Water vapor goes right through wax. A blank waxed all over will still eventually dry out, but slower than unwaxed. It can crack just as much as an unwaxed blank depending on the wood species, and whether pith, juvenile wood, or sap wood is present, how the grain is oriented on the various faces, the temperature/humidity in the environment, etc.
Waxing the wood properly does slow down the shrinkage which can minimize the cracks. Typically the end grain is coated with wax and the side grain is uncoated. Since water exits the end grain at a much high rate than the side grain, the wood right at the end grain surface can quickly get too dry and shrink too much due to the strong moisture gradient - the cells are literally pulled apart by the strong force of shrinking. Once such a crack/check starts at the surface, it only gets worse. Waxing the end grain slows down the drying at the surface and gives the moisture deep in the wood more time to migrate towards the end grain which creates a less severe moisture gradient and less chance of the strong tangential shrinkage pulling the fibers apart. The thicker the wax the better! When I use Anchorseal I usually decant some into a separate container (a plastic coffee can) and leave it open to the air for a couple of days so it gets thicker and goes on thicker.
The wood should be sealed as soon as possible after felling the tree or cutting up the log. If you wait until a check or crack already starts, even if it's so small it's invisible, it's too late. Best thing to do with an unwaxed or cracked blank is to cut the crack away then cut thin slices off the end grain until bending one doesn't separate easily. This proves you've cut deep enough. When I process logs into turning blanks I wax the end grain immediately, then check the blank in a few days. If any cracks have developed I cut them away, rewax, and check again after another few days. (Sometimes I end up with a considerably smaller blank but that's far better than a cracked blank!)
I've successfully dried that size of ambrosia soft maple by waxing just the end grain and air drying on shelves in the back room of my shop which I don't heat as much in the winter. But it depends on the wood - some will crack regardless of what you do, some is better. I have a block of 12x12x15" ambrosia maple drying for about 4 years now with no sign of a crack. On the other hand, I had a 3x3 block of maple split nearly in two! I usually have very little trouble with walnut blanks but recently one 4" thick blank developed a 1/2" deep crack all the way down the sapwood side. I cut this off, rewaxed and the next day it had 1/4" deep crack in the same place. I cut of some more and now the blank is down to about 3" and so far, no cracks. (keeping fingers crossed.)
When I rough turned bowls, I would always wax the end grain inside and out and sometimes wax the entire roughed bowl. I air dried on wire shelves for several months before finish turning.
BTW, there are several ways to preserve wet unturned blanks until you are ready to work on them. One is called "ponding" - immerse the blank completely in clear water, weighting it to hold it down. Change the water as needed. Wood will keep indefinitely this way. Another way is to wrap in plastic and freeze it. Another way is to boil it, discussed in detail recently in another thread.
BTW, I have some blanks that I cut from trees in 2006 and earlier. I try to process some wood every few months so I'll have a continuous supply of dry wood. (I far prefer turning dry wood to green.) I do make some bowl blanks, but mostly blanks for spindles, boxes, and other generally smaller things.
Freshly cut chunks ready to process, mostly cherry:
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Directly from the sawmill behind the barn:
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This guy's ready to go rough out some bowls:
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Processing on shop bandsaw
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Some ambrosia maple ready to put on drying shelves:
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Some drying on wire shelves:
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JKJ