Charlie Harley said:
I'm thinking about cutting a section of 6" diameter tree and turning a bowl with the pith in the middle and growth rings in concentric circles. I've seen finished bowls like this and it's a neat effect.
However, I've also heard that the pith is the weak part of the tree and tends to generate cracks.
Does anyone have suggestions how to make this happen without the pith generating cracks?
I use wood science rather than magic, but it's still sort of like concrete. You can treat it so badly it's guaranteed to crack, but you can't treat it so well that it's guaranteed
not to crack. Everything here following relates to green wood, which is to say at or above the FSP. Partially cured wood has a whole developed set of stresses which are more difficult to predict and therefore control for.
First thing to realize is that a stress crack in the center of a log is a fairly common thing. Avoid slabs containing them. You can check yours by putting some water on the surface and watching for wicking to reveal the crack.
Second is to realize that the heartwood is generally much dryer than the sapwood to begin with, and will likely reach FSP and begin contracting sooner, making pith cracks more likely to develop or open existing. Natural enough thing to clear the excess unbound water from the outside with some compressed air, and easy to do in thin section. See the weird cherry in my gallery for an example of simple centrifugal ejection, where the end grain is across the piece.
Third is to remember that the reason for radial checks in logs, the ones that go out to in, is that the sapwood contains few of the extractives and physical changes which make heartwood, so it contracts proportionally more than heart. Minimize the amount of solid it has to pull against and potentially pull itself apart by dishing in or curving out, increasing the rate of curvature toward center in things like goblets where you can't avoid leaving more wood so that the long fibers can't collaborate into a crack.
Last, remember that it's differential drying which begins surface checks that can grow and stress into full splits, so dry the piece at as close as possible to the same rate from all surfaces. For me, it involves elevating the piece so the bottom dries as fast as the top. Instead of leaving the base of say, a goblet sitting on a flat surface, set it up on stickers to allow air access below. Else the top will start drying and contracting while the bottom remains wet and expanded. The result is well known.
There are other hedges you can take, like running CA into the pith and juvenile wood areas. Firms up existing, and slows loss by clogging the surface. Make sure you do this both top and bottom, or you're in the situation which the fourth step aims to prevent. If you're not too concerned about time and mildew, you can contain and equalize the relative humidity within a full box or wrap which will slow the heart/sapwood drying differential. Remember to put the wrapped piece on its edge, or you get into situation four again!
