Metallurgical examination of ferrous artifacts has revealed the presence of derived structures such as martensite and tempered martensites. Conservation of these artifacts by the hydrogen reduction technique, involves the application of elevated temperatures for up to 144 hours. The paper describes the effect of prolonged exposure at high temperature on typical structures found in archaeological iron work; in particular, cutting implements such as swords, knives etc. Iron samples from 19th century bridges were tested (so that no artifacts were destroyed). It was concluded that 350 degrees C is an acceptable temperature to conserve artifacts which are suspected of containing derived microstructures. Micrographs, tables, references. -- ICCROM