Preventing veneer checks: proper maintenance of moisture content will lessen the chances of stress and checking in panels.

Wood failures, commonly known as veneer checking, are cracks which occur on the surface of veneer panels, veneered table-tops, etc. These cracks typically run parallel to each other and to the grain of the wood. If left untreated, veneer checking generally will also cause cracks in the coating on the veneer.

The dynamics behind the checking or cracking occur because of stress, The stress that causes veneer to check is the result of bonding together two materials that react differently to changes in moisture content.

A veneered panel (Figure 1) is typically manufactured by gluing a thin slice of natural wood to an engineered wood product, such as plywood, MDF or particleboard. The problem lies in the way in which the natural wood veneer and the engineered wood substrate react dimensionally to changes in moisture content.

[FIGURE 1 OMITTED]

Achieving Equilibrium

Wood, whether engineered or natural, will exchange moisture with its environment until it reaches equilibrium with that environment. In other words, if wet wood is placed in a dry environment, the wood will lose moisture until it is as dry as the environment it is in. Conversely, the same phenomena occurs if dry wood is placed in a wet environment. The exchange of moisture is ongoing.

Natural veneer, having been cut from wood, reacts similarly to changes in moisture content as does natural wood. When wood takes on moisture, it expands, primarily across the grain. There is little dimensional change along the length of the grain. When wood loses moisture, it contracts in the same manner, across the grain.

Engineered wood products, such as particleboard and MDF, also expand when moisture content increases. However, they do so in a different way and at a different rate. Because particleboard and MDF have no specific grain orientation, they expand almost equally in all directions and at a lesser rate than natural wood. Conversely, when moisture content decreases, engineered wood panels contract almost equally in all directions, again at a lesser rate than natural wood.

Therefore, when a veneer panel is manufactured, as long as the veneer and the engineered wood substrate are seasoned to the same environment, and excessive moisture or drying is not introduced during the manufacturing process, the two bonded components can exist together with little or no interracial stress in that particular environment.

Dealing with Unequal Moisture

Guess what happens when a balanced veneer panel is installed in an environment with excessively low relative humidity due to, perhaps, the use of a gas fired heating system. Upon installation, the veneer and the engineered wood substrate attempt to reach moisture equilibrium with their new environment. Both begin to lose moisture and shrink.

The engineered wood substrate shrinks a small amount, almost equally in all directions. The veneer, however, shrinks at a greater rate and mostly across the grain. This differential shrinkage between the two bonded components creates stress. If the veneer could, it would actually become physically smaller than the engineered wood substrate (Figure 2).

[FIGURE 2 OMITTED]

However, since the two are bonded together across the entire surface, that is not possible. Instead, the stress of this differential shrinkage is relieved in the form of cracks, which occur in weak spots in the veneer, such as areas of open grain or lathe checks (Figure 3). Because the veneer shrinks most across the grain, the cracks in the veneer tend to run along the length of the grain. Parallel cracks running along the grain are a typical symptom of veneer checking.

[FIGURE 3 OMITTED]

Cracks and Coatings A good quality coating can slow down the moisture exchange dramatically, thereby minimizing the destructive effects of uncontrolled expansion and contraction due to changes in the wood's moisture content. However, in the case of veneer panels, it can not prevent it from occurring completely.

For example, it is a common practice to manufacture veneer panels with solid wood edges for a variety of purposes, such as large table tops. In eases where the coating on such a panel also has cracked as a result of veneer checking, the cracks can be seen running parallel to the gram on the veneer portion of the panel, but not extending onto the solid wood portion of the panel.

This is further testament that the cracks in the coating are the result of stress cracks in the veneer. The solid wood portion of the panel has been exposed to the same variations in moisture content as the veneer portion. However, instead of cracking, the solid wood simply expanded and contracted with the changing moisture content and the coating expanded and contracted with the wood, without cracking or failure. Coatings companies are aware that wood will expand and contract with changes in moisture content, and coatings are formulated to tolerate this dimensional change.

Coatings are not, however, formulated to bridge open cracks and gaps which occur on the wood once in service. A coating with such elasticity would lack the hardness to be of much value as a protective wood coating.

It also should be noted that the expansion and contraction of wood is ongoing. This can lead to confusion when a contractor strips the coating off of a panel that has cracked as a result of veneer checking and sees no visible signs of checking on the stripped veneer.

In many cases, veneer checking is the result of a relatively short exposure to an environment of extremely low relative humidity, such as a cold spell in the dead of winter. With such low indoor relative humidity, the moisture content of the veneer panel drops, and cracks or veneer checks begin to form. As the veneer cracks and checks, so does the coating on top.

When the cold spell is over, in door relative humidity goes up and so does the moisture content of the veneer panel. As moisture content increases, the cracks and checks in the veneer begin to close up to the point where they are not visible to the naked eye. The cracks in the coating, however, remain visible and hence the confusion.

Although the cracks in the veneer panel are no longer visible to the naked eye, they are seen clearly under the microscope. Figure 4 is a cross-section of a coated, paper-backed veneer bonded with contact cement to an MDF substrate. At 50x magnification, cracks in the veneer clearly can be seen beneath the crack in the topcoat. Also visible in the veneer are stress cracks, which have not yet resulted in veneer checks.

[FIGURE 4 OMITTED]

Adhesive's Role

It can be noted in Figure 4 that the main visible crack in the veneer extends only to the paper backing and not all the way through to the MDF substrate. In this instance, improper use of the contact cement could be responsible, as some experts believe, for some of the veneer checking.

There is a school of thought in the woodworking community that contact cement, because of its inherent elasticity, is not a great adhesive choice for laying up veneer. I believe that PVA or UF glue, which form hard, non-elastic glue lines, are better choices, but of course these types of adhesives require some type of press.

If applied and cured according to veneer and glue manufacturers' instructions, contact cement is a reasonable choice for applying backed veneer products. Therefore, in viewing the cross section (Figure 4), it appears that the fact that the paper-backed veneer is adhered to the MDF with contact cement is incidental to the problem. The real cause of the failure seems to be a difference in the rate of expansion and contraction between the face veneer and its paper backing.

The backing of the veneer, being a paper material impregnated with, typically, a phenolic resin, would certainly be more stable under changing moisture conditions than the natural wood veneer. As moisture content drops, the veneer shrinks more than the paper backing and checks develop.

Stress vs. Strength

At the end of the day, veneer checking is all about stress vs. strength. If the strength of the adhesive and the strength of the veneer is greater titan the stress resulting from changing moisture content, then checking will not occur. In many cases, however, if moisture content reaches extreme highs or lows, the stress will overcome the strength and failure will occur.

Wood is a natural raw material with certain properties that are beyond human control. To avoid problems of wood cracking, checking and heaving, it is necessary to understand its properties and to play by the following cardinal rule:

Before constructing a project using natural and/or engineered wood components, all components should be seasoned together to a moisture content that closely reflects that of the end-use environment. Once the project is constructed at a suitable moisture content, this moisture content must be maintained within a reasonable range for the life of the constructed piece. That means during construction, finishing, warehousing, shipping, installation and daily use, exposure of the constructed piece to wide variations in relative humidity levels must be avoided.

8 Steps to Avoiding Veneer Checking

Following these simple steps will help avoid veneer checking and problems:

1. Determine the average moisture content range for interior wood in your climate. Wood that will be used in coastal climates, where relative humidity is generally high, will have a higher average moisture content than wood that will be used in dry, arid climates. The Architectural Woodwork Institute's Quality Standards Manual is a good source for such information--www.awinet.org.

2. Season all wood and engineered wood components to the correct average moisture content range.

3. Follow veneer manufacturers' instructions for the application of veneer to substrate.

4. Maintain a climate in the shop and warehouse similar to that of a properly set climate-controlled home or office.

5. Be careful not to soak or saturate veneers with stains or finishing materials. Initial applications should be conservative until the veneer is sealed.

6. When shipping is required, attempt to minimize the time the wood projects spend in non-climate-controlled trucks and vans.

7. Do not install wood projects in new or renovated homes or buildings where the climate-control system is not yet operational.

8. In the end-use environment, compensate for seasonal changes in relative humidity by employing humidification and dehumidification equipment.

Clearly, many of these steps would be difficult to follow in the real world, due to time and money constraints, especially for smaller shops. Wood cannot distinguish between large, well-equipped woodworking facilities, and smaller, not as well-equipped custom shops. Wood is a natural material with certain properties over which we have no control. Without proper understanding of these properties, or deliberate ignorance of these properties in order to save time and money, woodworkers will likely pay the price in terms of rework, repairs and lost business.

Editor's Note: Darrel Young is a Technologist for the ML Campbell Co. He investigates and reports on wood and film failures and is the developer and instructor for the ML Campbell Distributor Training Program. Young can be reached at [phone].