Sterling silver is an alloy of silver and copper. Real world pure silver is usually considered too soft for craftswork and so is merged with other metals to improve its strength and durability. Since sterling silver has a lower value and cannot be used to make weapons, it doesn't seem worth spending the fuel to create it, unless a dwarf has a preference for it, or a noble demands it. The total value of four bars of sterling silver (4 * 8) is the same as the value of 3 bars of silver and one bar of copper (3*10 + 1*2 = 32).
temperature values unknown, used silver
value is weighted average of 3 silver and 1 copper
[NAME:sterling silver][ADJ:sterling silver][COLOR:7:7:1]