For woodworkers, one of the most frustrating experiences can be to have a nearly finished project crack while you are turning it. You've spent time and money and effort to get to this point and now, instead of having a nice, handcrafted bowl or vase, you have a lesson in what isn't working. There are three main reasons that wood cracks while it is being turned: it is too dry, too hot or too weak.
Seal your freshly cut lumber until you are ready to cut the blanks for your project. Apply green wood sealer to each end and any exposed interior surface with the paintbrush. Store the wood in a cool, shaded location until you are ready to use it.
Cut the wood through the core, if your design requires it, by laying the log on its side and cutting through the bark, through the core and almost to the bark on the other side. Stand the log back on its end to finish the cut. This creates smoother cuts through the wood that are less likely to lead to stress cracks.
Cut the blank as needed, removing any cracked or rotted areas. Mount the roughly shaped blank on the lathe so that you can use the gouges to hollow out your piece.
Run the lathe, gouging out the center of the blank to a depth of 1 or 1 1/2 inches. Move the gouge out slightly and repeat the gouging to the same depth. Repeat until you have the first layer of gouging completed and a wall depth of 1/4 to 1/2 inch.
Repeat Step 4, returning to the center of the blank and gouging down another 1 to 1 1/2 inches, then moving the gouge out again to continue gouging this layer until you reach the walls. Maintain a consistent wall depth. Gouging the blank in this manner reduces the stress on the walls, making it less likely that they will crack.
Sand the inside and outside of the turned blank gently, never allowing the sander to rest in any one spot for more than a second or two. You want to prevent heat from building in any one area, which could crack the wood.
Place the wooden object inside a paper bag, loosely fold the bag closed and place that bag inside another paper bag. Repeat so that your wood is nestled inside three paper bags. The paper bags will become damp from the moisture in the wood. Set the bags in a cool, dry location until the paper bags are dry. This will allow the wood to dry slowly, which is less likely to result in cracks than heating the wood to dry it.
It is the two inches or so around the wood's core that is most likely to crack. Make sure your saw or chainsaw is sharp and oiled so that it cuts through the wood core as easily as possible.
Always follow the safety precautions and instructions included with your lathe and with any attachments.