Laser Cleaning of Silver Coin Surfaces

An analysis has been made of the reasons for the formation of internal and external corrosion of silver coins, which are based on copper (23–70 %) and iron (11 %) impurities of varying concentrations depending on the rank and denomination. Existing chemical cleaning methods using acids and salts are mainly effective in removing thin oxide films and can lead to destruction of the internal structure of coins. Experimental studies were conducted on two silvrt coins with a face value of 20 kopecks; their metal alloy: 50 % silver and 50 % copper in the original VF (Very Fine) rating condition with films of 0.1 to 0.5 mm thickness made of silver carbonate Ag 2 CO 3 , argentite Ag 2 S, and silver oxide Ag 2 O. A Wattsan FL TT laser marker with an IPG ytterbium fiber emitter (operating wavelength 1064 nm, output power 15.5 W) was used for cleaning. The EzCAD 2.14 software was used to control the parameters for moving the laser marker beam. The optimal parameters for processing silver coins are: speed of 5000 mm/s; the pulse repetition frequency of 200 kHz and the step between the processing lines of 0.01 mm, which ensures a two-fold overlap of the processing radiation along the x -axis and a five-fold overlap along the y -axis. In this case, for greater uniformity of the effect of laser radiation on the surface, it is advisable to carry out processing in several passes with alternating directions. The best surface quality was obtained with a total of 6 passes: 3 of which were parallel to the base axis х , 3 were at an angle of 45°. A visual assessment of the coin surface quality during cleaning process was carried out using a portable digital USB microscope GUANMOU AN104/F210 at 50x magnification. The quality of the formed mirror surface was assessed by measuring the reflection coefficient on the obverse side in the visible spectral range on a Photon RT Essentoptics spectrophotometer. The use of a laser cleaning method for silver coins has improved their condition from the original VF (Very Fine) to XF or EF (Extremely fine), and in combination with final polishing (Fluoropol M with a grain size of 0.6÷0.8 μm in a solution of Glycerin C 3 H 8 O 3 ) to the UNC (Uncirculated) condition, which is confirmed by a significant increase of 4–6 times in the reflection coefficient of the mirror surface.