Nd:YAG Laser Cleaning of Red Stone Materials: Evaluation of the Damage

Abstract

Lasers have been tested, during the recent past, as a useful cleaning method in conservation treatments: this is due to selectivity and precision of its performance. Nevertheless some colour changes have been detected using Nd:YAG laser sources, especially on white and red coloured substrates. Colour changes on white marble and other white architectural materials have already been widely surveyed. This chapter focuses on the interaction of laser radiation with two kinds of red materials: red Verona limestone and terracotta. These materials have been chosen because of their large use in northern Italian architecture and in statuary. Red Verona limestone is not homogenous in hue, owing to the presence of calcareous nodules (lighter in colour) and clay veins (dark reddish colour). On the other hand, terracotta is homogeneous in colour in most of the cases. Experimental tests have been carried out on specimens comparing two different Quanta System Nd:YAG laser devices: Michelangelo and a SYL 201 lab model equipped by a scanning computer-aided system. In this way a better radiation control has been obtained, both to address laser pulses on surfaces and to count exactly their number per surface unit. Fluences have been chosen on the basis of a previous survey, which indicated the harmfulness threshold fluence on red Verona limestone as 1.5 J cm−2. To better bring out laser effects, fluences have been increased up to 3 J cm−2. The interactions between laser radiation and substrates have been inferred comparing the changes of surface both in colour and morphology. Surface colour changes have been measured with a reflectance colorimeter (CIE L∗a∗b∗ system). Morphology has been examined by different microscopic techniques: thin and polished cross sections have been observed by optical microscopes; moreover, a close examination of surfaces has been carried out with the aid of scanning electron microscopy.