Q: We are getting some veneer that has some unusual color. It seems to be mostly brown. We also notice some gray at the edges, which would be the ends of the logs. Can you help?
A: Let me give you a quick lesson in wood color - if this is too brief, e-mail me with additional questions. The "natural" color of veneer can be modified by:
1) The presence of dark colored fungi (mostly dark blue or gray in color).
2) The presence of mold and mildew fungi on the wood's surface, often green, red, or white in color. The fungal reactions (#1 and #2) are most active at 80 to 90F. Heating veneer logs to 130F will kill these fungi, but any damage done prior to this sterilization heating will remain. So long as the wood remains hot, re-infection is not possible, but if the wood is cool and wet, the fungi can return. Without water, the fungi cannot be active; so there is no risk with veneer under 22 percent MC.
3) The interaction of chemicals in the wood with chemicals to which the veneer is exposed. Most often, tannic acid in oak reacts with iron in veneer handling equipment to form iron tannate, which is almost black in color.
4) The interaction of heat with certain chemicals in the wood, producing dark brown colors. Excessive heating (over 160F) quickly leads to permanent darkening of the wood, especially when the wood is quite wet. However, even drying at temperatures over 130F, when the wood is wet, can produce noticeable darkening.
5) The reaction of naturally occurring chemicals in the wood with oxygen (sometimes called an enzymatic oxidation reaction) leading to gray colors most of the time. The wood cells in the tree do not die when the tree is cut down. They stay alive for many months, slowly creating new chemicals that will oxidize as the wood dries, forming gray or brown color, primarily in the sapwood as the wood dries or at the end of drying. (Maple is typically mostly sapwood, so this color loss can be serious, while for oak the amount of sapwood is quite small.) Long storage of a log before cutting can make such coloring inevitable.
This reaction is accelerated if the wood is warm, about twice as fast for every 20 degrees F of heating.
In your situation, we are probably dealing with an enzymatic oxidation reaction, where naturally occurring chemicals in the wood oxidize. Perhaps these chemicals were formed due to long log storage. The oxidation is quite rapid because the veneer is warm. If the veneer can be quickly dried (within several hours after veneering) before the oxidation takes place (it does take time), then the color change is minimal.
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