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

Specific Gravity

 The one property of wood that all of the others seem related to.

 Specific gravity is the mass per unit volume compared to the mass per unit volume of water at 4 degrees C. In this country we cheat and use weight per unit volume.

Water's mass per unit volume at 4 degrees C is used because that is when water is its densest.  That is why ice floats.

If we were to eliminate all of the voids in wood and have nothing but a solid chunk of cell wall material - the specific gravity would be    1.5.    This means that cell wall material is half again as heavy as water and that it is due only to the pores and voids in wood that it floats.

Sometimes when a log is green, the free water fills all of the voids thereby giving the voids the same specific gravity as water and since the cell walls have a higher specific gravity, the log sinks when placed in water. Water logged.

 As a side note, mild rolled steel has a specific gravity of 7.8.

 As you can see, specific gravity is unitless.

 Density is sometimes considered the same as specific gravity.  After all density is mass per unit volume.

 Specific gravity is calculated using a special density value.

 With density, the mass value can be either the green value, oven dry value, or at any moisture content in between.

 With specific gravity, the mass value is always oven dry.

 The volume value can be either in calculating density or specific gravity.  You must be sure to state at what moisture content the volume was measured.


Specific GravityOD  =MassOD/ VolumeOD  / Density of water

 Specific Gravity12%  =MassOD/ Volume12%   / Density of water

 Specific GravityGr = MassOD/ VolumeGr   / Density of water

You must do the same for any density values stated.




Methods of determining specific gravity


The weight or mass is always determined after oven drying. 103 degrees C until weight of specimen stabilizes (24 to 72 hours)

 The volume can be determined at the moisture content desired.

 LAW OF PHYSICS: a submerged object will displace its volume.

 This leads to 2 ways of determining volume.

 No. I - submerge a piece of wood and measure the displaced volume of fluid


(A)                                                                    (B




No. 2 - set container of fluid on scales, submerge piece of wood, and record the increase in weight.  Convert weight of displaced fluid to volume by using its known density.




Wood being porous, will absorb some of the displaced water giving an erroneous value.

 Care must be taken to keep the wood from wetting.

Paraffin or other coating

mercury or some other non-wetting fluid.




Variations in specific gravity within trees

Growth rate:

1.Ring porous hardwoods- density tends to increase as growth rate increases

     Remember we discussed earlier in the term that ring porous woods lay down the same amount of early wood vessels regardless of the rate of growth and that the increase in growth rate is made up of dense late wood fibers.

Hickory and ash handles - large number of growth rings - weaker.


2.Softwoods with prominent latewood -- density tends to decrease slightly as rate of growth increases.  The correlation is weak.

     Remember the generalization that we discussed about softwood.  An increase growth rate results in an increase in the amount of large thin-walled earlywood tracheids.  This does not always hold true; therefore, the weak correlation.

SYP, Doug-Fir, W Larch - large number of growth rings - stronger.


3.Other woods - diffuse porous hardwoods & softwoods with no prominent latewood - no correlation between density and growth rate.

     Remember in these woods, most of the growth ring is made up of the same type of cells.


Effect of height:

Softwood - reported that specific gravity decreases with increasing height.

Yellow-poplar - highest value at stump decreasing to a level 10 to 20 feet above ground - then increasing


Distance from pith:

generally specific gravity increases with increased distance from the pith - in some cases, after a certain distance from pith, value becomes constant.


Other factors:  



    genetic (cov 10%)





Another useful term is Bulk Density:

This is the green weight per cubic foot for logs.   In today's industry, logs and trees are being bought by the ton. There exist many equations for calculating the cubic volume of logs and trees but a conversion factor is needed to change the volume to tons.  Studies have shown that bulk density is not constant but changes with size and position in the tree.




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