Fire sculpture and Fire rope create images in burning flame. They can make a tremendous impact, and can be a transitional and organic effect lasting several minutes. Images are bespoke and prices vary according to size and complexity. Standard or Yellow flame is the simplest and cheapest, but blue and green flame colours are possible and effective. More examples can be seen at firesculpture.co.uk
Another technique is lancework, which uses lots of individual coloured fire lances, and is particularly good for lettering.