Wood veneer, with us since the days the ancient Egyptians began making fine furniture, has evolved in production quality, consistency and application. Competing with decorative laminates and other alternatives, wood veneer is holding its ground, in part because of recent developments such as improved backings, and more durable surfaces. A trend toward natural interiors is also furthering the cause for those who see real wood veneer as preferable to non-wood-based substitutes.
Through traditional forms of sheet veneer (raw, paper and phenolic backed, laid up) and patterns (book matched, slip matched, radial matched and diamond matched, etc.) are adapted to shifting interior design trends (textured surfaces and reconstituted wood veneers), classic styles still dominate.
Veneer can be challenging. Australia’s Forest and Wood Products Research and Development Corporation catalogs common challenges in veneers: discoloration, checking or cracking, discoloration, zigzag telegraphing, glue line discoloration, glue bleed-through, veneer delamination, lacquer crazing, or blushing.
Improvements in manufacture allow veneer makers to slice or peel ever-thinner layers from logs. A Manual for Decorative Wood Veneering Technology, a wonderful reference published by the Forest and Wood Products Research and Development Corp. of (available free at the online version of this article), explains, “Why do we need veneer?” The answer:
• Veneer makes for economical use of figured wood, the thin slices delivering the maximum surface area from a log.
• Veneer permits use of highly figured timber with unusual and beautiful effects due to grain irregularities, timber that otherwise can’t be seasoned into boards.
• Veneered panels are less prone than solid figured timber to shrink, check, warp.
• Less expensive timbers can be used in the cores of veneered panels, which are built up to provide stability and strength and the most suitable foundation for displaying the veneers to best advantage.
• More extensive use can be made of figured timber by matching consecutively cut sheets of veneer to produce effects impossible to obtain with solid construction.
• Bent and curved panels are readily fabricated by gluing up veneer between shaped forms or in a vacuum press, often using accelerated curing methods such as radio frequency or electrical resistance heating.
The Timber Veneer Assoc. of Australia also posts a number of troubleshooting guides and technical papers. A recent one addresses the discoloration effect sunlight has on reconstituted and dyed veneers, which is attributed to UV radiation. “Generally, the application of protective clear coatings containing UV-absorbing additives significantly reduces the detrimental color change of veneers caused by UV radiation originating from the sunlight exposure,” says the paper. “High quality solvent-based acrylic-polyurethane coatings systems with UV blockers are the most effective protective systems which can be used on veneers.”
The Manual for Technical Wood Veneering also provides guidance on the performance of veneer in relation to humidity, and for “flattening.” Peeled or sliced from bulbous burls or gnarled stumps, some veneer sheets resist lying flat. Here are excerpts from the manual on humidity and flattening:
As a very thin material, veneer responds quickly to humidity changes. Therefore, it is critical that the value of the veneer moisture content is as close as possible to the average value of the equilibrium moisture content for the intended service conditions.
The moisture content of veneer must be in the range of 6% to 12% prior to making up into layon and also prior to pressing onto the substrate. However, it should be pointed out that according to the standard requirements the moisture content of the substrate must be in the range of 8% and 12%. The use of such a wide range of moisture content for the veneer and substrate could result in a high moisture gradient between two types of laminated materials.
For example, the use of veneer of 12% moisture content and the MDF of 8% moisture content would certainly cause severe veneer checking in service. Therefore, to avoid any possible failure it is recommended that the moisture content of veneer and substrate should be between 8% and 10%. However, some brittle veneers are difficult to handle when their moisture content is below 11%. In such cases, the moisture content can be increased to 11% or 12%.
Severe problems can occur if the moisture content of veneer is too high or too low. If veneer with too high a moisture content is used for production of furniture panels and the furniture is then used in a dry environment (such as an air-conditioned or centrally heated building) it will dry out and shrink significantly, resulting in splitting and cracking.
The moisture content of the veneer should be measured with special moisture meters available for measuring veneers. It is important to use correction factors for various veneer species. These can be obtained from any supplier of moisture meters. If the moisture content of veneers is too high they should be re-dried.
|Madison (WI) Area Technical College is the site for a Veneer Seminar, on March 22, 9:30 a.m. - 2:00 p.m. Conducted by Stiles Machinery and National Casein Co., "Tips & Techniques for Working with Veneers" is free, but you must register at stilesmachinery.com/events|
There are various methods that can be used for re-drying veneers and which keep them flat. One method is to put a stack of veneers in a warm press (slightly above room temperature) and leave them overnight. Another method is to put a few sheets of veneer in a hot press (about 60°C) for 2-3 hours, between two pieces of dry, absorbent board, which will remove excess moisture.
Flattening of Veneers
One of the main prerequisites for good veneering work is that the veneer must be flat. However, veneers often buckle or warp in various ways. The primary causes of general buckling of veneer are tension wood in hardwoods, compression wood in softwoods, irregular grain and non-uniform drying. In all cases, buckle is caused by unequal stresses across or parallel to the grain of a sheet of veneer.
Buckled veneer can be flattened by various methods, which are based on the application of moisture, heat and pressure. The most commonly used method involves applying a mixture of water and glycerin to the veneer to dampen the wood. Various proportions of the two liquids are recommended and a solution of 10% glycerin to 90% water is usually used.
A sheet of dry absorbent material (e.g. particleboard, brown kraft paper) is inserted between every 6-10 sheets of veneers (depending on the species and its density) to absorb excess moisture. The veneer is then kept flat in a warm press. The time in the press can be varied, but two hours at 60°C is thought to be adequate, provided sufficient absorbent material is included within the stack in the press.
Laying burls and curl veneers can cause difficulties because their surface usually is not flat, but presents a mass of brittle knots and short fibers. These veneers are often dried with a slightly higher moisture content than ordinary veneers, which makes them less likely to crack or break in handling.
However, it is almost impossible to prepare and handle burl veneer with low moisture content. To overcome this problem, the veneers need to be dampened to make them more flexible prior to flattening and, unless they are dried carefully, this treatment can increase the risk of cracking in later stages.
There are methods of flattening burls while minimizing the risk of cracking. The most effective procedure is:
■ Dampen every third or fourth veneer in a stack of 10 to 12 with a sponge or rag dipped in water.
■ Wrap stack in a plastic film for 24 hours to enable all veneers to reach equilibrium
■ After removing veneers from plastic film, place a panel of a dry particleboard in the center of the stack.
■ Lightly press the stack in a heated press at 800°C for two hours.
■ After removing the stack from the press, remove dampened particleboard from the center and replace with a similar dry panel.
■ Place stack under a light pressure between two panels of dry particleboard or plywood in a dry atmosphere for one or two days.
Sourcing sheet veneer, so different than buying lumber or panel, is almost as labor -intensive as its manufacture, involving much hand work and individual inspection.
Great for sourcing veneers in the U.S. is RedbookOnline.com, with listings of commercial veneer providers broken down by:
Pressure Sensitive Veneer
Veneer Rolls (natural/pre-finished)
Flexible sheets (pre- or unfinished)
RedbookOnline.com also lists veneer sources grouped under more than four dozen wood species, indicating which suppliers carry them.
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