This work investigates the influence of the pulse duration and the wavelength on the laser cleaning of thin silver plating
layers found in late Roman coins. Comparative cleaning tests were performed using Nd:YAG (1064 nm and 532 nm - 6
ns), GaAlAs diode (780 nm - 90 ps) and Ti-Sapphire regenerative amplifier (800 nm - 100 fs) laser systems. The
cleaning results on the plated areas were characterised by high resolution optical microscopy, SEM-EDX, XRF and
micro-profilometry.
Laser cleaning tests were performed on ancient (Roman and Byzantine) coins, which belong to the collection of the
Numismatic Museum of Athens, Greece. Coins with various types of surface corrosion were studied, using Q-switched
Nd:YAG, CO2 and Er:YAG lasers and a range of laser pulsing parameters on dry and wet surfaces. A section of each
object was cleaned mechanically, by the conservators of the museum in order to show the results of this method. It was
discovered that the results of laser cleaning was influenced by the type of corrosion of the surface of the coins. X-ray
fluorescence was applied as analytical technique. The results show that XRF could provide detail information about the
surface chemical nature of the treated objects, as well as about their past and present state and it leaded to
recommendations for restoration with the appropriate laser cleaning conditions.
The political problems in Late Roman Empire caused significant changes in the coin technology. The silver content
dropped severely and a new technology, in all the mints operating around the Empire, was introduced. For the production
of these coins, copper based quaternary alloys were used and their surface was covered by a silver amalgam plating
layer. Hoards of these coins have been recovered in thousands from across the Empire, however, their treatment has been
problematic. Both mechanical and chemical cleaning results in the damage or the complete destruction of the thin silver
layer. The use of laser technology in the cleaning of works of art has a wide range of applications which includes
metallic objects. The main aim of this work was to investigate the use of lasers in the cleaning of the thin silver plating
layers found in late Roman coins. The optimisation of laser parameters was achieved through comparative cleaning tests
by employing Nd:YAG (532 nm and 266 nm) laser systems. The cleaning results on the plated areas were characterised
by optical microscopy, and SEM-EDX analysis. Following a systematic investigation and many cleaning trials on two
different wavelengths and fluence values, optimum irradiation parameters were thoroughly demonstrated. Microscopic
observations of the cleaned areas evidenced complete removal of the encrustation and high selectivity of the laser
cleaning. Neither thermal or mechanical injuries, nor cuprite blackening were observed on the cleaned surfaces at the
optimum laser cleaning technique, using 532 nm of the Nd: YAG laser.
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