what wood hardness means?
All woods are not created equal and it is important when working with
wood or when selecting a wood species for an architectural application
or a piece of furniture that you understand some of the important characteristics
of wood so that an informed choice can be made.
In this month's fact page I will attempt to explain the issue of hardness
as a property of wood.
The most common way to categorize woods is to lump them into one of
two general headings; hardwoods and softwoods. While most people feel
this as much as they need to know when selecting wood for hardness,
these categories are not at all accurate when concerned about the hardness
or durability of a specific species of wood. The terms hardwood and
softwood refer the botanical classification of a wood species and has
nothing to do with its density.
Hardwood is a common term used to describe trees in the botanical division
Angiospermae or angiosperms. Angiosperms are flowering plants with seeds
born in a vessel, and with generally broad leaves. They more commonly
lose their leaves in the fall but some species do remain evergreen.
Examples of angiosperms are oaks, maples, cherry, and walnut.
Softwood is a term used to describe trees in the botanical division
Gymnospermae or gymnosperms. Gymnosperms are not flowering plants and
produce naked seeds, usually on the scale of a cone. Their leaves are
usually needle-like or scale-like and usually, although not always,
evergreen. Examples of gymnosperms are pines, firs, and cedars.
All woods which are sold commercially as lumber are given a rating for
their density. While density is the best single figure to determine
the hardness of a wood species, it is not without some glitches. Density
refers to a ratio of how much a certain volume of wood weighs and is
often given in the form of specific gravity, which gives it a comparative
listing to the scientific standard of water. A wood with a specific
gravity of .75, means that given the same volume of wood and water,
the wood will weigh 25% less than the water. If the wood species has
a specific gravity of 1.02, it will sink in water. Relative densities
of various woods mean nothing until one compares those densities with
species with known working properties.
Another way that density may be listed in the wood industry is using
the ratios of kg/m3 or lb/ft3. Kg/m3 is very easy to convert
to specific gravity since specific gravity of 1.0 equals 1,000kg/m3.
Here are the specific gravities of a few common woods.
Balsa .16, Western Yellow Pine .51, Poplar .45, Cherry .58, White Oak
.76, Maple .72, Ipé 1.08.
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