Purpleheart is the common name lumber derived from the genus Peltogyne, commonly Peltogyne paniculata, Peltogyne pubescens, Peltogyne porphyrocardia, Peltogyne venosa and Peltogyne densiflora. Alternative common names include Amaranth, Violetwood, Koroboreli, Saka, Pau roxo, Nazareno, Morado, Tananeo, Palo morado, or Guarabu. The family is indigenous to Mexico Central America and South America, and is used for marine construction, bridge beams, building construction, decking, flooring, furniture, and veneer. The trees may grow to 170 feet in height (50 meters), more commonly 125-150 feet (38-45 meters), with diameters of 4 feet (1.3 meters.)
A common topic of discussion amongst woodworkers is the conditions under which purpleheart changes color. Speculation and anecdotal evidence from a number of sources has lead to some confusion on the actual properties. A systematic and controlled study of the color changes is therefore needed. The following is a report of several simple experiments to bring data to the discussion.
Speculation holds that the wood, when freshly cut, is either bright purple or pale brown. Cutting with a table saw resulted is dark purple flashing where the blade created friction, while cutting with a band saw resulted in no coloring. The color of the freshly cut wood was found to be a light red-brown. Several shims were cut utilizing a band saw to avoid friction coloration. The shims measured 1/8" by 1" wide and either 6" long or 3" long. To be fair, I do not have any knowledge of which species of Purpleheart I obtained from the Kettle Moraine Hardwoods.
Directions for establishing a bright purple color from freshly cut purpleheart are perhaps the are of the most confusion and disagreement. Attention is now turned to this. Possible reasons that have been postulated include ultraviolet (UV) radiation, oxidation, pH, or heat. For the time being I will consider pH, heat and oxidation, UV will be addressed by the use of a Hg lamp UV reactor. The effect of heat upon the wood will be considered first.
Heat
Heat appeared to be a significant factor based on the response of the wood to friction heat when being cut on the table saw. Chemists in the 1930's investigated the extracts of Purpleheart and reported that at 240°C the compound, Peltogynol, decomposed. A color change was noted at 200°C (392°F) as well, but no specifics of chemical identity where given. This preliminary report serves as the starting point for the investigation. Samples of Purpleheart were subjected to temperatures of 350°F, 425°F,450°F, and 500°F for 10 minutes. As the temperatures were raised color change became more rapid and darker. At 500°F (260°C) the wood becomes almost black with purple tones, there is also burning smell generated. At 450°F (232°C) the purple color becomes dark, but no burning smell in observed. This is consistent withthe hypothesis that the colored compound in decomposing above 240°C (464°F) as stated in the old chemical literature. A consistent interpretation of these observations is that the initial compound, peltogynol, undergoes a reaction to become a deep purple compound, which decomposes at temperatures above 240°C (464°F.) The nature of the purple complex is unclear at this point.
Oxidation
It occurs that the purple compound may be the result of oxidation of peltogynol. Evidence for oxidation is the depth of the purple color, usually only the surface of the wood. Sanding the wood typically is sufficient to remove the color, and expose fresh red-brown wood. To test this hypothesis a thicker piece of wood was heated at 425°F for 25 minutes, during which time the wood became deep purple. After cooling, the test piece was sectioned utilizing a band saw. The cut revealed thatthe wood had become deep purple uniformly through out the sample. If oxygen is required to generate the purple color the inside of the sample wood be purple only on the surface. The experimental results are inconsistent with the hypothesis of oxidation by atmospheric oxygen.
pH
Some speculation about the effect of acids and bases on Purpleheart. In short, pH changes do not result in the intense purple color. However, there are some changes of interest when the wood in subjected to acids and bases. Many pigments found in plants function as indicators, and Purpleheart is no exception. Acid (Sulfuric Acid, Hydrochloric Acid aka Muriatic Acid, and Acetic Acid aka vinegar results in a redder color, while base (sodium bicarbonate aka baking soda, and ammonia) results in green. For comparison, purple grape juice was subjected to similar conditions and displayed the same color changes.
A piece of Purpleheart treated with sulfuric acid on the left side and ammonia on the right side.
Ultraviolet Light
I have not yet fully tested Purpleheart under UV light and will not comment at this point.
Conclusions
The pigment in Purpleheart wood is reactive to heat and becomes a deep purple when subject to heat less that 460°F (240°C) after which decomposition likely occurs.
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