I majored in ceramics. If a piece shatters into tiny fragments, it was still damp inside. If a big chunk blows out, it was either too thick (over an inch) or from the child placing a wad of clay over an indented surface to smooth it out. You can usually see the air bubble in the blown out pieces if that was the problem. Children need to be taught how not to add air pockets to their clay while they are working it. Kids are kids. Some will poke a finger into their clay and then just cover it up any old way. I tell my students that if they have a dent in their clay, they should smooth it out so that a smooth clay coil or smooth ball of clay could be pressed onto the dent to fill the void. Another thing I show them is how to use a serrated metal rib to "plow the field" or level out uneven surfaces if they want a smooth surface. What I mean by that is, use a serrated metal rib to comb over the clay with enough hand pressure to level off all of the highs and lows on the surface of the clay. Then turn the rib around in your hand and use the smooth side of it and run it over the leveled off toothy texture from the comb. This will get everything leveled off again and smooth in a way that little fingers just can't do by themselves at times. I tell kids to add texture AFTER they are finished forming their clay features so they don't smooth it off in the wrong way.
I also teach my students how to build a hollow form that won't explode beginning at 3rd grade level. We build a body of an animal with pinch pots joined together with slip and a smoothed in coil, and then add on a neck with coils. An air hole is poked into the place where the hollow neck joins the hollow body, and also under the body allowing the neck air out through the bottom of the body. The legs and head are formed from modeled solid clay, pinching out rough shapes, adding on muzzles starting with a ball of clay on the front of the face, smoothed into the face. Kids love to see how easy it is to make that anchor shaped muzzles/mouths (dog, rabbit,lion, cat, etc) from four little balls of clay that get rubbed into the face on the outer edges of the balls placed together. The lines in the middle where the balls touch are left alone to form that anchor shaped mouth after the outer edges of the balls are smoothed into the cheek, chin, and upper part of the muzzle. Picture a little ball for the nose, two slightly or greatly larger (depends on the animal) balls for the animals cheeks, and a little ball for the chin. Place those four balls at the front of the face, touching each other, where you want the muzzle to be. Then rub the outside edges into the face smoothly. What you are left with after rubbing in the outer edges is a muzzle with an anchor shaped mouth. Kids love the ease of making something so complicated looking in such a simple way. I also teach them to make little balls for eyes, and then a small coil over the edge of the ball to make an eyelid. Rub the outer edge or that coil into the forehead or face, and you have a sculpted eye. The only trick is to make the other eye match it in size and placement...another story.
Anyway, our kids are well versed in sculpture by the end of 5th grade. The blowups we have in the kiln are either my fault if I am rushing the firing (I'm very careful), or their fault if a big chunk blows out of the bottom. It might have been too thick to dry all the way through in that spot, or it might have had an air pocket because the kid just slapped the clay onto a rough surface. I tell my kids that all shaping is done from the outside without poking holes into the clay and covering them up.
When I dry their pieces, I give them one day to just sit on the shelf with a light plastic with holes in it thrown loosely over them. The next morning, I take off the plastic and start a gentle fan in the room. BY afternoon, the fan is directly on the pieces. The next day I can USUALLY put them in the kiln if they no longer feel cool and damp and aren't too thick. I preheat my kiln at 150 degrees for 3 hours. That is below boiling temperature, so they won't blow up at that gentle heat. After that, they go at slow speed for the rest of the firing at climbing temperatures. I rarely have blowups that way. But I don't put a fan on them the first day. That can cause cracks in parts of their clay that are uneven in thickness. Little handles will pop off if you fan dry them too soon, for example.
I had to change clay bodies one time in my life. I was using a lot of underglazes in my own work, and for some reason, some change they made to the clay body, those underglazes were just flaking off the rim of a pot, a few times, from ANYWHERE on the pot. I multi fired the pieces, added layers of different things, had been doing that for a long time. INTENSE labor in glazing. The problem never showed up until the final firings when I put the clear glaze coat on top of all of that work. It was VERY disappointing, and since I had used this clay body for so long in the same way with my underglazes, it was very disheartening. The seller of the clay and I decided it was something to do with too much talc in the white lowfire clay body, that the makers had changed their formula somewhat. Perhaps due to a shortage of something they usually used in their formula, who knows. Clay makers have machinery that compresses the air out of their clay. It's usually not their fault if the piece has air bubbles. Kids probably did it inadvertently in their construction, or you might have fired it too soon.
HOpe this helps,
--- In email@example.com, Gayle Parent <gayleparent@...> wrote:
> In recent years I have heard from several sources that most projects that "blow up" in the kiln do so because there is moisture still in the clay, more than due to air bubbles. Anyone agree or disagree? I have several years of experience with clay, but I'm not a clay specialist. Could we hear from some high school ceramics teachers?