Evaluation of laser cleaning of ancient Greek, Roman and Byzantine coins

Laser cleaning tests were performed on ancient coins, which belong to the collection of the Numismatic Museum of Athens, Greece. Silver and copper alloy coins with various types of surface corrosion and ground deposits were studied, using Q-switched (QS), long pulse (LP) and free-running (FR) Nd:YAG lasers and a range of laser-pulsing parameters on wet surfaces. A comparison among the different laser types was made, cleaning with an optical fibre delivery system and an optical focusing subassembly, or with the help of x-y microadjustable stage. It was discovered that the results of laser cleaning was influenced by the type of corrosion and the corrosion products or the ground deposits on the surface of the coins. The results, evaluated by objective and SEM-EDX observations, indicated that on silver alloy coins, different wavelengths could achieve a successful laser cleaning for several types of corrosion. On the other hand, laser cleaning could result in loss of the original surface in various cases of copper alloy coins, followed by frequent appearance of the oxidation layer underneath and partial removal of corrosion products and ground deposits. The optimum laser cleaning procedure was associated with the different types of coin corrosions. Special emphasis was stressed on the side effects of the procedures and, in particular, on the minimization of any chemical modifications and thermal results induced to the original surface of the coins. Copyright (C) 2010 John Wiley & Sons, Ltd.