One of the most beautiful characteristics of natural lumber is its color. One of the most frustrating aspects of natural lumber is, again, its color. What’s so frustrating about lumber color? Chances are, you don’t have to ask; you know. Color varies within each species, often greatly. And color changes, sometimes drastically.
Wood changes color on its journey from the sawmill to the job site, and beyond. Even years after being sawn and installed, wood continues to endure chemical changes that prompt visible transformation. As your wood matures, however, the matching you always wanted can become either closer to or further from reality.
The Natural Graying Process
Most wood species naturally turn a silvery hue as the wood ages. While that scenario might be frustrating to someone looking to match older, unstained lumber, it should provide hope, as well, since the new lumber will eventually mature into the same color family. On the other hand, you can sand or plane aged wood in order to remove the outer layer, and it will look new again.
How quickly a board will change colors depends on the species. Tropical species often have high oil content, which allows them to put off the graying longer than other species. What happens when wood turns gray is that, basically, the sun bleaches the surface and dries it out — which helps explain why natural oil content (and oily finishes) slow the graying process.
The same bleaching process often creates surface cracks, or checks, which will be virtually unavoidable if your project is an exterior one subjected to direct sunlight. Bleaching oils can escalate the process, but the results are usually a little different from the results of the natural weathering process, which includes a combination of water, heat, wind, UV and longer wavelength light.
The Chemistry of Color Change
Wood used in interior settings can still endure color change, due to the complex and variable chemical components of the wood. Many aspects of the tree and the soil chemistry in which it grew affect the wood’s chemical makeup. Particularly, extractives and lignins make the biggest difference.
Extractives are exotic compounds that are often extracted from wood in order to make other products, such as pharmaceutical products, Tannins, and Turpentine. Lignins are water-resistant compounds that serve to bind cellulose wood fibers.
Before a tree is harvested, these chemicals were responsible for creating the grain, which is a result of the color change in sapwood as it’s converted into heartwood. Both extractives and lignins combine with longer wavelengths of lights found indoors to prompt color change. They break down when they come into contact with either heat or chemicals, such as furniture waxes, varnishes, or lacquers.
As we mentioned earlier, some species endure more extreme color change than others. We’ll look at some species of special interest in Part 2.