12"x12" isn't that big of a slab. dunno what firing schedule you're using but likely it's too fast.
slabs and similar work with large flat surface areas against a shelf (like slabs, and big platters/pots without a foot trimmed into it) tend to have two main issues with firing.
-One is that they are a big surface area, sitting flat against a shelf, thus the heat cannot easily penetrate to the bottom, center, of the clay. Because it was cooler than everywhere else during the time you were candling your kiln, by the time you go to your next stage of firing, this cool section kinda rapidly heats up and thus you get blow-outs. Raising/elevating the piece will help tremendously. Also slow down your firing to compensate for lack of heat penetration. Kinda the same concept if you were to stack a really tight, low height shelf in the bottom of your kiln - the heat can't penetrate very well.
-Second issue is more likely with larger work than the scale we're talking about, and the problem is usually cracking due to the friction and drag caused by the physical weight of the mass as it's trying to expand/contract in the kiln. Something under the piece
Yes, silica sand can work if you need to sit it flat. Grog will also do the same thing. Putting a layer under your piece will act as ball bearings and facilitate the movement as the clay expands/contracts. Since it's not a solid, it will also allow moisture and heat to pass through the interstices and make its way to/from the bottom. Another alternative is raising the piece on pieces of broken kiln shelf or clay pucks. Most of the time with big work, I use either the broken shelf bits or balls of kiln wadding (1/3 each: grog, silica, clay) - they do the same thing, but they have one main factor that makes me go toward them before loose sand/grog......Grog/sand can fall through the shelf crack/joint to the work below, the wadding stays put. Another benefit to the wadding and sand is that they will self-level out which is great if your kiln shelves aren't perfectly flat, but your piece was built on a flat surface.
Yes, slabs fired at an angle on their side can definitely warp. Depends on how thick/thin, how heavy, how big, the temp you're firing to, etc etc.
I lean stuff on soft brick all the time as long as there's no glaze.
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