Publication Type:Journal Article
Source:Studies in Conservation, Volume 60, Number S1, p.S12-S18 (2015)
Laser cleaning trials using a Nd:YAG (1064 nm) device (El. En. Combo) were performed on outdoor sculptures at Chartres cathedral, France. Two laser regimes were compared: long Q-switched (LQS), which corresponds to a regime wherein the pulse length reaches 100 nanoseconds, and short free-running (SFR), with a pulse duration of 100 µseconds. Surfaces cleaned with the LQS mode appear clean and slightly yellow while the SFR mode leads to a darker but less yellow appearance. Colour measurements were performed on a low-porous stone from Berchères, used as the main construction material of the cathedral and commonly found on flat background architectural elements and pediments. Samples were collected from a range of laser cleaning windows, and a small sample from a limestone column was cleaned with both laser regimes, leaving a sharply delimited untreated band between the two regimes. Polished sections were prepared perpendicularly to the stone surface after having embedded the samples in epoxy resin. Samples were observed using an optical microscope (reflected light, dark field) and a scanning electron microscope (SEM) and analysed with energy-dispersive X-ray analysis. As far as mm to cm scale samples may be representative of the reality of hundreds of square meters of sculptures and background architectures, we could see that in the operating conditions selected, SFR mode at 300 mJ output energy with fluence 5.4 J/cm2 gave fairly good preservation of the oxalate layer lining the stone surface, and left in place some fragments of black crust, while LQS at 150 mJ with fluence 0.37 J/cm2 preserved the layer well and left no trace of black crust.