Wood identification

Fluorescence: A Secret Weapon in Wood Identification

by Eric Meier
While certain woods can appear basically identical to one another under normal lighting conditions, when exposed to certain wavelengths—such as those found in blacklights, (which are mostly invisible to the naked eye)—the wood will absorb and emit light in a different (visible) wavelength. This phenomenon is known as fluorescence, and certain woods can be distinguished by the presence or absence of their fluorescent qualities.
One of the best examples of fluorescence is found in Black Locust (Robinia pseudoacacia), which is very similar to Mulberry (Morus spp.) in both appearance and weight. But one way to easily distinguish the two is by observing them under a blacklight; Black Locust will emit a strong yellow-green glow, while Mulberry will be non-reactive.

The process of detecting fluorescence in wood samples need not be intimidating or limited to the scientific community—blacklight bulbs are available in many hardware stores for only a few dollars and can be used in standard lamp sockets. (These bulbs should never be confused with germicidal ultraviolet bulbs such as those used in UV sterilizers, which emit UVB or UVC  light, which can pose serious health hazards.)

Even though the process of detecting fluorescence is very simple, a few suggestions will help to maximize the effect and make the identification process as reliable as possible:

• Buy the brightest (highest wattage) blacklight available.
• Freshly sand/expose the wood grain before testing: this can be critical in detecting fluorescence in some woods.
• Turn out all other lights for the clearest detection of fluorescence.
• Be careful not to mistake simple reflections or illumination of the wood surface for fluorescence: the wood should literally glow.
• The most common colors of fluorescence are green and yellow, but some woods can fluoresce orange, pink, red, etc.
• While some species will give a strong fluorescent response, many others will only be faintly fluorescent, while the majority of species will exhibit no fluorescence at all.
• Species that are shown to produce a fluorescent response may still vary from piece to piece in the color and intensity of the fluorescence.

Here is a list of woods that exhibit some level of fluorescent properties when exposed to a black light:

Wood Species	Fluorescence notes:
Afzelia	faint to medium yellow-green; dependent on species
Amendoim	bright streaks of yellow
Avodire	faint to medium streaks of yellow/green
Blackwood, Australian	medium streaks of yellow/green
Bloodwood	faint to medium uniform blue; not always apparent in wood, but vivid in ethanol extract
Buckthorn	medium uniform green
Canarywood	faint to medium yellow streaks
Cebil	medium to bright streaks of green; surrounding dark areas in heartwood
Chechen	medium to bright streaks of green
Coffeetree	bright uniform yellow/green
Cumaru	faint to medium green streaks
Elm, Wych	faint to medium yellow/green streaks
Gidgee	medium to bright uniform green
Goncalo Alves	faint to medium streaks of yellow/green; not always present
Greenheart	faint to medium uniform green
Guatambu	faint to medium streaks of yellow/green
Hububuli	medium to bright streaks of green
Ironwood, Black	faint yellow/green streaks
Ironwood, Desert	faint yellow streaks
Jatoba	faint to medium yellow/orange
Koa	medium to bright streaks of green; not always present
Lancewood	faint to medium green streaks (not present in sapwood)
Locust, Black	bright, uniform green
Locust, Honey	bright, uniform green
Machiche	medium, uniform green
Mahogany, Santos	faint to medium green streaks
Mango	bright green streaks in certain portions of heartwood
Merbau	medium, uniform green, with bright yellow/green streaks in areas of mineral deposits
Mesquite	faint to medium green streaks, dependent on species
Mimosa	medium green streaks
Monkeypod	faint to medium yellow/green streaks
Myrtle	bright yellow/green streaks in certain portions of heartwood
Olive	faint to medium yellow streaks in certain portions of heartwood
Olive, Russian	medium uniform green
Orange	faint to medium yellow streaks
Pink Ivory	medium streaked pink/red in certain portions of heartwood
Piquia Amarello	bright, uniform yellow
Purpleheart	faint to medium yellow/orange streaks; varies by species
Quebracho	faint green streaks
Quina	faint to medium uniform yellow/green
Raspberry Jam	faint to medium uniform yellow/green in heartwood, with bright streak at sapwood/heartwood transition
Redbud, Eastern	medium green streaks
Redheart	medium uniform orange
Rengas	faint to medium uniform yellow/orange
Sassafras, Blackheart	medium orange/red streaks in colored heartwood areas
Satinwood, East Indian	faint to medium yellow streaks
Satinwood, West Indian	faint to medium yellow streaks
Sugi	faint uniform pink/red
Sumac, Staghorn	medium to bright streaks of green
Tali	bright uniform yellow/gold
Tanga Tanga	bright, uniform green
Timborana	faint to medium green green
Tornillo	medium uniform yellow/green
T’zalam	faint to medium uniform green
Witels	faint to medium uniform yellow
Yellowheart	faint to medium uniform yellow

Also, another trick to do with a blacklight is what is called water extract fluorescence and ethanol (alcohol) extract fluorescence. Some woods, while they don’t physically fluoresce under a blacklight, will have very bright reactions when wood shavings and/or sawdust is mixed in a small vial or other clear container and held up to a blacklight.
An excellent example of this is Wenge: the wood doesn’t fluoresce, but something as simple as wiping the surface with denatured alcohol will serve to draw out and concentrate the heartwood extractives, and reveal a bright green fluorescence. The proper way to test for this is to take some small shavings (or sawdust) and place them in a small glass vial with either water (for a water-extract test), or denatured alcohol (for an ethanol-extract test).
Alcohol typically results in faster, more vivid, extract fluorescnce than using simply water, but water extract tests can be useful when some woods are known to not produce a water-extract fluorescence color, such as Brazilian Rosewood.