Why does the pith cause cracking? What exactly is going on there? Does the pith cause some sort of pressure that's being relieved?
Also, if I cut out an 8 x 8 x 15 inch vase turning blank from a chunk of wood that contained the pith down the center, would I be successful in drilling out the pith with a Forstner bit, and then gluing a dowel into its place, and then turning?
I'm curious as to whether or not the removal of the pith via drilling and replacement with glued dowel (or similar) would be successful.
(Sawing and Drying Forum)
From contributor I:
The wood doesn't shrink evenly. It may shrink by 4% in the radial dimension, but 6% in the tangential dimension. If both shrinkage rates were the same, then you could dry a round without it distorting. But back in the real world, the radius of the round has shrunk 4% and the circumference has decreased 6%. The missing 2% is the crack. If the pith isn't included in the blank, the effect is much reduced and the piece usually just pulls itself slightly out of square.
Your idea of drilling out the pith may work. I'd suggest you drill out something like a 3 inch hole removing the pith. Then dry your blank. Once dry, the 3" hole would probably be 2 3/4 (just guessing here). Then you should be able to insert a dry and stable plug and hopefully it will all hold together.
This effect occurs in all large timbers, but it is the pith where the shrinkage forces are the greatest. It is also greater with a round shape than with a square or rectangle - more volume in a circle per area, etc.
I was hoping to drill out the pith and glue in a replacement using Gorilla glue, because of its reaction to moisture. If drilling maple, I was hoping to dowel it with a contrasting color wood, like walnut, and make the final turning decorative.
Contributor I, I want to sell these, and was hoping the blank would be whole and complete before I sell them, but what you suggest may be the only way.