Other notable silver bullion coins include the Australian Silver Kookaburra, Chinese Silver Panda, and the Russian George the Victorious. Minting coins, whether gold or silver, always entails the risk of having the value of the metal used in the coin greater than the coin's face value. This is especially true in coins of low denomination. 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. Platinum coins are internationally recognized forms of currency and have the ISO code XPT. However, there are only a few varieties of platinum coins minted, largely because of the cost of platinum and of the difficulty in working with it. Bullion coins minted from platinum include the Manx Noble (minted from 1983 to 1989), the Canadian Platinum Maple Leaf (1988 to 1999), the Australian Platinum Koala (1988 to the present), the Mexican Libertad (1989 only), and the American Platinum Eagle (1997 to the present). Also called fire assay, the method involves treating ores or alloyed metals under high temperatures and carefully controlled operations to separate gold from base metals (copper, zinc, or lead), which may be present in the ore. Once the base metals are heated at high temperatures, the gold (as well as the other precious metals that may be present) remains apart and the other non-precious metals react forming other compounds. A silver certificate allows an investor to buy and sell the security sans the inconveniences associated with the physical silver's transfer. The Perth Mint Certificate Programme, which is fully guaranteed by the Government of Western Australia, is the only silver certificate program in the world that is guaranteed by a national government. mol-1 (second) Platinum is known to occur as only three thousandth parts-per notation (0.003 ppm) in the Earth's crust. This makes it an extremely rare metal. Compared to gold, platinum is more precious, although its price is considered more volatile. One of the reasons for this is that its demand is driven by industrial uses or applications.