Definitions of Terms- |
 |
These definitions attempt to skim over the whole subject of hardwood lumber and may well contain some minor inconsistencies or omissions.
Hardwood lumber is produced from trees which drop their leaves each year and are known as deciduous trees. This is in contrast to softwood lumber which is produced by evergreen or coniferous trees. The terms hardwood and softwood do not necessarily refer to the hardness of the lumber.
This section is divided into three general subjects.
- Manufacture of lumber.
- Moisture content of lumber.
- Grading of lumber.
Our product here at Kettle Moraine Hardwoods is hardwood lumber. The structure of a hardwood log is such that all of the higher grade (quality) 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 of low quality and is used for pallets, beams, fencing , etc. In order to obtain the high quality lumber in the perimeter our sawyer will continue to turn the log as soon as he sees that the quality of the face is dropping. This tends to result in random lengths and random widths which is the normal standard for hardwoods.
| This process tends to produce plain sawn lumber.The end of the board will show grain pattern where the annual rings tend to run parallel to the flat sides of the board. This grain is known as tangential grain. |  |
| 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. |   |
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.
Return to the top of this page
What's the big deal with moisture content anyhow?
Any woodworker who has completed a beautiful project and has it crack, split or separate
will tell you that shrinkage or swelling is a big deal! This term refers to the
percentage of water (by weight) which is present in wood. With the exception of
construction grade lumber or special orders all of our hardwoods are kiln dried to
approximately 8% moisture content. This is the recommended value for our area. Other parts
of the nation will vary from 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 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, 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 a point of 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% is the objective for woodworking which is
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.
Return to the top of this page
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 recta 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 we supply 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% "
" Grade
established on poor side of board.
Selects- 83 1/3%
" " Grade
established on best side of board.
#1 Common- 66 2/3 % "
" Grade established on
poor side of board.
Several other factors are considered in grade determination:
The top three grades are often combined into one known as Select & better(S&B).
The standard unit of measurement is the board foot which is one foot long, one foot wide and one inch thick or the equivalent (i.e.- 1 foot long, 6 inches wide and 2 inches thick). Length is rounded down to the nearest whole foot. Surface measure (length x width) is half rounded to the nearest whole foot. Surface measure times thickness equals board measure in board feet.
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".