In the spring of 1961, Klein was able to gain access to one of France's major destructive testing laboratories (much like NIST here in the USA), whose gas powered flame propagation equipment Klein made terrific use of. The 'Fire Paintings' today remain along wth the IKB Monochrome paintings and the frighteningly existential 'Leap into the Void' some the most elemental and spiritual of his works, counterpointing the overwhelming and fiercely intellectual power of his work.
Using the bodies of young women as a mask (he had them covered in flame retardant and transfer their nubile forms onto the receiving paper - such that the impressions their corpulence left on the paper would serve to hinder the action of the flames - leaving a negative impression against the figure of the flame's action.
Fascinatingly, the action of 'burning' a figure onto the paper receiver is innately photographic - not only is there strong similarities in the sense that an energy(light in the case of photography) used to darken or create a figure in the substrate - but the masking technique (like 'dodging' in the darkroom) functions in an identical fashion. The net result isn't at all unlike a 'heliograph', or 'light painting'.