Basic knowledge of metal commemorative coins
Under normal circumstances, gold is more resistant to corrosion than silver; silver is more resistant to corrosion than copper; metals such as zinc, aluminum and iron are more susceptible to corrosion. For alloys, sometimes depending on the chemically active metal, such as pure gold, the stability is better, and when copper is added to the gold to form an alloy, black spots tend to appear on the coins. But brass (copper-zinc alloy) is more resistant to corrosion in the atmosphere than pure copper. Polyvinyl chloride is harmful to coins in packaging materials. Water vapor, sulfide and carbon dioxide in the air are also very harmful to coins. For example, sulfur reacts with silver to form silver sulfide, which makes the surface of silver coins gray and black. The moisture in the air also accelerates the oxidation process of the coin. During the appreciation process, the bare coin is touched by hand, so that the sweat marks leave a fingerprint on the coin, and the ...