Q. We just received some lumber that has a color issue. We see that the core is normal in color, but the core of the wood is considerably whiter. This contrast in color, shell is darker than the core, is giving us fits and looks poor in the finished product.
A: When wood is dried, an oxidation reaction will occur in the wood and sometimes, when drying is slow, the wood will change color slightly. When dried more rapidly, the reaction does not occur or does not occur to the same extent, so the color is brighter and whiter. The time when the color is established in a piece is almost always when the wood is above 45 percent moisture content.
Here is my guess (I am fairly certain about this) as to what has happened in your case. The fresh lumber was put outside for air drying, where it dried slowly and at cool temperatures. This slow drying encourages the somewhat normal darkening. However, before air drying was complete the lumber was moved into the kiln where low humidities and higher temperatures were used. This fast drying occurred so rapidly, that the normal color reaction did not occur, leaving the wood lighter in color. One of the rules of drying is that once air drying begins, let the wood air dry down to 25 percent MC (approximately) before going into the kiln.
Special note: With the white colored woods like maple, it is possible that air drying can be rapid and can develop a white outside. Then when the partially air dried lumber is put into the kiln, the heat and high humidity (incorrect setting most likely) result in slow drying and darken the core. Heating wet wood over 110F has been shown to also darken the color.
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