Monte Kali is an unusual landmark in the small town of Heringer in eastern Hesse, Germany. It’s a spoil heap containing nothing but sodium chloride or common table salt, which is a byproduct of potash mining.
For over a hundred years, potash mining has been a major industry in the region. It started with the opening of Wintershall potash works, which began mining in 1903, and today is the world’s biggest potash mine with an operational area about the size of Greater Munich’s.
Potash mining produces a mixture of potash and sodium chloride, with potassium content between 20% and 35%. Thus, for every ton of potash recovered, several tons of sodium chloride is produced. This is dumped at several sites around the region. The dumps contain up to 96% sodium chloride. (Source)
Egyptian blue – a bright blue crystalline substance – is believed to be the first unnatural pigment in human history. Ancient Egyptians used a rare mineral, cuprorivaite, as inspiration for the color. Cuprorivaite was so rare searching and mining for it was impossible. Instead, using advanced chemistry for the time, Egyptians manufactured the color. It was made by mixing calcium compound (typically calcium carbonate), a copper-containing compound (metal filings or malachite), silica sand and soda or potash as a flux, then heating to between 850-950 C.
Egyptian blue was widely used in ancient times as a pigment in painting, such as in wall paintings, tombs and mummies’ coffins, and as a ceramic glaze known as Egyptian faience. Its use spread throughout Egypt, Mesopotamia, Greece, and the far reaches of the Roman Empire. It was often used as a substitute for lapis lazuli, an extremely expensive and rare mineral sourced in Afghanistan. After the decline of the Roman Empire, though, Egyptian Blue quickly disappeared from use.