M. Montoto

Laser cleaning of stone materials: an overview of current research

Abstract This paper presents an overview of current knowledge and recent advances in the application of pulsed laser radiation to the cleaning of works of art, particularly ornamental stone. Special emphasis is put on the critical review of publications from the last six years. The recent history of laser cleaning in art conservation is summarised and a description of laser fundamentals, laser types and practicalities for use in conservation is given. Existing explanatory models of the underlying physics of laser cleaning are briefly described, as are the advances in laser cleaning of different stone types. Common techniques, including on-line monitoring used to detect material damage after laser irradiation, are outlined and examples are given of their application. Finally, current trends and improvements in laser-cleaning efficiency and the reduction of damaging side effects are addressed. It is hoped that this overview will bring greater attention to the issues that are important for future research on the laser cleaning of stone.

Rock Matrix Diffusion as a Mechanism of Radionuclide Retardation: A Natural Analogue Study of El Berrocal Granite, Spain

Blocks and cores of El Berrocal granite intersected by hydrogeologically-active fractures have been studied in order to determine (1) the extent to which diffusion of natural radionuclides has taken place from fractures into the rock matrix, and (2) the microstructural controls of diffusion. The results suggest that, in many granitic rocks, matrix diffusion (of uranium, at least) may be limited to a zone extending only a few cm from water-conducting fractures and that not all of the rock may be available for diffusion. This should be taken into account in establishing radionuclide migration/retardation models for repository sites.

NON-DESTRUCTIVE ULTRASONIC PROCEDURE TO EVALUATE IN SITU THE RELATIVE DETERIORATION OF MONUMENTAL STONES: PRELIMINARY RESULTS

To evaluate in-situ the relative deterioration of monumental stones, a non-destructive ultrasonic procedure is under development: ultrasonic waves are introduced into the rock and after travelling through it are processed for evaluation of their Vp, amplitude, duration, energy and count number. The procedure was applied to two Spanish Monasteries, one built with a granodiorite (El Escorial, Madrid, which results are included here) and the other built with sandstone (Sta. Maria de Ripoll, Gerona). Laboratory ageing tests, involving monitoring of the above mentioned ultrasonic parameters, were performed as a basis for interpreting the in-situ measurements. The energy seems to be the most sensitive parameter as an indicator of the rock deterioration, so, the value obtained for a masonry block in the monument can be used to classify the rock into one of preset deterioration groups.

Mechanical Stresses Generated by Crystallization of Salts Inside Treated and Non-Treated Monumental Stones; Monitoring and Interpretation by Acoustic Emission / Microseismic Activity.

The crystallization of salts in building stones is one of the most common and severe deteriorating phenomencndeveloped on Historic Buildings. To evaluate their mechanical effects in treated and non-treated stones, experimental salt crystallization tests and the monitoring of the corresponding acoustic emission / microseismic activity, AE/MS, have been planned. Salt crystallization tests have therefore been performed on a high porosity (30%) limestone used in the building of the Cathedral of Murcia (Spain). Each test consisted of 11 and 20 cycles (for the non-treated and treated specimens, respectively) of immersion of specimens in salt solution (14% Na 2 SO 4 . 10H 2 O) for 4 h., drying in an oven at 60 °C for 14 h. and final cooling at room temperature for 6 h. The treated specimens were consolidated by total immersion in ethyl silicate -Tegovakon V- and protected with a water repellent, oligomeric polysiloxane -Tegosivin HL- 100-, (both from Th. Goldschmidt, FRG). The AE/MS, was monitored in the 100-300 KHz range. The preliminary results show a very different AE/MS behaviour between the non-treated and the treated specimens.

Acoustic Emission Monitoring of the Cathedral of Palma de Mallorca (Spain)

The presence of fractures and fissures in a building is the consequence of an inadequate response of its structure and/or materials to the stresses affecting them. Frequently, primarily in large buildings, the soil-structure interaction modifies the distribution of stress on the structure: differential settlement, unforeseen strains, underground instability, etc. are some of the causes that can induce fractures in the structure or in building materials. Generally speaking, the fractures and fissures caused by these factors cease to grow after a short time and do not represent a problem for the buildings stability.