Just to round things out a bit, I mentioned stump killer only as a source of potassium nitrate, the stuff that keith had mentioned earlier. I'd be a little careful with top openers and salt baths, you may mess up your kiln, have a tough time accessing color changes, but most of all, you have to look in right in the line of fire if some thing splatters.

Consider checking out the pdf. Temp changes as low as 10* and certainly 20* will change the colors. Maybe think camp stove style burner setup and an appropriate material pot to dip in, maybe.