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:
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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Interesting how the sapwood doesn’t fluoresce in this section of a small branch from Black Locust.
The new 365-nm flashlights (~$50) should be fantastic for observing wood fluorescence
These slabs are from the very hardwood trunk which we didn’t encounter until we started digging to make a planting area. We hacked at it and ultimately used a reciprocal saw.
I tried the black light test on this piece with both ethanol and water. Enthanol extract glowed blue/green. water = no glow. The heartwood sank in water. It has a floral sweet smell but not really roses. Any idea what it could be?
Tough to say. You tested ethanol and water extract fluorescence, which would be something only really useful if you had it narrowed down to East Indian rosewood vs. Brazilian rosewood. But to me, it doesn’t look like a rosewood species based on how wide the rays are. I can’t recall ever seeing a rosewood endgrain with rays that were that conspicuous before.
That looks like pau ferro to me.
Black locust, Normal lighting and black light. Thanks to my brother-in-law Eli Brown for the sample.
Various laminates under 365nm black light. Left to right:
tigerwood (broad yellow stripes)
purpleheart (faint orange)
canarywood and padauk (yellow and orange)
mix (bubinga purpleheart tigerwood padauk)
morado (2 different boards, both faint purple; the blacker board being more reactive) and padauk (orange stripe)
I just subscribed and look forward to receiving your updates. I bought your book a couple years ago and it’s become a most valuable resource.
I picked up this wood a few months ago at Woodcraft but forgot to write down the species name. It’s now becoming a small box. I can’t find it in your book after a page by page search. It’s medium weight and rather hard.
Can you help with ID? Thanks so much!
Port Huron, Michigan
Thanks for the good pictures. It looks like lati.
Thanks so much, Eric!
What if just the knots/burls of the wood glow. Used a black light on the decking of the wood in my front porch and just the knots/burls glow. Has me reallly curious.
This is such a good question and I can think of like 3 possible answers. But I’m not an expert, I just found out the bark I boiled to dye yarn glowed and now I’m diving in to researching, so I’ll keep you in mind and then never be able to find this again to reply.
Have you seen the MIT research on creating bioluminescent trees to replace streetlights? I’m curious if you have anything to add on such science. Is there any way to use light (any) to reflect the sap circulation within a tree? Love your info and definitely buying the book too! Thanks in advance!
So, I was gifted a piece of “padauk” from a person who had purchased it several decades ago. Once I cut it open, it didn’t seem to match. I did a few different density tests and found it to be just around 66 lbs/ft3. I thought it might be bloodwood, so I got a sample of it to check as well against the fluorescence and the difference was as night and day.
That being said, any clue what I got here?
Edit: can’t seem to add photos. Link to imgur here: https://imgur.com/a/P9EYuGp
How would you characterize the smell? Does it resemble anything else you’ve worked with before? I can’t quite make out very many details in that endgrain shot, is it possible to get a closer shot?
I will try to get a better shot of the endgrain as soon as I can. My nose is notoriously bad, but I will see what I can get. Thank you!
I tried to sand this as best as I can up to 600. I am terrible at identifying smells, but it is very mild and somewhat pleasant? I don’t know what I can compare it to. It doesn’t last long at all. It’s not like I would make a perfume out of it. It is not like anything else I have worked with. It is super dense however. The only other woods I have worked with that are this red are paduk and bloodwood, but it is too dense for paduk, and I confirmed it wasn’t bloodwood with UV. The… Read more »
Well, it doesn’t look too unlike pink ivory. Pink ivory does have it’s own unique scent, though I don’t think most people would find it pleasant. But it may just some other obscure wood. It appears to be very fine grained with medium to small pores, so that rules out a lot of other tropical hardwoods. Add in the color and weight, and there doesn’t really seem to be any known matches.
does this interesting phenomena have any correlation in the woods other properties?
I work at Woodcraft and every so often I look through our lumber with a UV flashlight after we close.
Mango has by far the most interesting fluorescence I’ve come across yet, with astonishingly bright streaks of yellow, green, and sometimes even blue.
Oregon Grape will also fluoresce a bright yellow when under a blacklight.
I want the hard copy of this book! how & where to buy?
But WHY does this happen? What are the chemicals responsible for producing fluorescence in wood?
Lots of [minerals fluoresce](https://www.naturesrainbows.com/), with the right impurities, including (table) salt, aka [“halite”](https://www.naturesrainbows.com/photo-archive/tag/Halite), calcium (calcite/apatite/etc). I don’t know the specifics of tree biochemistry, but it would likely vary widely among species, given the differences in colors. There are probably scientific papers behind a paywall somewhere that analyze individual species (or components, like pine needles), although there might not be a comprehensive list. See https://en.wikipedia.org/wiki/Autofluorescence.
Wonderful article…I used a black flash light for the first time and walk past my firewood pile one night and almost fell over when 20% of it went PURE BRIGHT YELLOW! I had NO idea what this meant…was it dangerous, could I get sick, is it OK to burn?…or maybe from outer space?
Happy I found your information…THANK YOU!
still so confusing, how to define faint, medium etc. and so many species with similar appearances !
Many years ago now my children and I used UV light to explore the wooded area around our house at night for fluorescent response. Insect egg cases, mycorrhizal fungi strands, and other material which was unidentifiable would floresce. I still have a collection of material from that time. At the time, 30 years ago, I called this phenomenon biofluorescence. I wondered what the purpose of biofluorescence was in pine duff and decaying plant material which would never be exposed to UV light from the sun. The article about fluorescent response to UV light in wood left out the American tulip… Read more »
El cedro que color muestra ante la luz negra ?????????
El cedro no fluoresce. No mostrará ningún color. (Cedar doesn’t fluoresce. It won’t show any color).
cedar apparently does not fluoresce
Would I ask you a question? My friend found a special kind of wood in Taiwan, and the wood has a strong fluorescent reaction in blacklights. He has collected a lot of kinds of wood and the unknown wood is the only wood that has fluorescent reaction in blacklights. Would you please tell us what the wood’s name is? Thanks!
ps. the right/ fluorescent one is the special wood whose name we are curious about.
Can you get a closer picture of the endgrain? Could you also describe the appearance and weight of the wood in question? Any noticeable odor? There are a fair number of fluorescent woods, even within Taiwan, so need more info.
check out this link for “pequia”. looks like this is piquia amarello.
The scientific name listed is Caryocar villosum, while piquia amarello is generally belonging to Aspidosperma genus, so not the same.
I am wanting to make a end-grain cutting board. What type of wood is this ifs heavy tight grain.
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!
Not true acecombs. I have personally owned and used several incandescent black light bulbs and they do in deed produce fluorescent effects.
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