The fire will his own way - Up, Up, Up and the direction the wind is pointing. The hamper completion of the work. Requires that you calculate these factors. Film: http://www.youtube.com/watch?v=Si0XSgKNRU4
There are actually many ways to go; the entirely controlled where gas or power blocs representing a physical work, or you Built skeleton into laths, bound with straw. The last method makes it easy to create moving fire sculptures.
See also: www.ildskulptur.dk