A: What you are seeing is called casehardening or drying stress. It occurs because at the beginning of drying, the outside tried to shrink, but could not because of the wet core. So, the outside dried to a larger shape than if it had been free to shrink. This creates a stress condition that, when the pieces are ripped, will cause the pieces to bow; the left piece will be a mirror image of the warp in the right piece. (Sometimes there is stress in the tree and this stress will cause immediate warp but it will not be balanced, left to right.) It is not uncommon that construction lumber will have casehardening; it does not hurt anyone and would cost money to remove during drying. Casehardening does not affect nailing as it is a stress condition so nothing is harder.
So, how do you remove casehardening? Existing casehardening is removed by steaming the material for a few hours. (Water misting might work too.) As you only need a few pieces to be stress free, I wonder if you can find someone with a steamer for cleaning car engines, etc. or a rental place where you can borrow a steamer when needed?
Tips to avoid stress development and cracking
Q: We are having a problem with shrinkage. We make furniture, but someone else sells and delivers it. This person claims he did everything correctly, including opening the furniture wrapping (we wrapped the furniture with shrink-wrap and it was fairly well sealed) and letting it acclimate to the house climate. When the customer moved in, they said the furniture looked really wonderful, but within a week, it started to warp, open joints and crack in a few places. We are so careful to keep our plant at 40 percent RH and check the MC of the lumber. This is frustrating! Can you help?
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