I have two kilns that both run on standard household current. One is tiny - a six inch cube on the inside - and it heats quite nicely to at least cone 5 so I can successfully fire either stoneware or low-fire clay. The other one is larger, but is only rated to go to 1900, which is a few degrees lower than cone 04 - the recommended bisque firing temperature for low-fire clay. So, theoretically, I can make larger pieces as long at they're low-fire clay. The problem is that the kiln doesn't really go to 1900. Anything much over 1800 is really pushing it.
So I think the crack either came from firing too low - perhaps I fired a bit lower on the bisque fire than on the glaze fire. Or maybe the piece was too thin.
Well, I'm not too crushed about the crack because it just means I'll keep this piece and not put it in my Etsy shop - I don't mind the crack at all.
But it does give me a new resolve to get a new kiln. I'm planning on ordering it tomorrow. It's pretty large inside (though, at 19" high and 17" in diameter, it's not huge). It takes a special outlet on a dedicated 30 amp circuit, so I'll need to get an electrician - it will be a while. Once I get that kiln I'm planning to set up a firing schedule and fire no more than one bisque fire and one glaze fire a week. Meanwhile, I'm limiting myself to small pieces so I can use my little kiln. Hence, the smaller version, below, of my bear icon.