Sawing, Drying, Grading

Manufacture of Lumber

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Our product here at Kettle Moraine Hardwoods is hardwood lumber.  The structure of a hardwood log is such that most of the more valuable higher grade lumber is found in the outer portion of the log's circumference.  The center of a hardwood log which contains the pith (about 4 to 5 inches round) is less valuable because it produces boards with more knots and splits. That material can be used for rustic grade boards & also for pallets, beams, fencing, etc. In order to obtain the most high quality lumber from the perimeter of the log, our sawyer will continue to turn the log until they see that the quality of the face dropping.  This tends to result in boards of random lengths and random widths, which is the standard way hardwoods are sold.

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This process also tends to produce plain sawn lumber. The end of the board will show grain pattern where the annual rings run parallel to the flat sides of the board.  This grain is known as tangential grain.

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Special sawing procedures can be used to produce quarter sawn and rift sawn lumber. This is more difficult and reduces the yield from the log resulting in higher prices.  In these boards the grain tends to run at right angles or 45 degrees to the sides of the  board.  This grain is known as radial grain.

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Plain sawn lumber is generally preferred by woodworkers because of it's lower price and more decorative appearance.  It is, however, more likely to warp or crack because of different shrinkage rates of the annual rings.  Tangential shrinkage is always greater than radial shrinkage.  In the case of Red Oak, a board will shrink 8.6% tangentially and 4.0% radially.  Quarter sawn and rift sawn boards are more stable and less subject to uneven shrinkage since all of the grain is radial.

 

Moisture Content of Lumber

 

What's the big deal with moisture content anyhow?  Any woodworker who has completed a beautiful project and had it crack, split or separate will tell you that shrinkage or swelling is a big deal!  Moisture content refers to the percentage of water (by weight) which is present in the wood.  With the exception of construction grade lumber, industrial beams & some turning stock, all of our hardwoods are kiln dried to approximately 8% - 10% moisture content. This is the recommended value for our area.  Other parts of the nation will want different moisture contents than this (Arizona - 4% and Florida - 13%) depending on the average relative humidity at their location.

The following comments attempt to define green, air dried, and kiln dried lumber:

 
Green lumber - A fresh-cut log will have a varying moisture content depending on the species.  Ash at about 45%, Red Oak at about 80%, and Basswood at about 130% (yes, a green Basswood board has more water than wood by weight).  If the log is sawn immediately after cutting the tree down, it will of course yield green boards.  The term "green" can still be considered to apply until the fiber saturation point is reached.  At this point (about 30% M.C.) all of the free water in the board is gone.

Air dried lumber - This is a difficult category to define because of the variety of opinions that exist.  Lumber will not dry below the point where it reaches equilibrium with the temperature and relative humidity that is present in the air at the time.  When this condition exists, we say that equilibrium moisture content (EMC) has been reached.  In Wisconsin, this mean equilibrium point is about 15%.  This will vary with the seasons but, in general, a moisture content between 15% and 20% is the best we can expect from air drying.  We hear of 1 inch thick lumber that has been air dried for three or more years - it would be more accurate to say it was air dried for 1 year and sat there for 2 more years since it will not dry beyond the current equilibrium point.  In fact it takes on moisture whenever the relative humidity rises.

Kiln dried lumber - This term refers to lumber which has been dried to a desired moisture content depending on the use intended.  As previously stated, in Wisconsin 8% - 10% is the objective for woodworking projects which are to remain in a heated home.  This is a good compromise between the relative humidity on a damp summer day, and the much lower relative humidity found in the winter in a heated home.  Our target moisture content might be higher or lower depending on the expected use of the lumber.  The drying process is generally performed in a dry kiln which allows us to control the temperature, relative humidity, and air movement in order to dry the wood as rapidly as possible without the damage which can occur if the wood is dried too rapidly.  It would be possible to attain the same result in your basement or garage if the ability to control relative humidity existed, but it would take longer than can be done in a dry kiln.

 

Grading of Lumber

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Our lumber is measured and graded according to rules established in 1897 (and maintained and updated ever since) by the National Hardwood Lumber Association (NHLA).  The grading rules will in general define an expected standard for the amount of usable (defect free) wood in a board.  It is not possible to do more than skim the surface of these rules in this space since they are very extensive and detailed.  We will concern ourselves with the top four grades although there are lower grades which may be satisfactory, depending on the expected use.  Remember that the grade is designed to establish a predictable amount of defect-free wood so that the buyer can obtain what is needed.  Special consideration regarding length, thickness, grain patterns, color, etc. can always be specified in an agreement between buyer and seller.
 
The surface of a board to be graded is divided into imaginary rectangles of defect-free areas known as clear cuttings.  The minimum size of the cuttings and the number of cuttings allowed in a board is also specified.  The percentage of the board's total surface is considered to determine the grade of the board. The grades in kiln dried lumber for woodworking are as follows:

     Firsts -     91 2/3% clear cuttings.  Grade established on poor side of board.
     Seconds -  83 1/3% clear cuttings.  Grade established on poor side of board.
     Selects -   83 1/3% clear cuttings.  Grade established on best side of board.
     #1 Common - 66 2/3% clear cuttings.  Grade established on poor side of board.

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NOTE:  The best grade we offer in our store is a combination of the top 3 grades above called Select & Better (S & B)

Several other factors are considered in grade determination:

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1. Minimum length and width.

2. Number and size of clear cuttings.

3. Amount of wane (bark edge or lack of wood on edge).

4. Special exceptions to rules based on species.

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We also have a supply in some species of a "Rustic" grade.  This grade allows some of the knots, mineral streak and color contrast in the wood.  The boards are sound, kiln dried and can be used in woodworking projects to show the natural character of wood. 

 

The standard unit of measurement in our lumber is the board foot, which is 1 foot long, 12 inches wide and 1 inch thick or the equivalent.  In a 1" thick board, every sq ft equals a board foot.  As the boards get thicker, the board footage goes up proportionately.  So, a 1 foot long board that is 12" wide and 2 inches thick, measures 2 board feet.

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Standard thicknesses are defined by NHLA rules. For lumber sold as 1" thick it must be possible to surface it on both sides to 13/16 inches. Since about 1/8" is lost in drying, the rough thickness from the sawmill is at least 1 1/8".