Q: We have some sticker stain or sticker shadow that showed up in hard maple lumber after kiln drying. There was no stain when the lumber went into the kiln, except for a little log stain on the ends of the lumber. We also had lumber from other suppliers in the kiln and their lumber didn't stain. The lumber came to us in June and was on sticks within 24 hours. It was on the yard for a week waiting to go into the kiln. When it went into the kiln it was about 42 to 45 percent MC. We start our kiln at 100 F and a 12 percent EMC. Can you explain what has happened?
A: First, it is difficult to see stain at higher MCs, because the final color reaction only takes place under 20 percent MC. The stain is set to occur at much higher MCs and cannot be stopped under 35 percent MC. But the color hasn't fully developed yet when the lumber is over 20 percent MC!
Second, stain is caused by what happens (slow drying at warm temperatures) at very high MCs - above 40 percent MC. The lumber was sawn at the mill at about 65 percent MC, so the lumber going into the kiln at 42 to 45 percent MC has been dried somewhere else during the critical stage. Therefore, in this case the lumber is out of your control when stain is developing or is set up to occur, as the lumber reached the kiln at the low 40 percent MC.
Third, there is no question that poor log storage (two months during warm weather) is the number one cause of sticker stain. The log stain you noted points to log storage being a problem. In fact, November and December are when I get the largest number of complaints - summer stored logs were sawn in September, then kiln dried in October. The lumber is then cut up in November and December!
Fourth, the kiln settings of 100 F and 12 percent EMC (or 110 F and 12 percent EMC) are perfect. The real question is whether you got those conditions in the kiln within the first six to eight hours, and then maintained them.
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