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 SpeciesFluorescence notes:
Afzeliafaint to medium yellow-green; dependent on species
Albiziabright, uniform green
Amendoimbright streaks of yellow
Araracangamedium uniform yellow/green
Avodirefaint to medium streaks of yellow/green
Blackwood, Australianmedium streaks of yellow/green
Bloodwoodfaint to medium uniform blue; not always apparent in wood, but vivid in ethanol extract
Bubingafaint to medium yellow/green; not always present
Buckthornmedium uniform green
Camelthornmedium uniform green
Canarywoodfaint to medium yellow streaks
Cebilmedium to bright streaks of green; surrounding dark areas in heartwood
Chechenmedium to bright streaks of green
Coffeetreebright uniform yellow/green
Cumarufaint to medium green streaks
Ebony, Texasmedium uniform yellow/gold; not always present
Elm, Wychfaint to medium yellow/green streaks; not always present
Espavefaint to medium yellow/green streaks; not always present
Gidgeemedium to bright uniform green
Goncalo Alvesfaint to medium streaks of yellow/green; not always present
Greenheartfaint to medium uniform green
Guanacastemedium uniform yellow/gold
Guatambufaint to medium streaks of yellow/green
Hububallimedium to bright streaks of green
Ironwood, Blackfaint yellow/green streaks; not always present
Ironwood, Desertfaint yellow streaks
Jatobafaint to medium yellow/orange
Koamedium to bright streaks of green; not always present
Kopiefaint uniform yellow/green; not always present
Lancewoodfaint to medium green streaks (not present in sapwood)
Lebbeckbright, uniform yellow/green
Lemonwood, Africanfaint to medium yellow/green
Locust, Blackbright, uniform green
Locust, Honeybright, uniform green
Locust, Waterbright, uniform green
Machichemedium, uniform green
Mahogany, Santosfaint to medium green streaks
Mangiummedium to bright streaks of yellow/green; not always present
Mangobright green streaks in certain portions of heartwood
Merbaumedium, uniform green, with bright yellow/green streaks in areas of mineral deposits
Mesquitefaint to medium green streaks, dependent on species
Mimosamedium green streaks
Monkeypodfaint to medium yellow/green streaks
Monkeythornmedium to bright streaks of yellow/green
Mopanefaint to medium yellow/green; not always present
Muningafaint to medium yellow/orange; not always present
Myrtlebright yellow/green streaks in certain portions of heartwood
Narra/Amboynamedium yellow
Olivefaint to medium yellow streaks in certain portions of heartwood
Olive, Russianmedium uniform green
Opepefaint to medium yellow/green
Orangefaint to medium yellow streaks
Padauk, Africanfaint to medium yellow/orange
Panga Pangafaint uniform yellow/green; not always present
Partridgewood (Andira spp.)faint yellow streaks; not always present
Peroba Rosamedium uniform yellow/green
Pink Ivorymedium streaked pink/red in certain portions of heartwood
Piquia Amarellobright, uniform yellow
Pistachiobright, uniform green
Purpleheartfaint to medium yellow/orange streaks; varies by species
Quebrachofaint green streaks
Quinafaint to medium uniform yellow/green
Raspberry Jamfaint to medium uniform yellow/green in heartwood, with bright streak at sapwood/heartwood transition
Redbud, Easternmedium green streaks
Redheartmedium uniform orange
Rengasfaint to medium uniform yellow/orange
Sassafras, Blackheartmedium orange/red streaks in colored heartwood areas
Satinwood, East Indianfaint to medium yellow streaks
Satinwood, West Indianfaint to medium yellow streaks
Sugifaint uniform pink/red
Sumac, Staghornmedium to bright streaks of green
Talibright uniform yellow/gold
Timboranafaint to medium green green
Tornillomedium uniform yellow/green
T’zalamfaint to medium uniform green
Witelsfaint to medium uniform yellow
Yellowheartfaint to medium uniform yellow

 

Water extract fluorescence

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 fluorescence 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.

  • Per

    WARNING
    Unfiltered ultraviolet radiation can be harmful to the eyes and skin. Black lightbulbs SHALL NOT be operated without filters. Cracked, chipped, or ill-fittingfilters SHALL be replaced before using the lamp

  • viper010

    Not true acecombs. I have personally owned and used several incandescent black light bulbs and they do in deed produce fluorescent effects.

  • Leonard Carter

    Need to chime in here – I just bought a flashlight LED blacklight, and I can assure you, it is very powerful and can damage your vision. Most of them are pretty weak and operate around 405 nm (which is practically violet), and barely work as UV lights, but the one I found emits light around 375 nm, which is very effective. It emits enough UV that it can temporarily damage your eyes if you use it long enough and are too close to the subject. I speak from experience!