The U.S. glass industry has made significant improvements in energy efficiency over the past two decades. Much of the achieved efficiency improvement is the result of improvements in the glassmaking process, as well as conversion of glass furnaces from electricity to natural gas. Additionally, oxy-fuel furnaces now account for about 20-25% of all glass furnaces and use considerably less natural gas fuel; these reductions, however, are offset by the energy required to produce the oxygen used in an oxy-fuel furnace. [Ross 1999]