**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

As
a side note, mild rolled steel has a specific

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
Gravity_{OD} =Mass_{OD}/
Volume_{OD }/ Density of water_{
}

Specific
Gravity_{12%} =Mass_{OD}/
Volume_{12% }/ Density of water_{
}

Specific
Gravity_{Gr} = Mass_{OD}/ Volume_{Gr }_{
}/ 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:

site

geographic

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.