Reducing Drying Defects in Bowls & Platters
Let's face it - turning green wood is a pleasant experience, but drying can sometimes be difficult. While success can't always be guaranteed, here are some ways to put the odds in your favor:
Perform all rough turning at one time
A mistake many people make is to rough out one side of a bowl or platter, then come back later to finish turning the other side. While sometimes this may appear to work without small checks or cracks appearing on the already cut surface, they will almost inevitably started to form (even if not visible). These microscopic checks will often remain closed until the roughed out forms are almost fully dried, at which point they will quickly become larger cracks. If for any reason a piece must be left on the lathe before fully being roughed out, try taking a plastic draw string trash bag and placing it over the unfinished piece until you can get back to finish it up.
Some woods must dry more slowly
Denser woods, such as oaks, pecan, hickory, birch and black walnut require slower drying rates (and therefore more drying time) to reduce the chances of defects occurring. Woods with a very low initial moisture content (typically cedars) must also be dried more slowly to reduce the chances of checking. If using a paper bag to enclose a piece that is drying, adding a second or third bag around the first will reduce the drying rate. The addition of shavings from the freshly turned piece into the bag can also help to slow the drying rate even more if necessary.
Wider pieces need special attention
Pieces beyond 12" in diameter can often cup or twist more than smaller pieces, which reduces the eventual size of the finished product. This is especially evident in platters where the rim can sometimes raise significantly, making the final turning much more difficult. To combat this problem, these pieces should be dried more slowly, particularly when the piece is within the last few weeks of its drying cycle. Consider adding additional bags (or layers of kraft paper) to slow down the drying process. This will help to control warpage and help you get the most out of your wood!
Pay attention to wall thickness
Having an uneven wall thickness in roughed out bowls or platters creates conditions which will cause uneven drying to occur. As thinner parts dry more quickly, the thicker surrounding areas will not be able to dry at the same rate, causing stress between these areas of wood. Pay special attention to the transition areas between the rim and bottoms of pieces, as this is usually one of the more problematic areas.
Pay attention to wall shape
Our final tip this month is concerning the shape of walls, particularly in bowls. The best shape is a smoothly flowing curve from rim to base. The worst possible shape, however, is a 90 degree corner. In shallower pieces (3" thick and less) the likelihood of problems occurring is somewhat low. In deeper pieces (greater than 3" thick), however, the probability of cracking is greatly magnified. If a 90 degree corner in the bottom of your piece is desired, though, try reducing the thickness of the bottom of your piece to increase your chances of success.