Southern yellow pine
Southern yellow pine (SYP) consists of four major species: loblolly, slash, longleaf and shortleaf. They grow abundantly throughout the 13 southern states. In fact, over 15 billion board feet of lumber are produced every year when the economy is strong. Some of the growing sites have been harvested five times over the past 200 years. The soil nutrients are in the needles and small twigs, so soil depletion is not an issue when logging these lands, as long as the small items are returned to the site and soil.
This fast-growing species produces some of the strongest wood in North America. Structural uses, such as roof trusses and decking, are the more common uses for this wood. Estimates are that over 60 percent of the SYP lumber is treated to resist decay and is then used outdoors, where it will last for centuries. Due to SYP’s high weight and difficulty nailing, some of the lighter weight Canadian species (spruce and fir) are preferred for 2x4 and 2x6 studs for house walls.
Note that most SYP on the market will be loblolly. Longleaf pine was abundant when the Europeans first came to North America, but it is no longer harvested in any quantity today, partly due to environmental concerns. Occasionally an old building is demolished and the beams and lumber, which are often longleaf, are recycled, especially when building authentic historic buildings.
Uses for furniture, cabinets, and millwork are limited, as the wood is prone to warping, has very heavy grain, and presents special problems in processing, compared to our native hardwoods. Also, drying seldom achieves the low MCs required for furniture and cabinets. Yet, with experience, this wood can be an excellent wood, especially where strength is critical or when the heavy grain appearance is needed. The key is proper drying, using many of the techniques that would be used for hardwood lumber drying.
The density is 34 pounds per cubic foot at 8 percent moisture content. This is one of the heaviest softwoods. Lumber, after drying and surfacing to 3/4-inch, will weigh about 2-1/4 pounds per board foot.
The wood dries quickly, often within a few days in the kiln. However, as most drying of SYP is for 2-inch thick construction grade lumber, it may be difficult to find someone that has the experience and time to dry the wood to 9 to 10 percent final moisture content, using equalizing to achieve the required moisture uniformity and conditioning treatments to remove drying stress. In many cases, quality for lumber intended for remanufacturing is better if you dry it yourself, using hardwood drying procedures and schedules designed for furniture-grade pine and similar products.
Brown stain, common in most pine species, can occur during drying if logs are old, temperatures are high (over 130F), or humidities are high (over 75 percent RH) initially. Anti-brown stain schedules are available.
The resin or sap in the lumber must be "set" in the kiln to avoid subsequent bleeding of the resin. This is done by using 160F or higher temperatures at the end of drying. Shrinkage of flatsawn lumber in drying is about 5-1/2 percent; this is moderate compared to many other species.
Gluing and Machining
Southern pine glues okay, but the dark colored part of each annual ring is hard to glue with many standard "furniture-type" adhesives; good results have been reported with some PUR adhesives. More expensive adhesives, such as phenol-formaldehyde, also work well. Best gluing is achieved if the lumber has not been dried at temperatures over 160 F. As always, with a dense species, gluing surfaces must be flat and true.
Machining is very difficult because the dark-colored portion of each ring is very hard, while the lighter-color portion is very soft. Knives must be very sharp and feed rates modest. Excessive feed or removal will push the hard fibers into the soft fibers. With time and moisture, these areas will pop back up and give a rippled or corrugated surface. The wood will machine best at no lower than 8 percent moisture content; in fact, 9 or 10 percent is even better.
Avoid excessive pressure when sanding, in order to avoid grain raising and gumming sandpaper.
Due to unusual grain (especially compression wood), some pieces will move excessively when their moisture changes. Eliminating compression wood visually (which is very easy) is prudent in many operations. Normally, Southern pine is as stable as most other species.
The strength of Southern pine is very high; MOR is 12,800 psi when dry. The stiffness is also very high; MOE is 1.8 million psi. In fact, this high strength and high density mean that fasteners will perform very well, but will be hard to insert without predrilling. Hardness is 690 pounds; this is somewhat low due to the softness of the light-colored wood in each annual ring. This strength means that for structural components, SYP is ideal.
Color and Grain
Most Southern pine lumber is all sapwood. The sapwood is yellowish, with the darker-colored wood being light brown. The annual ring color contrast is high, adding character. Heartwood, if present, is reddish brown.
The grain is fairly straight, except around knots and when compression wood is present.
Tips to avoid stress development and cracking
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Q: We are having a problem with shrinkage. We make furniture, but someone else sells and delivers it. This person claims he did everything correctly, including opening the furniture wrapping (we wrapped the furniture with shrink-wrap and it was fairly well sealed) and letting it acclimate to the house climate. When the customer moved in, they said the furniture looked really wonderful, but within a week, it started to warp, open joints and crack in a few places. We are so careful to keep our plant at 40 percent RH and check the MC of the lumber. This is frustrating! Can you help?
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