Whether you are buying a slab from us, or somewhere else, never, ever buy a “kiln dried” wood slab without proof of the moisture content. If they refuse, or say they do not have a moisture meter, then do not buy it, because they have no way of verifying their claim that it is kiln dried. Do NOT just take their word for it. A moisture meter is a very basic tool that every competent, legitimate seller of kiln dried slabs should possess. A slab dried to at least 6-9% moisture in the middle (NOT on the very end or on an edge of the slab) is generally ready to use. A slab with a moisture content in the upper teens or even the 20s is nowhere near dry enough and will warp and crack if you move it indoors. A slab over 30% moisture is still soaking wet and above the fiber saturation point and has not even begun to even shrink yet. Unfortunately with the latest wood slab furniture craze there are all sorts of newbies coming out of the woodwork claiming to be selling thick kiln dried slabs that were cut only a month earlier. Never, ever assume that a “kiln dried” slab is at the proper moisture content. Do not just take their word for it. Buying a wood slab without verifying the moisture content is like buying a used car without taking it for a test drive. Demand proof or shop somewhere else.
A slab is generally safe to put inside after it reaches 6-10% moisture. Just like with lumber though, you will want to let it acclimate for a couple weeks before you make it into a piece of furniture.
Always finish every surface of a slab, both top and bottom. If you do not, one side will continue to lose moisture and shrink while the other doesn’t. The result will be a slab that curls up like a banana or cracks. And no, after a slab dries and warps it will not flatten out unless you move it outside to a warm high moisture environment (like an active greenhouse) to absorb moisture so the cells swell back up. Spraying it with water will not fix it.
You must allow for seasonal movement of a slab wood top. On slabs over four feet wide they might change in size as much as 3/4″ when gaining or losing just a couple percent in moisture. If you try to constrain the movement with hard fasteners you could crack your slab. I recommend figure eight fasteners or any other wood joinery system that allows for seasonal wood movement.
Once a slab has been dried to a safe moisture content it will generally stop cracking. While it is then not necessary to put butterfly keys in the cracks, they are a great way to highlight cracks as accents instead of defects