i've done a few barrel firings in a galvanized trash can. did burn out the zinc first. punched holes in the sides and bottom. use bisqued highfire stoneware clay and have had no cracking problems. best results from whitest clay you can get. burnish or terra sig before bisquing for a nice shine on the finished pot. beware the fumes, especially when unloading. this is a really fun technique, so enjoy!