Nd:YAG laser systems (1064 nm) have been widely applied in the removal of protective layers from stones, and many studies have been conducted both on laboratory model sample and real cases, investigating different laser parameters, among them the pulse duration length. The Er:YAG laser (2940 nm) effectiveness of cleaning of stones has been mainly investigated on model samples and for the removal of natural varnishes [1]. This study aims to make a comparison of the ablation rate, effectiveness and safeness of the laser assisted removal of synthetic protectives and consolidants from stones, by using Q Switched Nd:YAG laser with different pulse duration and Er:YAG laser (250 ?s). Different limestone and sandstone model samples have been chosen as representative of the most common stone artifacts which have been subjected to conservative interventions in the past, The selected products (Pulvistop and Idroblock by Geal S.r.l., Hydrophase Acqua by Phase Italia) were aqueous emulsions of silanes/siloxanes having low and medium molecular weight, with water repellent and consolidant (Pulvistop only) properties for stone materials, and Paraloid B72 as consolidant. The products were applied by absorption until refusal on the 5x5 cm and 10x10 cm l model samples. Two set of laboratory models have then been submitted to different artificial aging procedures: i) in Solar Box (CO.FO.ME.GRA 3000e, equipped with Xenon lamp, ?>295 nm) for 400 h at 500 W/m2 and ii) in climate chamber for the (Challenge 500, by Angelantoni) with -20/+70°C temperature cycles. To evaluate the performances of the materials, the evolution of ageing tests and the removal of products a multi-analytical approach was chosen: as: -preliminary characterisation of the materials and investigation of the physical properties of the model samples; a Pyroprobe EGA/PY-3030D (Frontier Lab) coupled with a Gas Chromatograph/Mass Spectrometer (PY-GC-MS, Agilent technology) and a portable Fourier Transform Infrared Spectroscopy (FT-IR ALPHA, Bruker) have been used for the characterisation of the not aged products. The water repellence imparted to the laboratory models has been tested by means of capillary absorption measurements following the UNI EN 15801:2010 standard. Colorimetry data (using a ChromaMeter CM-700d by MINOLTA) have been obtained on different spots of the model samples following the UNI EN 15886:2010 standard; - all the preliminary tests have been performed before and after artificial aging; - the investigation of the residue of protective/consolidant after laser removal has been performed by means of capillary absorption measurements and FTIR analysis. The degradation phenomena has been investigated by means of PY-GC-MS, both on the removed material (after Er:YAG laser application) and on the residue of material, collected by scalpel on a few selected area. Finally Environmental Scanning Electron Microscopy (ESEM) has been used in order to follow the laser removal in a some test areas. This paper shows how the optimization of the different laser parameters can lead to a safe removal of those materials used for conservation purposes, which over time have led to yellowing or darkening phenomena on a stone artifacts.
Comparison between Er:YAG and Nd YAG laser for the laser assisted removal of degraded protective from stone
S Bracci;A Sansonetti;B Sacchi;S Vettori;MP Colombini
2016
Abstract
Nd:YAG laser systems (1064 nm) have been widely applied in the removal of protective layers from stones, and many studies have been conducted both on laboratory model sample and real cases, investigating different laser parameters, among them the pulse duration length. The Er:YAG laser (2940 nm) effectiveness of cleaning of stones has been mainly investigated on model samples and for the removal of natural varnishes [1]. This study aims to make a comparison of the ablation rate, effectiveness and safeness of the laser assisted removal of synthetic protectives and consolidants from stones, by using Q Switched Nd:YAG laser with different pulse duration and Er:YAG laser (250 ?s). Different limestone and sandstone model samples have been chosen as representative of the most common stone artifacts which have been subjected to conservative interventions in the past, The selected products (Pulvistop and Idroblock by Geal S.r.l., Hydrophase Acqua by Phase Italia) were aqueous emulsions of silanes/siloxanes having low and medium molecular weight, with water repellent and consolidant (Pulvistop only) properties for stone materials, and Paraloid B72 as consolidant. The products were applied by absorption until refusal on the 5x5 cm and 10x10 cm l model samples. Two set of laboratory models have then been submitted to different artificial aging procedures: i) in Solar Box (CO.FO.ME.GRA 3000e, equipped with Xenon lamp, ?>295 nm) for 400 h at 500 W/m2 and ii) in climate chamber for the (Challenge 500, by Angelantoni) with -20/+70°C temperature cycles. To evaluate the performances of the materials, the evolution of ageing tests and the removal of products a multi-analytical approach was chosen: as: -preliminary characterisation of the materials and investigation of the physical properties of the model samples; a Pyroprobe EGA/PY-3030D (Frontier Lab) coupled with a Gas Chromatograph/Mass Spectrometer (PY-GC-MS, Agilent technology) and a portable Fourier Transform Infrared Spectroscopy (FT-IR ALPHA, Bruker) have been used for the characterisation of the not aged products. The water repellence imparted to the laboratory models has been tested by means of capillary absorption measurements following the UNI EN 15801:2010 standard. Colorimetry data (using a ChromaMeter CM-700d by MINOLTA) have been obtained on different spots of the model samples following the UNI EN 15886:2010 standard; - all the preliminary tests have been performed before and after artificial aging; - the investigation of the residue of protective/consolidant after laser removal has been performed by means of capillary absorption measurements and FTIR analysis. The degradation phenomena has been investigated by means of PY-GC-MS, both on the removed material (after Er:YAG laser application) and on the residue of material, collected by scalpel on a few selected area. Finally Environmental Scanning Electron Microscopy (ESEM) has been used in order to follow the laser removal in a some test areas. This paper shows how the optimization of the different laser parameters can lead to a safe removal of those materials used for conservation purposes, which over time have led to yellowing or darkening phenomena on a stone artifacts.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
