Abstract
Nd:YAG Q-Switched (1064 nm) laser cleaning of soiled stone often results in yellower appearances than other cleaning techniques. This yellowing effect is partly due to the creation of iron-rich nanostructures through the interaction between the laser radiation and the black encrustation to be eliminated. However, the iron speciation in these newly formed compounds is still unknown. Therefore, in this paper, X-ray absorption and visible optical spectroscopies have been performed on hematite based model crust samples to (i) determine the iron speciation in the newly formed compounds and (ii) evaluate the optical properties modifications before and after laser irradiation. It was found that the yellow color is mostly linked to the presence of octahedral (Oh) Fe3+ ions at the surface after irradiation. A small amount of octahedral (Oh) Fe2+ ions is also detected, highlighting a reduction process occurring during the laser irradiation. In conclusion, the iron contribution to the yellowing of hematite crusts has been addressed.
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