditions around the wood, you can develop the best moisture gradient.

See Tables 8.6 and 8.7 on Page 193 in the Textbook



Why kiln dry?

 A) reduce moisture content below attainable levels by air drying

 B) dry wood faster (less inventory cost)

 C) with high temperature, can kill insects and fungi

 D) because of the above, it has a higher market value.


 The denser the wood and the more refractory the wood, the longer it takes to dry the wood and therefore, energy is required to dry it.  In some woods, 70 percent of the energy required to produce finished lumber from trees is tied up in drying.



Dehumidification (DH) Kilns

 DH kilns usually operate at a lower temperature than normal kilns; therefore, a cheaper and less insulated chamber is required.  No boiler is needed, so that great capital cost is eliminated.

Generally speaking, if you are going to dry less than 1 MMbf per year, DH is more economical than normal steam kilns.  Over 5 MMbf per year, steam kilns are more economical.

DH kilns run on electricity.

DH kilns take as long or longer to dry wood as a conventional kiln.

DH kilns are cheaper to buy and install but the electric bill makes them more expensive to operate.

There appear to be less drying defects in DH kiln dried material than conventional dried material.




 Solar drying

Uses solar collectors to concentrate the suns energy to heat the wood.  May not be able to dry below air drying moisture content.

The only energy needed for these kilns is the electricity to run the fans.

In this country, the kiln should face South and the angle of the solar collector should be the same as the degrees of latitude so as to maximize the exposure to the sun's rays.

There are many solar kiln designs. The following is just one example.



 Other Types of Drying


        VACUUM Drying - by lowering the atmospheric pressure, you lower the boiling point of water and increase the diffusion rate.  Less heat energy is required but a chamber with a vacuum seal is required.


          RADIO-FREQUENCY Drying - the wood is dead stacked on a metal platen and another metal platen is placed on top.  Microwave energy is transmitted between the platens.  Heats the wood internally.


          Radio-Frequency/Vacuum (RFV) drying - a combination of the above.


          Boiling in oil - normally used in treating cylinders prior to treating green hardwoods.


          Steaming - heats wood and drives off a lot of free water.  Used in treating cylinders prior to treating green softwood poles.



Up Wood & Water Specific Gravity Mechanical Properties Enemies of Wood Tree Growth & Wood Quality