Q: I am using hard maple lumber and our design requires the wood to be as white as possible. Indeed, the lumber looks white when it is kiln dried and rough, but after we plane it, it is darker (almost gray). The light color we want is only on the outside. I have several questions about our drying operation. 1. Can hard maple be kiln dried in the summer when it is very humid and hot? 2. Should the sawmill sticker the lumber right after they saw it, dry it for a few days to a week, then unstack it and ship it to us, and then we restack it? 3. Is there a part of the country that grows whiter maple?
A: The stain is a chemical stain (called enzymatic oxidation) and is identical to sticker stain. It results because naturally occurring starches and sugars are oxidized. This oxidation reaction begins soon after the tree is cut. The wood cells containing starches and sugars will still be active for many months after harvesting, converting the starches and sugars to chemicals that can be easily oxidized. The final color change occurs at very low MCs in drying; but the color change is inevitable due to mistreatment (that is, slow drying or no drying) of the lumber before low MCs are reached.
First, let's consider why there is a dark core and light surface. The surface apparently has been dried fast enough to be white - fast drying means white colors. The dark cores are because the core was dried too slowly. If the logs from which the lumber was sawn were freshly harvested, then differences in drying would not result in differences in color - the wood would be quite white.
Therefore, an important contributing factor to gray color is old logs. In fact, this is why some suppliers, those with long log storage times, will have more poorly colored lumber than others. As the discoloration oxidation reaction doubles for every 20 F increase in temperature, logs stored in warmer weather have a much higher risk of lumber color problems.
In answer to your specific questions, it is critical when drying maple to dry it rapidly. Fast drying is not just fast for a day or two at the beginning. It means fast drying every day for a week or longer, until the lumber is well under 40 percent MC. When the outside RH is high, it is critical to get the lumber into a fast drying situation (low RH and high air flow) ASAP after sawing. Even though the mill does not have a truckload of green lumber, they have to ship a partial load to a kiln operation to avoid storing it at the mill. Air drying in hot, humid weather (slow lumber drying weather) is not going to produce white lumber very often either.
The kiln, when loaded with green lumber, will also have to be run hot enough to produce the desired humidities initially and for the next five to seven days. Some kilns do not vent well enough to achieve low RHs, so then the kiln is loaded with less lumber, and uses higher air flow and higher kiln temperatures.
The sawmill that stickers the lumber, air dries for a few days, then tight packs it and sends it to you is guaranteed to have color problems in warm weather. The wood surface may dry fast enough for a day or two to develop good white color, but then when the lumber is tight packed and shipped, drying in the core stops: perfect discoloration conditions for the core.
Hard maple color varies from region to region and mill to mill. Where the climate is cold, the oxidation reaction in stored logs or lumber proceeds slowly. When warm, it proceeds rapidly. Further, the length of storage time for logs has a large effect. Nevertheless, you can purchase beautiful hard maple lumber from any region of the country if the logs and lumber are properly handled prior to drying.
In summary, for the whitest color through and through, use fresh logs, try to put the freshly sawn lumber into the kiln within 48 hours after sawing and use low RH conditions in the kiln promptly after loading to achieve fast drying for several days.
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