Just natural clay from the hobby store instead of the really, really natural clay from the river. Has to be natural clay though, not one of the plastic or oil based clays. Just regular natural clay like potters use, nothing special, can sometimes get it cheap at colleges. The storebought is a lot smoother, so I'm thinking I may want to try making a slurry and seeing if I can get the larger bits to settle out a bit more with the riverbank stuff.
As to how long, I just covered them in charcoal, lit it and left it until the coals burned themselves out and cooled. So from a few hours to several. If you look close (because most of your piece should be covered), the clay gets hot enough to actually glow. Since the fire is pretty small, stick to small simple pieces, anything large and you'd need a real pit or kiln.
Going to try something a bit fancier next time, wish me luck.