As wood dries the size along the grain stays pretty much the same but across the grain it shrinks. This can be a problem when producing bowls.
Two ways to avoid this is to make sure the wood is properly dried before turning another popular solution is to “twice-turn”. A twice-turned bowl is initially turned while the wood is still wet but is only rough turned down to around 10% of the overall diameter leaving a wall thickness of around 20 – 25mm. This unfinished piece is then set aside for possibly several months until the wood has dried. Part-turning the bowl like this allows it to dry more quickly due to the reduced timber thickness. Once the piece has dried it is remounted in the lathe and turned to its final finished size when it should be stable.
That all said sometimes it is quite nice to turn a bowl from wet wood all the way to a finished piece. Doing this there is always the risk that the bowl will split in places and end up in the bin. But if the final wall thickness is quite thin one can usually get away without any splits. What cannot be avoided when turning wet like this though is the inevitable warping that will occur due to the cross grain shrinkage. However, this can produce a quite striking and unusual bowl.
This bowl is very oval!
