For the totally nerdly, read on!
Leo Frilot said:
Alright, I'll have to look for this book at my library. Thanks for some info. Anybody else?
Leo:
I had a look at Hoadley's Chapter 7 paragraphs on wood fluorescence. He describes the process in very (very) basic terms, but does not elaborate on
why the wood fluoresces. He includes a table (p. 140) of some of the known species of wood which have demonstrated fluorescence properties, and notes that yellow is the predominant color of visible fluorescence when irradiated with UV light. Recall that there are UV light sources that are termed "short-wave" and "long-wave", depending on which wave bands of UV light are emitted by the lamp. The properties of fluorescence (including the visible light color given off by a compound) may differ depending on the light source used. Regardless, the property is really neat, and this post has piqued my interest again in this phenomenon. I guess I'll have to dig-out my UV light sources from my lab's store room and see whether there is some utility in describing fluorescence colors, perhaps as an aid to wood identification.
As to the chemical nature of the compounds found in the wood that cause fluorescence under UV illumination, my first suspicion would be a class of phenolic compounds collectively termed "flavonoids" which are produced by plants as part of a biosynthetic chemical pathway which imparts different coloration to various plant parts, protection from damaging light, herbivore/predation protection, and some other functions. [For those interested in these chemicals, see:
http://en.wikipedia.org/wiki/Flavonoid]
I did research on these plant-derived compounds in the mid-1980s; they routinely fluoresce under both short and long UV irradiation, and may be part of the complex mixture of chemicals found in wood, collectively termed 'extractives'. Hoadly notes that in some species, differences in color due to early-wood or late-wood position is observed; this may be due to the tree depositing different chemical compounds at different times throughout the growing season. Very interesting indeed!
For those that have a UV light source and would like to experiment, you can find wood species that fluoresce a particular color under "black light" (test as many different species as you can!). [Be careful with short wave UV light, as it can cause eye/skin damage.] Once you have identified which species "glow" in the visible spectrum with a particular color, then fume the wood surface with ammonia vapors, and re-examine under the same UV light for any color changes. If the fluorescence color does change, the compound(s) is/are very likely from the group of flavonoids (incl. flavonols, flavones, coumarins, chalcones, and related phenolic compounds). This was one of the standard visual "chromatic shift" methods used to identify these compounds following chromatography. Most compounds were actually identified from isolated and purified samples using UV spectrophotometry and known absorption spectra.
Finally, Hoadley mentions that the wood fluorescence characteristics (i.e. intensity) decline with age, or upon prolonged exposure to light, even just "room" light (...suggesting oxidation or photo-oxidation of the compounds responsible for fluorescence). He recommends storing the fluorescent woods in the dark if the wood's fluorescence quality is to be maintained. (If a dark place is not available, wrap the wood samples in aluminum foil.)
I'll see what I can do to find out more about this, but at least I hope I've provided some additional information about the topic.
Rob
