Mesquite Properties

An Overview of Wood Properties

Printed with permission from the National Wood Flooring Association Technical Publication No. A200, pp 3-8. Copies and reprints are available by calling the NWFA at (800) 422-4556 or 314-391-6137 or through their web site at www.woodfloors.org.

Wood is a dynamic medium. Like all organic materials, it has character and quirks, responds to its environment, and should be treated with understanding and a certain amount of care. For wood flooring professionals, knowing about the properties of wood in general, as well as those of individual wood species, is critical to proper installation. For consumers, it's important to have realistic expectations about how wood will perform.

The Character of Wood

As a flooring material, wood is superior to vinyl or carpet, both practically and aesthetically. A solid wood floor is more than a covering; it adds strength and stability to the floor system. A one-inch thickness of wood has the same insulating value as 15 inches of concrete. Wood is durable and long-lasting - occasional sanding and refinishing essentially results in a brand-new floor. Wood floors don't retain mildew or absorb dust, simplifying cleaning.

The most appealing characteristics of wood flooring may be its attractive appearance and natural warmth. A beautiful wood floor can enliven a drab room, enhance any architectural style, complement furniture and design schemes, and add value to any home or building.

A combination of qualities should be considered when selecting a species for flooring: appearance-related attributes such as texture, grain and color, as well as mechanical properties like dimensional stability, durability, machinability and ease in finishing; and finally, availability and cost.

Properties Affecting Appearance

Many different factors, from the nature of the living tree to the way the lumber is sawed, affect the way the finished floor will look.

Heartwood is the older, harder central portion of a tree. It usually contains deposits of various materials that frequently give it a darker color than sapwood. It is denser, less permeable and more durable than the surrounding sapwood.

Sapwood is the softer, younger outer portion of a tree that lies between the cambium (formative layer just under the bark) and the heartwood. It is more permeable, less durable and usually lighter in color than the heartwood.

The relative amounts of heartwood and sapwood in a flooring batch may affect the way it accepts stain and finish and, therefore, the finished appearance of the floor. Heartwood is also more dimensionally stable than sapwood.

Wood grain and texture: "Grain" and "texture" are loosely used to describe similar properties of wood. Grain is often used in reference to annual growth rings, as in "fine" or "coarse" grain; it is also used to indicate the direction of fibers, as in straight, spiral and curly grain. The direction of the grain, as well as the amount of figuring in the wood, can affect the way it is sanded and sawed.

Grain is also described as either being "open" or "closed", referring to the relative size of the pores, that affects the way a wood accepts stain and finish.

Mechanical Properties

Moisture Content and Dimensional Stability: Moisture plays a large part in how wood behaves, both during the machining process and after installation. Installers would do well to study moisture's effect on wood in some detail; however, a brief discussion is worthwhile here.

Moisture content is defined as the weight of water in wood expressed as a percentage of the weight of oven-dried wood. Weight, shrinkage, strength and other properties depend on the moisture content of wood. In trees, moisture content may be as much as 200 percent of the weight of wood substance. After harvesting and milling, the wood will be dried to the proper moisture content for its end use.

Wood is dimensionally stable when the moisture content is above the fiber saturation point (usually about 30 percent moisture content). Below that, wood changes dimension when it gains or loses moisture.

The ideal moisture content for flooring installation can vary from extremes of 4 to 18 percent, depending on the wood species, the geographic location of the end product and the time of year. Most oak flooring, for example, is milled at 6 to 9 percent. Before installation, solid wood flooring should be acclimated to the area in which it is to be used, then tested with a moisture meter to ensure the proper moisture content.

The individual species descriptions that follow include an indication of dimensional stability, from "below average" to "excellent", as well as a comparison to red oak. For example, mesquite (the most dimensionally stable species in this publication) is rated as "excellent", with a notation that it is 65 percent more stable than red oak, that is, mesquite is likely to shrink or swell 65 percent less. The percentages noted are based on comparing a factor called the "dimensional change coefficient" of each species with that of red oak. Red oak was chosen as the benchmark because of its widespread familiarity and use in the flooring industry.

Hardness and Durability: Probably the most important strength property for wood used in flooring applications is its side hardness, also known as Janka hardness. Side hardness represents the resistance of wood to wear, denting and marring. Refer to the following chart.

Machining: The workability of a wood depends on several factors, including density, extent of interlocked or variable grain, hard mineral deposits and tensions in wood that may cause fibrous and fuzzy surfaces. The degree of checking and separation present will also affect machining ease.

Hard deposits in the cells, such as calcium carbonate and silica, may have a pronounced dulling effect on cutting tools. This effect generally increases as wood is dried to normal working requirements.

Nailing: When nailing some of the denser woods with hand or air nailers, installers may encounter splitting tongues, as well as failure to secure the fastener even after repeated attempts. This can sometimes be corrected by changing the angle of the nail's point of entry. On certain exceptionally dense species, pilot holes may have to be drilled to ease nailing. Blunting the ends of fasteners may also help prevent splitting.

Though dense, heavy woods normally offer higher nail-withdrawal resistance, less dense species allow the use of more and larger-diameter fasteners to compensate for their lower holding ability.

Sanding: Some wood species are highly resinous and tend to clog sandpaper. When working with such species, it may be necessary to use a coarser grit of sandpaper than normal, or to change the sandpaper more often than with other species.

Also, the wood dust created by sanding some species tends to cause an allergic reaction in some people. This is more likely to occur with imported species than with domestic. However, even North American oak has been known to cause a skin rash or respiratory difficulties in some people. Where applicable, known tendencies to cause allergic reactions are noted.

As a precaution, flooring mechanics should wear long sleeves, dust masks and eye protection when sanding.

To test for possible allergic reaction to a species, perform a skin-patch test by placing a small amount of wood dust under a round adhesive bandage on the inside of the forearm. If serious skin irritation is present when the bandage is removed after 24 hours, consider not working with that species.

Finishing: Some woods contain oils and chemical compounds that may adversely react with certain types of finishes to inhibit drying, dramatically change the color of the wood, or both. In addition, many finish formulations are undergoing continual change as their manufacturers move to comply with evolving environmental regulations, making hard-and-fast finishing rules difficult to come by.

Water-based urethane finishes have gained in popularity over the past 10 years. They are quick-drying and increasingly durable. Some flooring professionals also believe they tend to inhibit the color change certain woods undergo over time, which may be desirable. These finishes tend to leave wood lighter in color. Non-ambering urethanes are often recommended for finishing white or pastel floors.

Water-based finishes tend to adhere well to most woods, including exotics, whereas some solvent-based finishes have adhesion, drying or color change problems with woods like African cherry, rosewoods, teak, Brazilian and African walnut, purpleheart, padauk and wenge.

For floors that are to be stained to alter the natural color of the wood, flooring professionals should be aware that some species (hard maple, pine and fir, for example) do not accept stain as readily or as evenly as other species.

A grain filler is sometimes used for wood species with large pores, such as oak and walnut, if a smooth finish is desired.

When working with a new species for the first time, installers should test stains and finishes on a small sample of flooring before attempting an installation.

Relative Hardness of Selected Wood Flooring Species
(Ranked by Janka hardness rating)

The Janka (or side) hardness test
measures the force required to embed a
.444-inch steel ball to half its diameter
in wood. It is one of the best measures
of the ability of a wood species to
withstand denting and wear.

Editor's Note: It becomes very clear why
Mesquite is a superior floor material
when these mechanical properties are
compared to other woods used in
flooring. Mesquite is well above average
in density and hardness. As shown in the
accompanying charts, Mesquite is 150%
as stable as Brazilian cherry and nearly
350% that of Oak. Walnut's side
hardness in only 50% that of Mesquite's.

Relative Stability of Selected Wood Flooring Species
(Ranked by dimensional change coefficient)

Stability is the most important aspect
when considering the type of material to
be used for flooring. Most of us
consider stability only in the sense of
how much a certain measure of wood
shrinks as it dries. There are other
factors that are necessary to review
when considering something as stable.

Wood is dimensionally stable when the
moisture content is above the fiber
saturation point. Wood changes
dimensions as it gains or loses moisture
below that point. It shrinks when losing
moisture in the cell walls and swells
when gaining moisture. This shrinking
and swelling may result in warping,
either sideways developing a "crook,"
bow, or twist; or across the grain
showing itself as a "cup."

Wood shrinks most in the direction of the annual growth rings (tangentially), about one-half as much as across the rings (radially), and only slightly along the grain (longitudinally). The combined effects of radial and tangential shrinkage can distort the shape of wood pieces because of the difference in shrinkage and the curvature of the growth rings.

Editor's Note: Mesquite's rates of shrinkage in both directions are almost equal. This fact, in addition to the very low shrinkage values, sets mesquite above all of the other flooring woods when considering stability.

The numbers in the chart reflect the dimensional change coefficient for the various species, measured as tangential shrinkage or swelling within normal moisture content limits of 6-14 percent. Tangential change values will normally reflect changes in plainsawn wood. Quartersawn wood will usually be more dimensionally stable than plainsawn.

The dimensional change coefficient can be used to calculate expected shrinkage or swelling. Simply multiply the change in moisture content by the change coefficient, then multiply by the width of the board. Example: A mesquite board (change coefficient = .00129) 5 inches wide experiences a moisture content change from 6 to 9 percent a change of 3 percentage points.

Calculation: 3 x .00129 = .00387 x 5 = .019 inches.

In actual practice, however, change would be diminished in a complete floor, as the boards¹ proximity to each other tends to restrain movement.

The chart is best used for comparison.

* Source: Hardness and Stability ratings for most species taken from Wood Handbook: Wood as an Engineering Material (Agriculture Handbook 72, Forest Products Laboratory, Forest Service, US Department of Agriculture; revised 1987), except for Australian cypress, wenge, African padauk, merbau and Santos mahogany, which were provided by International Hardwood Flooring; heart pine by Mountain Lumber, and mesquite by Mesquite Products of Texas.