The striking green coloration found in wood is a rare aesthetic feature resulting from two fundamentally different natural processes. This color can be caused by the biological activity of specific fungi that stain the wood fibers with pigment, or it can be an inherent chemical property of the tree’s heartwood. The appearance of a genuine green hue distinguishes this wood from common “green lumber,” a term used for wood with high moisture content.
How Fungi Create Green-Stained Wood
The most common source of intense, bright blue-green coloration in wood is caused by the activity of a fungus, a process known as spalting. This specific coloration is primarily attributed to the soft rot fungus Chlorociboria aeruginascens, often observed on decaying wood like oak and maple. The fungus colonizes the wood and produces a metabolic compound that permeates the wood cells, resulting in the distinctive hue.
The vibrant blue-green pigment responsible for this discoloration is xylindein, a quinone derivative. As the fungal mycelium grows, it secretes this compound as a secondary metabolite into the wood structure. Xylindein is highly stable and remains permanently embedded in the wood fibers even after the wood is dried and treated.
The fungal staining process does not significantly compromise the structural integrity of the wood, unlike other types of decay fungi. Chlorociboria aeruginascens is a soft rot fungus, meaning it breaks down the cellulose component of the wood slowly, but its main effect in this context is purely cosmetic. The resulting wood, known historically as “green oak” or “green wood cup,” is highly sought after for decorative applications.
The concentration of the pigment varies depending on the species of wood and environmental conditions, such as nitrogen levels and moisture content, which influence the fungus’s growth. The fungus needs a high moisture content to thrive and produce the pigment, usually working on dead or fallen trees.
Wood Species That Are Naturally Green
In contrast to fungal staining, some tropical species possess heartwood that is inherently green due to the presence of specific chemical extractives. This coloration is genetically determined and is a permanent feature of the wood’s inner core, independent of external biological agents. The most prominent example is Greenheart, scientifically named Chlorocardium rodiei, a species whose very genus name translates to “green heart.”
The heartwood of Greenheart typically presents an olive-green to pale olive color, often intermingled with darker streaks. This hue is a result of the complex organic compounds, or extractives, that the living tree deposits in its non-living heartwood cells as a natural defense mechanism. The presence of these extractives contributes significantly to the wood’s resistance to decay and marine borers.
Another species with a notable greenish cast is Lignum Vitae (Guaiacum officinale or G. sanctum), known for being one of the world’s densest woods. Its heartwood typically ranges from an olive to dark green-brown color, which can deepen upon exposure to light, particularly ultraviolet radiation. This color change is a chemical reaction involving the high concentration of resins and oils, which can make up nearly 30% of the wood’s weight.
The heartwood extractives in these species are complex secondary metabolites. These compounds not only impart the color but are also responsible for the wood’s unique properties, including its density and durability. The color is a visual indicator of the wood’s chemical composition.
Practical Uses and Safety Considerations
Green-colored wood is prized for its aesthetic appeal and is utilized in applications where its appearance can be highlighted. Fungal-stained wood, or spalted wood, is primarily used for small, decorative items where structural integrity is not a concern. Common applications include:
- Turning blanks for pens
- Jewelry boxes
- Decorative veneers
- Intricate inlays, such as the historical Tunbridge ware
Uses of Greenheart
In contrast, naturally green heartwoods like Greenheart are employed where extreme performance and durability are required. Greenheart is a traditional choice for heavy marine construction, including docks, pilings, and lock gates, due to its ability to withstand constant exposure to saltwater and resist marine borers.
Uses of Lignum Vitae
Lignum Vitae is historically famous for use as self-lubricating propeller shaft bearings and bushings in ships. This is owing to its exceptional density and high resin content.
When working with any wood, safety precautions are necessary, especially for the green-colored varieties. Although the fungus causing green spalting is largely inert in the dried wood, standard respiratory protection should be worn. The inhalation risk of this wood dust is considered similar to that of non-spalted wood.
The dust from chemically-rich tropical species poses a different type of risk. Greenheart dust is known to be a respiratory sensitizer and can cause throat irritation and wheezing. Furthermore, splinters from Greenheart can cause severe and persistent infections, necessitating diligent protection for the skin and eyes. High-quality dust extraction and personal protective equipment are required to safely process these dense, extractive-rich timbers.