People in this group work in refineries and plants that deal with raw materials such as copper, lead, zinc, uranium, and precious metals, and plants and mills that produce materials / make products out of materials such as steel, aluminum, cement, glass, concrete, clay, and stone.
Here are the average annual salaries.
2010: $39,582
2005: $35,984
2000: $27,458
1995: $27,140