These other characteristics of gold and silver make them both well suited for coinage: They are not radioactive. They are more ductile or softer than most other elements. They are less reactive compared with other elements. They have excellent luster. They have higher melting points compared with other metals. cm-3 Melting Point: 1964 C, 3567 F, 2237 K Boiling Point: 3695 C, 6683 F, 3968 K Heat of Fusion: 26.59 kJ.mol-1 Heat of Vaporization: 494 kJ.mol-1 Atomic: Oxidation States: 6, 5, 4, 3, 2, 1, -1 Electronegativity: 2.28 (Pauling scale) Atomic Radius: 134 picometre Covalent Radius: 142 7 picometre Ionization Energies: 719.7 kJ. Valuable minerals from the ore are separated by the combination of water and mechanical action. The small pieces of ore bearing the valuable minerals pass through the screen and are concentrated further in sluices. The larger ones (those that do not pass through the screen) are moved to a waste stack using a conveyor. Six are presented here: Buying silver coins: This is a popular way of taking hold of silver - physically. Perhaps the best example of a silver coin is the Canadian Silver Maple Leaf, which consists of 99.99% pure silver. Silver coins may either be "fine silver" or "junk silver". Junk silver coins are older coins with a lower percentage of silver. The United States alone, on the other hand, has an estimated total resource of 5,000 tonnes (5 million kilograms). These are identified in the states of Arizona, Miami, and Utah. But in spite of these significant resources, the U.S. continues to import a big part of its total consumption of the precious metal from some of the countries mentioned above. In fact, this is shown even in temperatures of as high as 2000 C (3632 F or 2273.15 K). Iridium was discovered in 1803 by the English chemist Smithson Tennant. It was identified from the residue of platinum ore which was dissolved in nitro-hydrochloric acid (also known as aqua regia). Platinum ores are still the main sources today of iridium.