EVALUATING LASER CLEANING OF CORRODED ARCHAEOLOGICAL SILVER COINS

Abstract

The present work aims at evaluating the use of laser in cleaning and analysis of archaeological corroded sil- ver coins. The selected corroded silver coins used in this study have been excavated from burial soil in Najran excavation site where there were more than 2500 rare coins dating back to pre-Islamic eras.. Laser cleaning tests were performed on 2 different types of corroded coins using a Q-switched Nd: YAG laser at 1064 nm. LIBS diagnostics was used to monitor the laser ablation process during the removal of different kinds of corrosion products with the aim of stopping the process once the cleaned surface is reached. XRD analysis was done to characterize the corrosion crust on the coin surface. For evaluating the usefulness of using laser in this study for analytical and cleaning of the selected corroded coins, the same coins were in- vestigated, before and after laser cleaning, with (SEM - EDX). The results showed that for analysis of the cor- rosion products on the crust on the surface of coin, the best LIBS spectra have been obtained after the first laser shot. The number of laser shots needed for surface cleaning depends essentially on the corrosion layer thickness. Only 5 laser shots were needed for the removal of thin corrosion layers while for thick layers 15 laser shots were used. Laser is successful, effective and safer cleaning technique for removing corrosion products of the studied archaeological coins.