Chapter 1. Basic Wood Properties

Moisture Content

      Formulas

            •Moisture content on an oven-dry basis
            (MC_od)

            •Moisture content on a wet or original
              basis (MC_w)

      Conversion Between MC_od and MC_w   

      Fiber Saturation Point (fsp),  Shrinkage, and Swelling
 

            •Calculating shrinkage (Sh) percentage

            •Calculating swelling (Sw) percentage

      Typical Moisture Contents of Wood  

            •Equilibrium moisture content (EMC)

            •Kiln-dried products

            •Air-dried products

             •Green products

Density and Specific Gravity 

      Density

      Specific Gravity     

Calculating the Weight of Wood Products 

      Basic Calculations

(Go to calculation sheet to calculate weight; Go to calculation sheet to calculate volume)

      Simplifying the Calculations

Product Examples

      Logs              

      Cords   

      Lumber

      Plywood

       Veneer 

Chapter 1. Basic Wood Properties

        This chapter explains how moisture content, shrinkage, and specific gravity of wood are meas­ured and gives procedures for estimating weight density in pounds per cubic foot or kilograms per cubic meter of solid wood. Examples using informa­tion in the remaining chapters illustrate methods for estimating weight for various products.

Moisture Content

        Water is naturally present in all parts of a tree and permeates the wood structure. Water com­monly makes up more than half the weight of a living tree, a fresh log, or wet chips. The term green refers to this initial condition. However, when a tree dies or is harvested, bucked into logs, or pro­cessed into products, wood moisture content adjusts toward equilibrium with the temperature and humidity of its surroundings. It eventually reaches an equilibrium at which it still retains some mois­ture. The only case when wood contains no moisture is when it is kept in an oven above 100^oC. In this environment all water is eliminated and the wood is referred to as oven dried. In contrast, kiln drying is designed to bring wood to a moisture content similar to the environment expected in service. As the environment changes, wood will take on or lose moisture as it adjusts to these new conditions. These adjustments may be accompanied by shrinkage or swelling.

        To illustrate calculation of moisture content and other properties, consider a hypothetical piece of lumber 16 feet long with nominal thick- ness and width of 2 and 4 inches respectively. Assume that a researcher carefully measured its volume and weight when green, kiln-dry, and oven-dry, obtaining the following results:

Condition	Volume (ft3)	Weight (lb)
Green	0.78	40.0
Kiln-dry	0.73	22.3

Oven-dry

0.68

19.4

 

Formulas

        Wood moisture content may be expressed in either of two ways, hence it is important to know which is being used. Using the data above, the following formulas show how to find the percent­age of moisture on an oven-dry basis (MC_od) or on a wet or original basis (MC_w).

Moisture Content on an Oven-dry Basis (MC_od)

MC_od       =  100  _*  weight of water / oven-dry weight                   
                =  100  _*  (22.3  –  19.4)/19.4  = 15%   kiln-dry

                =  100  _*  (40.0  –  19.4)/19.4  = 106%   green

MC_od is commonly used in solid wood industries such as lumber and plywood. Note that on this basis, moisture content can exceed 100%.

Moisture Content on a Wet or Original Basis (MC_w)

MC_w =  100  _*  weight of water/original weight                                                        
=  100  _*  (22.3  –  19.4)/22.3  =  13%  kiln-dry
=  100  _*  (40.0  –  19.4)/40.0  =  52%   green

MC_w is commonly used in the pulp and paper and the wood energy industries. Note that on this basis, moisture content can never reach 100%.

Conversion Between MC_od and MC_w

MC_od =  100  _*  MC_w / (100  –   MC_w)
MC_w    =  100  _*  MC_od / (100  +   MC_od)

Fiber Saturation Point (fsp), Shrinkage, and Swelling

        When wood dries below a certain moisture content, referred to as the fiber saturation point (fsp), it begins to shrink and continues to do so until it is oven-dry. Conversely, wood that is below fsp will swell as it takes on moisture and this will continue until fsp is reached. Changes in moisture content above fsp have no effect on shrinkage and swelling. Fsp varies among species, but a value of 30% MC_od (23% MC _{style='font-size:9.0pt'>w}) is commonly assumed.