1) There are no guarantees with wood. And especially with forms like bowls and hollowforms. You're setting up geometric stresses (and stabilities) beyond just the normal stresses in the wood.
2) Some woods are simply more stable than others. Sycamore doesn't tend to crack as much as other woods but it does warp. Mesquite is quite stable and you can do anything to it at any time and you'll not have too much trouble. Red Oak cracks terribly. These are just generalizations and each stick is going to behave differently depending on so many other factors.
3) Moisture content is only one variable having to do with cracking and warping (stresses). This just happens to be one that we can, somewhat, control and, mostly, measure. The other forces/stresses within a piece of wood can only be guessed at most of the time and are completely not under our control except to simply not use that piece of wood (which would be after the fact, unfortunately).
Using a moisture meter is going to help a bit. Without a doubt it will. You'll get close to what you want with it but don't expect exact accuracy between readings and pieces of wood. Ideally, it'll measure down below the environmental MC (moisture content) which the other, simpler methods can't. Those other, simpler methods are simply touch (when it no longer feels cool to the touch) and weight measurements (when it no longer loses weight). Both of these measurements should be taken after the drying method of choice has been done and the wood allowed to be brought back to equilibrium with the environment. These two methods are fine for drying bowls, for example, because all you need is to get it "in tune" with the environmental MC. It'll take on and release moisture along with the environment it's in relatively equally at that stage.
In kilning we shoot for about 6% MC (Moisture Content) in hardwood lumber that's going to be used for milling. It has to be stable. But, these are thinner pieces of wood and it can be done fairly easily. For this area, especially lately with the amount of moisture we've had here, if I take my rough turnings down to that MC, final turn them and apply a finish rather quickly, I'm setting myself up for problems later. Yes, wood can be TOO dry. However, I do want my pen blanks and trembleur, most spindle and many turned box blanks to be that dry.
You have a choice of whether to ...
1) dry your wood (or have it done for you, buy it that way, etc.) before you start working it
2) rough it, dry it, final turn it
3) final turn it and then possibly dry it.
The first option is going to be wasteful, expensive and hard to find for large blanks like bowls, hollowforms, or big boxes. That's because most kiln operators aren't going to do thick pieces of wood. You get high losses. I sell my kiln dried thick blanks for at least double the green stuff. The second option, depending on how you chose to dry it, can be done very well but can take some time. Time, unfortunately in some cases, is going to be your friend. ANY shortcuts are going to have their pitfalls. The third option, the one that I most often take for free-form pieces, is going to get you done quicker but demands more attention to wall thickness but, most especially, even-ness throughout the piece so that it doesn't crack. Warpage is something I don't mind in these pieces. If it's a piece that I can't have that in, I take either option #1 or #2 and give it more time.
Did you microwave that locust for 15 minutes on high? Surely not. Must have been on low. Either way, that's just too much at once. Locust is a hard wood when dry. That and Osage Orange aren't something I don't turn dry. If I can't drive a nail into it, I don't turn it.
Yep, lots of micro-cracking is a good sign that you've gone too far, too fast.
The un-comforting answer to your last question is, simply, that you won't know. You can increase your odds by getting the moisture content down to the 10% range AND picking a species of wood that is more stable AND picking a piece of wood that is straight grained, uniform, boring AND picking a piece of wood that was from the main trunk of the tree about 8 feet off the ground AND picking a piece of wood that was from a tree that was grown on flat ground, without other trees around it, that wasn't subjected to too much wind or drought or flood AND AND AND. Otherwise, it's going to be a guess. And I'm most thankful that it is. Otherwise, I might as well be turning epoxy.
