of the Bronze Age. Arsenical bronze objects appear first in the Near East where arsenic is commonly found with copper ore, but the health risks were quickly realized and the quest for sources of the much less hazardous tin ores began early in the Bronze Age. This created the demand for rare tin metal and formed a trade network that linked the distant sources of tin to the markets of Bronze Age cultures. Cassiterite (), the oxide form of tin, was most likely the original source of tin. Other tin ores are less common sulfides such as stannite that require a more involved smelting process. Cassiterite often accumulates in alluvial channels as placer deposits because it is harder, heavier, and more chemically resistant than the accompanying granite. Cassiterite is usually black or dark in color, and these deposits can be easily seen in river banks. Alluvial (placer) deposits may incidentally have been collected and separated by methods similar to gold panning. Compounds and chemistry In the great majority of its compounds, tin has the oxidation state II or IV. Compounds containing bivalent tin are called while those containing tetravalent tin are termed . Inorganic compounds Halide compounds are known for both oxidation states. For Sn(IV), all four halides are well known: SnF4, SnCl4, SnBr4, and SnI4. The three heavier members are volatile molecular compounds, whereas the tetrafluoride is polymeric. All four halides are known for Sn(II) also: SnF2, , SnBr2, and SnI2. All are polymeric solids. Of these eight compounds, only the iodides are colored. Tin(II) chloride (also known as stannous chloride) is the most important commercial tin halide. Illustrating the routes to such compounds, chlorine reacts with tin metal to give SnCl4 whereas the reaction of hydrochloric acid and tin produces and hydrogen gas. Alternatively SnCl4 and Sn combine to stannous chloride by a process called comproportionation: SnCl4 + Sn → 2 Tin can form many oxides, sulfides, and other chalcogenide derivatives. The dioxide (cassiterite) forms when tin is heated in the presence of air. is amphoteric, which means that it dissolves in both acidic and basic solutions. Stannates with the structure []2−, like [], are also known, though the free stannic acid [] is unknown. Sulfides of tin exist in both the +2 and +4 oxidation states: tin(II) sulfide and tin(IV) sulfide (mosaic gold). Hydrides Stannane (), with tin in the +4 oxidation state, is unstable. Organotin hydrides