Throughout the past 17 years since re-opening the Gitche Gumee Museum (after it was closed by its founder 21 years previous), each summer many people bring pieces of slag in for identification hoping that the specimens are agate. Slag can be found on the beaches west of Munising, MI, as well as at several other places in Michigan in other areas where blast furnaces were used to purify ore. I can understand why people think these specimens are agate since oftentimes like agates; they have conchoidal fractures, structure or other patterns, and translucency. I have learned over the years how to let people down easily and educate them at the same time.
Originally there were 29 blast furnaces in the Upper Peninsula of Michigan that were used to melt down and purify iron ore. Only two sites remain including the Bay Furnace in Christmas, and Fayette on the Garden Peninsula east of Escanaba. Iron ore was first discovered in the U.P. in the 1840s. Although the iron ore rock was up to 72 percent pure, it was necessary to remove the impurities and extract the iron.
Iron is purified from iron ore in a huge container called a blast furnace. Beginning in the 1840s in Michigan’s Upper Peninsula, iron ores such as hematite and magnetite were mined and then transported to blast furnaces. To purify the ore, the rock was added to the blast furnace along with limestone and charcoal. The mixture was heated to around 2282°F (1250°C) almost 300 degrees below iron’s melting point of 2786°F (1538°C).
In this reduction reaction, the charcoal was used to heat the mixture and add carbon to the chemical reaction. The limestone served as a flux that helped to catalyze the desired reaction and chemically bind to and remove impurities, such as silica. In this reaction, the iron oxide was reduced to iron, the carbon was oxidized to carbon dioxide, and the impurities were formed into glass-like slag, which was separated and removed. A picture of the Bay Furnace in its reconstructed condition is shown below.