Roberto Pini, Salvatore Siano, Francesca Fabiani, Renzo Salimbeni (Istituto di
Elettronica Quantistica, Firenze, Italia), Marco Giamello, Giuseppe Sabatini (Istituto di
Geochimica Ambientale e Conservazione del Patrimonio Culturale Lapideo, Siena, Italia).
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 façades 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:
- Basic studies on physical and chemical processes involved in laser cleaning of stone
- Laboratory tests on various type of stones to determine the optimum laser parameters.
- Design and development of laser systems dedicated to stone restorations.
- 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:
- laser cleaning is very precise and progressive because it removes layers of few microns
for each laser pulse. This means that cleaning operations follow the microstratigraphy of
the alteration layers and can be interrupted at predetermined stratigraphic levels. As a
consequence, original patinas, even of few micron in thickness can be preserved.
- very weak and highly altered surfaces can be successfully treated. This allows to
perform the cleaning before consolidation.
- chemically complex surfaces can be cleaned, such as those subjected to previous treated
with fluosilicates, where typically the cleaning by chemical means is impossible.
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 façades, 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.
Articles
Photoreports
Trade fairs
Stockmarket
Dictionary
Moralemeter
Print Edition
Subscription
Contact Us
HOME PAGE
Last Updated: July 2008
