ABSTRACT
Yellowing after laser cleaning was hypothesized to be due to the uncovering of a layer existing below the soiling. In fact, Vergès-Belmin & Bromblet (2000) found that when limestone or marble sculptures were cleaned, yellowish calcium oxalate residues were often revealed beneath gypsum black crusts. Moreover, stones and terracotta statues may contain brownish residues of oxidized organic matter below a black crust and laser cleaning will keep this colouration (Larson & Cooper 1996, Zafiropulos et al. 2003a). Cooper (1998) reported that the laser appeared to be unable to remove the greasy component from soiling on a stone sculpture, but rather caused it to yellow. In some other cases, yellowing was assigned to the presence of iron-rich spheres of 100-200 nm arising from the transformation of hematite (Klein et al. 2001). In this context, Gracia et al. (2004) used Mössbauer spectroscopy and XRD (X Ray Difraction) to study the changes produced in the pigment hematite by Nd:YAG, 1064 nm laser irradiation. Reduction of hematite into magnetite, formation of ferric species which are paramagnetic at room temperature, and dehydratation of gypsum to bassanite were the observed changes.
