Had you read the chapters mentioned you would recognize that you are getting bad information from several folks already. Water is either
unbound, carried in the vessels and lumens of the cell structure, or it is
bound, which is to say it forms weak Hydrogen bonds with the sugars making up the cellulose we call wood. The first is contained as in a sponge, and its loss is like letting it drip; the structure still maintains its shape. If, however, bound water is lost, the wood will shrink like the sponge left in the sun. With wood, the break point is around 30% moisture content compared to oven dry. Referred to as the FSP (Fiber Saturation Point), its where wood begins to lose bulk.
More air in the earlywood, between the latewood rings, so the bulk of shrinkage occurs there. Our "expert" in another thread referred to tangential/radial differences as if it were the reason wood splits. Mistaking the effect for a cause. If the % of earlywood is greater, the shrink will be greater. Depending on the orientation of the rings, it follows a pattern recognizable to anyone who's ever bought a 2X4.
What you see here happens in macro and micro, which is why wood with thinner earlywood (reaction wood) and gnarly figure distorts differently than straight-grained wood, though for the same reason. The reason a spindle turning ovals is obvious, in spite of deformation toward sapwood, no matter what is said, and the frown in the bottom should remind you of any bowl ever cut across the heart.
The first growth in woody plants doesn't really form "pith" as we find in non-woody plant stems. Juvenile wood is a better way to refer to it, as its composition is the same as the rest of the surroundings, just a bit less organized. You may, as I show in another thread, use the full slab cross-section if you take advantage of your scientific knowledge of the mechanical stresses imposed by differential shrink. The two most common, though of secondary concern after shape - are providing an environment with controlled RH like the bagging methods, and slowing the loss rate (10-12X versus endgrain on the face/quarter) at the surface of end grain through coating. This keeps the rate of loss slower, and the wood on the outside plastic, so it can adjust to changes in shape without forming end checks. No such thing as "heat" checks, BTW, the cause is shrink due to loss of moisture and wood contraction. Heated air carries more absolute moisture, so lowers the RH and increases the rate of loss on bound water. You can boil wood dry without checks, you can even oven dry it. Not the heat, the humidity.
REALLY good scientific information in the reference. I highly recommend it for any turner, because it will dispose of some of the mythology being repeated, and minimize the luck required to avoid self-destruction of your turnings.
Oh yes, you can avoid cracks by keeping the wood bulked. Water or PEG, polycryl, and so forth promise that. Or, you can dry it and keep it that way.
Better to follow the leader than the pack. Less to step in.
