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[Industry Knowledge]Terms Commonly Used to Grade Wood

Here is a brief description of some commonly used terms used to grade heart pine and other woods. Should you have any specific questions, our customer service representatives will more than happy to help you.

Checks  Surface checks occur naturally in Heart Pine. If the product is properly air-dried and slowly kiln-dried, checks can be sanded out or filled during installation.
Grain pattern There are three distinct grain patterns: plain sawn, vertical and curly. Plain sawn has an arching grain. Vertical has pinstripes with no growth rings over 45 degrees perpendicular to the face. Curly is the rarest.
Growth rings  The pair of light and dark growth rings denotes a growing season. The highest grades of heart pine require an average of eight growth rings per inch. Other grades may average six growth rings per inch or less. Dense growth with at least 1/3rd in the dark ring means stronger wood. Longleaf pine often lived 400 or 500 years or more.
Hardness  The scale used to measure wood hardness is called the Janka (“yahn-kah”) scale. The Janka measure for Heart Pine is 1225, compared to red oak at 1290. New Heart Pine is about one-half as hard and comparable to Southern Yellow Pine at 670. (To measure, a reading is taken from a hydraulic press after a .433" steel ball is pressed into the wood to half its depth.)
Heart content  Heartwood is formed when sapwood becomes inactive and is infused with additional resin compounds. It develops slowly in the center of the tree as the tree matures. The older the tree, the higher the heart content. According to the Forest Service a 200-year-old longleaf pine averages only 65% heart content (all the 200-year-old trees are now protected and cannot be cut). Longleaf heartwood turns a rich red color when exposed to light and oxygen. As heart content decreases, color tones can vary widely from pale red to yellow.
Kiln drying  A process by which moisture is removed from wood with heat and dehumidification. This ensures the wood can easily acclimate to a building interior and avoid excessive shrinkage when properly installed.
Knots Clear is the highest grade and has no knots larger than a rare ½” ‘pin knot’. Standard knots occur infrequently in the next best grade, often called select or select and better, and may be up to 1-1/2”. A ‘pith knot’ can be either a pin knot or a standard knot that has a small hole through the knot.
Longleaf pine  Longleaf (Pinus palustris) is the legendary ‘antique heart pine’ wood. The Longleaf ecosystem was once the largest contiguous forest on the North American continent. It is the quantity of resin in the heartwood that gives antique heart pine its uncommon hardness and durability. It takes 90 to 125 years to develop any significant amount of heartwood. Most of the trees were 200 to 500 years old when originally cut. 
Nail staining  Caused when the metal “bleeds” around the nail hole. Nail holes are ¼” in the select grades of Heart Pine, but may be larger in other grades. They can be filled onsite.
Natural Color  Heart Pine is yellow when first cut and turns red when exposed to oxygen and ultraviolet light. Beginning almost immediately, the heartwood will ripen within weeks and will continue to grow richer in color over the first several months. The heartwood portion of building salvaged heart pine is usually already red except for some ‘yellow heart’ areas. These areas commonly occur next to a more resinous area that may have prevented the ‘yellow heart’ area from oxidizing. Once cut the yellow heart will turn red also. If you want to retain the initial light color, a finish with UV inhibitor may slow the change.
Pitch pockets  Small pockets of crystallized resin occur seldom in Heart Pine. In the best grades, pitch pockets will be no larger than 1/8” wide, but can be up to 3/8” or more in other grades.
Resin  Oleoresin, the type of resin from longleaf pines, made the U.S. the world leader in naval stores production until the middle of the 20th century. Longleaf sapwood contains from 1 to 3% resin while the heartwood contains from 7 to 24% resins. The resin build-up is mostly in the latewood or the dark ring of the pair that make up a growth ring. The percentage of latewood is the factor most closely linked with weight and strength. Longleaf has the heaviest concentration of resin of any of the pines.
Sapwood Sapwood (non-heart) is the lighter colored wood on the outer perimeter of the log. It does not deepen in color and is not as hard as the heartwood. The best grades do not contain any sapwood. Lesser grades can have up to 50% sapwood and may today still be called heart pine. Source: Hanbao

Posted and edited by Ran, Hanbao News Department