Anhydrous ammonia helps bend wood.

Wood bending is of key importance in modern wood product designs and specifications. It is used in the manufacture of furniture, baskets, sporting goods, and other components.

Wood, like most materials, does not always come in the desired shapes or properties. As a product of nature it varies greatly from batch to batch and is highly anisotropic (materials with greater strength in one direction), with its greatest strength and stiffness parallel to the gram structure.

Early civilizations shaped and formed wood to meet a variety of needs. Ancient sarcophagi were laminated with cross-banded and cross-plied wood veneers to form beautiful shapes that have withstood centuries. Ancient man used wood for various tools and weapons. Green wood was easily coaxed into curved shapes. Once the curved shape cures or dries, it will hold its shape. This is the oldest, least precise, but easiest way to bend wood. Heat and steam was used to help straighten or further shape tools.

Bending Methods

Bent wood can be made into lightweight, strong, curved shapes by three board bending techniques or methods: mechanical, steam or hot water and chemical.

Bending wood into the desired shape may be the most economical method to produce curved shapes. Wood properties such as strength and strength-to-weight ratio may be improved by bending. Most bending techniques waste little or no material. Some methods require no machining or removal of material to attain the desired product design.

Mechanical Bending Methods

Bending green or partially seasoned wood is not new. Moisture content and curing must be carefully controlled if green stock is to be used as shrinkage may result in checks and other dimensional problems.

Shaping curved wood by band-sawing wastes considerable wood and produces curved pieces that have cross grain or grain that runs out along the surfaces. Solid pieces may be bent by making a series of deep saw kerf cuts on the concave side of the piece. Splines, veneers or other pieces are then glued to the concave side to strengthen and keep the required curvature.

Laminating or veneer bending is probably the most familiar method used to make curved products. Curved, laminated beams and other structural members are made by bending and gluing parallel grained layers of wood into the desired shape. This produces a highly anisotropic component with its strength and stiffness parallel to the grain. This is about 20 times what they are perpendicular to the grain.

Laminated, curved beams are far superior to solid wood that is cut to the same size and shape. Curved structural timbers have been used to span more than 91 meters. The term "structural glued laminated timber" refers to an engineered, stress-rated product of a timber laminate used as load-carrying structural framing in roofs, bridges, towers and marine installations.

Steam or Hot Water Methods

Wood cells are held together by an adhesive composed of lignin and hemicellulose. In green wood, this matrix adhesive is moist and semiplastic. This allows wood to be bent or stressed without fracturing. The stretched fibers on the convex face tend to fracture long before the compressed concave face reaches maximum stress limits. In green wood, rupture may be due to hydraulic pressure built up in the moisture-laden cells.

In air or kiln-dried wood, the matrix that bonds fibrous cells together is hard and tough. When cured wood is bent, it is stressed on the outer or convex side and compressed along the inner or concave side of the bend. This distortion caused by the stress tends to bring the bent piece back to its original shape.

The purpose of softening the matrix in wood is to reduce the development of these stresses and allow the wood to have a stress-free bent shape.

A combination of moisture and heat can produce a degree of plasticity roughly 10 times that of dry air or kiln dried wood. This combination softens the matrix sufficiently to take the deformation necessary to bend the softened wood. Forest Products Laboratory research shows that the optimum moisture content for bending southern oak in a hot-plate press is approximately 20 percent.

Boiling water or steam has been an accepted method for softening and bending purposes. Steam softening requires about two to three minutes per millimeter of thickness. It is commonly accomplished by placing wood in low pressure vessels or boxes. Boiling requires about three minutes per millimeter.

Boiling water treatment results are equivalent to steaming at atmospheric pressure. It is preferred when only a portion of a piece is to be bent. Wood with a moisture content of 20 to 30 percent is sufficient to render it compressible, especially when heated.

Heated plates or forms are used to force the steamed or water-treated piece to the desired shape. Pieces must be held until cooled and dried. Matrix plasticity is reduced as it cools and moisture is lost. With this change, new stresses are created that will hold the bent piece in the desired shape.

Moisture loss also creates shrinkage. With this shrinkage, internal stresses may cause surface checks and dimensional changes. Curvature increases with moisture loss. Once the wood is dried, the curvature is fixed and the wood properties are again similar to the original untreated state.

Chemical Methods

Chemicals such as urea, urea-adehyde and ammonia have been used to soften the matrix that holds wood fibers together. In tests conducted at the Forest Products Laboratory, urea-treated wood did not bend as easily and developed more stress failures than steamed wood.

Back in 1964, scientists at the New York State College of Forestry at Syracuse discovered that ammonia would soften the matrix bonds in wood, with both anhydrous ammonia and compressed gaseous ammonia used in varying degrees of success. The scientists found that ammonia breaks down the hydrogen bonds within cell walls and within the lignin matrix. This allows molecular movement when subjected to tensile or compressive forces. Once the ammonia evaporates from the wood, new bonds are made creating a permanent set; in simple terms, this can be considered similar to the process of adding a permanent wave to hair.

The process is reversible by further ammonia treatment. Chemical set is not recoverable by treating with hot water or steam; unlike with steam and hot water bending treatments, no stresses or strains are bent into the wood. Thus, the new shape is like other kiln or air dried wood. It is ready to be machined, glued and finished.

Several experiments with ammonia have shown that wood, especially hardwoods, can be used to make numerous unique furniture designs. It has also been used to help disintegrate wood in the manufacture of hardwood panels and chipboards. The process may also clear the way for better utilization of raw materials and new, economical designs.

Experiments with Anhydrous Ammonia

Several experiments with anhydrous ammonia in pressurized vessels have been highly successful. Pressure vessels are used because the anhydrous ammonia vaporizes and dissipates at atmospheric pressures and temperatures above -28F.

However, processing in pressure vessels has several drawbacks, especially for special, or low production activities. Personnel must be trained and certified to compress and use pressure vessels. Pumps, vessels large enough to hold various sizes of wood lengths and recovery systems are expensive.

Anhydrous ammonia is kept in a pressurized tank to keep it liquid. It is highly corrosive to aluminum, brass and copper, and presents considerable health hazards. Iron or steel must be used anywhere ammonia vapor or liquid contacts metal. It can cause frost burns and serious damage to the nose, lungs and eyes.

Alternate Experiment

For short runs or one-of-a-kind special designs, a simple soak tank can be used. [ILLUSTRATION FOR PHOTO 1 OMITTED]. Standard filling fittings used for charging anhydrous ammonia can be welded to a steel tank/pipe. Since this is not a pressure vessel, a filler pipe should be extended to within 50mm of the bottom to reduce spillage and boiling of the liquid ammonia during fill.

The tank should be slowly filled with anhydrous ammonia in an open area or field. A vent pipe on the lid of a pipe tank allows fumes to be directed away from the operator during filling. Remember, ammonia evaporates at room temperature. The colder it is outside during this operation, the slower the anhydrous ammonia vaporizes. During hot weather, vaporization will be more rapid. It may be necessary to refill the tank in order to keep the wood covered at all times.

Wood specimens should be soaked in the ammonia for about five or more minutes per millimeter of thickness. It may be necessary to use iron weights to keep the buoyant wood pieces submerged. At least 90 minutes soak time are required for 18mm thickness. Additional soaking time is required for 18mm thickness. Additional soaking time may be required for some species or denser woods. In any bending process, the stock should be as straight grained and free from knots or grain figure as possible.

Results of using a "soak" chemical method were quite successful. Wooden strips approximately 6mm by 600mm were covered for about 35 minutes. The anhydrous ammonia was then drained onto the ground and allowed to evaporate. Specimens could be bent within one hour after removal from the soak tank. Red oak, American cherry, western cedar, Philippine mahogany and American black walnut were easily bent in a variety of jigs and by freehand. [ILLUSTRATION FOR PHOTO 2 OMITTED].

Allow bent pieces to "de-gas" for 48 hours in a well-ventilated room before removal from bending jigs. Bending jigs can be as simple as a "peg mold." [ILLUSTRATION FOR PHOTO 3 OMITTED] A 6mm American cherry dowel was simply bent around a coffee can and held in place with hand clamps. The result is a cherry spiral or spring. [ILLUSTRATION FOR PHOTO 4 OMITTED].

The most common bending failure resulted from a buckling or wrinkling in the fiber walls caused by compressive failure on the concave side. Walnut and oak were reluctant to bend as sharply as birch, cedar and Philippine mahogany. Some oak and walnut specimens broke from tension failures on the convex side. Wood can be stretched only about 2 percent in tension, but more than 30 percent in compression.

You can expect color changes in all specimens. Many of the walnut specimens lost much of their color. These results were actually positive because all specimens took stain and finish very evenly. Light colored species absorbed color from darker species.

One contractor used a soak tank on-site to bend previously milled 18mm thick oak strips. The softened pieces were simply attached to a box frame for the bar and overhead shadow box/glass holder. The bar was then finished and hardware installed. [ILLUSTRATION FOR PHOTO 5 OMITTED].

In another example, the contractor used the wall as a forming jig. Previously milled oak was soaked for more than two hours and then simply nailed to the wall. Sanding and finishing was completed in place. [ILLUSTRATION FOR PHOTO 6 OMITTED].

To summarize, use of anhydrous ammonia will make wood thermoplastic enough to be bent. Heat treating specimens may aid in the bending process. Working in a cold climate will slow the evaporation of ammonia but reduce some of the plasticizing effects.

Terry Richardson, PhD, is a professor in the Industrial Technology Department at Northern State University, Aberdeen, SD. He can be reached at [phone] or by E-mail at: [e-mail]