BASIC RESEARCH AND DEVELOPMENT OF AN OPTIMISED LASER SYSTEM

Laser cleaning is a very promising technique in conservation that has been recently employed in operative tests for the removal of alteration layers from the surface of stone artefacts, as statues, architectural elements and decorations on the facades of historical buildings. This technique shows potential benefits, such as controlled and selective cleaning operations, that allow to face conservation problems that are still unsolved or present difficult solutions with standard conservation techniques.

In the frame of the Special project "Safeguard of the Cultural Heritage" we started since 1997 a research program aimed to investigate the application of lasers to restoration. The program is being presently carried out according to four research lines:

1) Basic studies on physical and chemical processes involved in laser cleaning of stone.

2) Laboratory tests on various type of stones to determine the optimum laser parameters.

3) Design and development of laser systems dedicated to stone restorations.

4) Field tests and operative interventions on statues and monuments.

Principles of laser-stone interaction

The first investigation step to understand the basics of laser cleaning of stone concerns the optical characterisation of the stone surface, including alteration layers and patinas. In other words, the response of the stone to low power laser irradiation has to be determined in terms of absorption and reflection of laser energy, in order to quantify the fraction that is effectively transferred to the material and is utilised in the cleaning process. Considering that these data are not typically available from the literature, one has to measure them directly on stone samples.

As an example, fig. 1 shows the complex microstratigraphy of a thin section of altered marble and Table 1 reports the measured values of the optical absorbency of the stone substrate and of the alteration layers at the wavelength of Nd:YAG lasers, usually employed for stone cleaning. This different optical absorption exhibited by alteration layers and substrates can be utilised to obtain a significant selectivity in laser cleaning operations. In fact, one can find an optimum value of laser energy which is effective in removing black crusts.

The response of stone materials to high energy laser pulses at levels suitable for stone cleaning was studied by means of coherent imaging diagnostics that permitted the observance and analysing of the very fast processes occurring during laser-stone interaction. In details, the analysis provided quantitative information on the evolution of physical parameters, such as local pressure and temperature, during the very short transient of laser-stone interaction. With this diagnostic tool we compared the mechanism of laser cleaning induced on stone by different laser systems. In particular, we evidenced the dependence on the duration of the laser pulse with respect to the morphology of the surface resulting after laser treatment. Regarding the control of possible side effects, we found that short laser pulses in the nanosecond range are more like to cause mechanical damage to the surface of the irradiated material, as local cracks, microfragmentation and increased porosity of the substrate, whereas the risk of thermally-induced modifications is greater with longer duration pulses up to the millisecond range, where the typical effects of continuous irradiation can be recognised. The problem of the increase local absorption due the presence of impurities that could induce colour changes to the treated surfaces are also being studied.

Laboratory tests

A large collection of stone samples was obtained from Italian monuments, especially from Siena and Florence, presenting various degradation conditions. Stone types subjected to laser tests were: white marble (Carrara and others), rosso ammonitico limestone, cavernoso limestone, Pliocene sandstone, pietra forte sandstone, travertine and Aurisina limestone.

Three different Nd:YAG laser types were employed. One of them (SFR) is the prototype version of the system developed, as described in the following. The analytical methods to evaluate the effects of laser cleaning were those of mineralogical and petrographic characterisation, performed before and after the treatment: 1) observation of the stone surface by stereomicroscope and SEM; 2) X-ray diffractometry; 3) observation of ultra-thin sections by polarising microscope.

Figure 2 shows phases of laser cleaning and analysis on a sample of marble with black crusts collected from the Baptistery of Siena. Usually, samples were prepared by selecting different areas where different irradiation conditions were tested, as shown in Fig. 2 a. Figure 2 b is a stereomicroscopic image of the transition between cleaned uncovered a well preserved patina of calcium oxalate, where traces of grooves from the original working are still recognisable. This figure was confirmed by ultra-thin sections that showed a fully preserved Ca-oxalate film.

In summary, laboratory tests pointed out important intrinsic aspects of this technique:

Design of an engineered system for field applications

Based on laboratory results, we developed (in co-operation with Electronic Engineering of Calenzano, Firenze, EL.EN. SpA) an innovate laser system, aimed to improve intrinsic performances, as well as handling capabilities, in order to optimize laser cleaning procedures and to facilitate applications in the restoration yard. The laser is a Nd:YAG with fiberoptic transmission, called "Smart Clean".It was designed to emit laser pulses with a duration of 20 ms, that allows to substantially reduce the risk of both photomechanical and heat damages to the stone, that are more likely to occur with shorter and longer laser pulses, respectively. Moreover, this pulse duration allows transmission of high laser energy through long optical fibers (50 m), which provides easy applications to facades, by leaving the laser cabinet at the ground level.

The laser system was used in operative cleaning interventions and tests on important Italian monuments, such as Palazzo Rucellai and Duomo in Florence, Capella di Piazza del Campo in Siena, the mausoleum of Theodoric in Ravenna and the church of S. Giovanni in Zoccoli in Viterbo. In most of the cases laser operation was required to complete the cleaning after the application of chemicals (such as ammonium carbonate), which removed the black crust but were unable to treat the dark superficial layer of the underlying calcium oxalate patina encapsulating carbonaceous particles. In one case, laser cleaning was successfully performed on graffiti produced by a red synthetic paint.

 

Conclusion

Future developments of this research will regard a critical analysis of laser cleaning in comparative studies aimed to validate this technique as an integration or substitution of conventional cleaning methods, with particular concern to the status of conservation of stone surfaces after long periods of time. Laboratory studies will be extended to a larger variety of stone types. Technological improvements are expected in the development of laser systems more suitable for the application on larger surfaces and with lower operative costs. Finally, a crucial point for the safe and correct application of laser cleaning will be training of expert restorers by multidisciplinary courses regarding all the aspects of this new procedure.

The authors wish to thank the Special Project "Cultural Heritage" of the CNR and the High Technology Network of Tuscany for financial support to this research.