Publication Type:Journal Article
Source:Studies in Conservation, Volume 61, Number 3, p.136-145 (2016)
Synthetic mixtures of gypsum/hematite and gypsum/lamp black at different concentrations are irradiated using a Q-switched Nd:YAG laser at 1064 nm. On irradiation, red to pink hematite-based mixtures turn either to a bright yellow and/or to black, while gray to black lamp black mixtures turn to pale yellow and/or gray depending on fluence and pigment concentration. Based on these results, artificial gypsum crusts with a microstructure very similar to the one of natural black crusts are produced from 1% hematite and 0.5% lamp black in hemihydrate. It is shown that these artificial crusts show a bulk discoloration on laser irradiation very similar to the one occasionally observed on laser-irradiated natural black crusts. A scanning electron microscope study of these artificial crust samples shows that irradiation residues distribute as nano-scale deposits on the surface of the individual gypsum crystals. Such artificial crusts are good substitutes for natural black crusts to study cleaning techniques. In the case of laser cleaning, they are able to scavenge a significant amount of irradiation by-products, and thus provide an interesting material to further study the laser-induced yellow effect on natural gypsum crusts containing iron oxides and soot.