The optical effect known as chatoyancy is a visual phenomenon. Derived from the French phrase “œil de chat,” meaning “cat’s eye,” it describes the unique way certain lumber appears to shimmer and move when viewed from different angles. This three-dimensional quality, often referred to as “figure” in wood, is not merely a surface texture but a display of the material science and physics embedded within the wood’s structure. This dynamic appearance is a result of an unusual growth pattern that fundamentally changes how light interacts with the wood’s cellular components.
The Visual Characteristics of Chatoyancy
Chatoyancy manifests as alternating bands of light and dark that appear to glide across the wood’s surface. This shimmering effect gives the wood a lustrous, watery, or holographic quality that seems to possess depth below the finish. As a viewer shifts their perspective or as the light source changes, the bands reverse their intensity, with previously dark areas becoming bright and vice versa. The overall effect is distinct from a simple sheen or gloss, as it involves a dramatic shift in the figure’s contrast and brightness.
Basic Wood Structure and Light Interaction
Wood is composed primarily of elongated, hollow cells, which are aligned parallel to the trunk’s axis in a standard, straight-grained piece of lumber. These cellular structures determine the direction of the grain. When light strikes a surface of uniform, straight-grained wood, it is scattered or reflected in a relatively consistent manner. This uniform scattering results in a flat, predictable appearance where the color and texture remain stable regardless of the viewing angle.
The Underlying Cause: Undulating Grain
Chatoyancy originates from a structural anomaly in the tree’s growth known as undulating grain. Instead of growing straight and parallel, the wood fibers undulate and interlock in a continuous, wavy pattern. This deviation is often called “curly figure” or “fiddleback” and is thought to be caused by environmental factors, such as wind stress or genetic predisposition. The chatoyant effect is best seen when the wood is cut perpendicular to the peaks and troughs of these waves, exposing the cross-section of the curved fibers.
Optical Illusion: How Reflection Creates Depth
The wavy structure of the undulating grain translates into the visual effect through the principle of differential reflection. When light encounters the wavy surface, the microscopic cell walls function like a series of tiny, angled mirrors. The fibers on the upward slope of the wave are angled to reflect light directly toward the observer, making that area appear bright. Simultaneously, the fibers on the downward slope of the wave reflect light away from the observer, causing that adjacent area to appear dark. As the viewer’s eye moves, the angle of incidence changes, and the light reflects off a different set of cellular slopes. This continuous shift creates the illusion of movement and perceived three-dimensional depth.