Re: Purpleheart ques...
Jon Arno
>Bubba, I haven't seen any research on this, but just from personal experience, I suspect that the purple pigmentation in this wood results more from oxidation (or other atmospheric gas)than from exposure to light. When the wood is first cut, it is a light brown in color, but in a matter of hours it turns purple.
The pigments are, however, sensitive to light, but I think the effect of light is to convert the purple pigmentation toward redish brown and this process is much slower than the one which causes the initial purple color to develop. The effects of light can be retarded by using a UV blocking varnish, but it is virtually impossible to preserve the purple color indefinitely.
The chemistry of this wood is indeed mysterious. Purpleheart is often used for dock pilings...and when used in this marine environment the wood turns almost jet black. Whether it is a reaction to salt or some dissolved mineral in sea water, I don't know...but I suspect the black pigmentation results from a mineral (probably iron), since iron has this effect on woods that are high in tannin, such as the oaks.
As for what heat does, it might play some minor role in accelerating the chemical processes that actually form the pigments...but the darkening effect it seems to contribute probably relates more to what heat does to the gums in the wood. As in cherry and other gummy woods, purpleheart does seem to be a little sensitive when it comes to friction burning. If the rate of feed isn't steady and/or the cutters are dull, you quickly pay for your inattention.