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
Albizia bright, uniform green
Amendoim bright streaks of yellow
Araracanga medium uniform yellow/green
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
Bubinga faint to medium yellow/green; not always present
Buckthorn medium uniform green
Camelthorn 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
Ebony, Texas medium uniform yellow/gold; not always present
Elm, Wych faint to medium yellow/green streaks; not always present
Espave faint to medium yellow/green streaks; not always present
Gidgee medium to bright uniform green
Goncalo Alves faint to medium streaks of yellow/green; not always present
Greenheart faint to medium uniform green
Guanacaste medium uniform yellow/gold
Guatambu faint to medium streaks of yellow/green
Hububalli medium to bright streaks of green
Ironwood, Black faint yellow/green streaks; not always present
Ironwood, Desert faint yellow streaks
Jatoba faint to medium yellow/orange
Koa medium to bright streaks of green; not always present
Kopie faint uniform yellow/green; not always present
Lancewood faint to medium green streaks (not present in sapwood)
Lebbeck bright, uniform yellow/green
Lemonwood, African faint to medium yellow/green
Locust, Black bright, uniform green
Locust, Honey bright, uniform green
Locust, Water bright, uniform green
Machiche medium, uniform green
Mahogany, Santos faint to medium green streaks
Mangium medium to bright streaks of yellow/green; not always present
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
Monkeythorn medium to bright streaks of yellow/green
Mopane faint to medium yellow/green; not always present
Muninga faint to medium yellow/orange; not always present
Myrtle bright yellow/green streaks in certain portions of heartwood
Narra/Amboyna medium yellow
Olive faint to medium yellow streaks in certain portions of heartwood
Olive, Russian medium uniform green
Opepe faint to medium yellow/green
Orange faint to medium yellow streaks
Padauk, African faint to medium yellow/orange
Panga Panga faint uniform yellow/green; not always present
Partridgewood (Andira spp.) faint yellow streaks; not always present
Peroba Rosa medium uniform yellow/green
Pink Ivory medium streaked pink/red in certain portions of heartwood
Piquia Amarello bright, uniform yellow
Pistachio bright, uniform green
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
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

 

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.

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