17th-century Ruins Could Unravel Mysteries of Earth’s Magnetic Field

Jesuit church ruins from the 17th century, like the São Miguel das Missões, were built with clay bricks that contain magnetite, which realigned with earth's magnetic field when it was fired in kilns. (Credit: Wikimedia Commons)  It seems safe to say that the laborers firing clay pavers, bricks and tiles to build the Jesuit mission of Santo Ângelo over 300 years ago had no idea that their toils might someday bear relevance to spacecraft orbiting 600 miles above what is now southern Brazil. As the bricks and pavers were fired in kilns, magnetite in the clay abandoned its inherent magnetic properties and realigned in response to the magnetic forces exerted by the earth itself. The point at which this occurs – 580 degrees Celsius, in the case of magnetite – is known as the Curie point. After these building materials cooled and were stacked to form a church, school and other buildings at Santo Ângelo, the magnetite inside retained this reshuffled alignment, a record of the magnetic past sealed away like a proverbial mosquito in amber. Along with several dozen other Jesuit missions built in the same era, Santo Ângelo flourished briefly along what was then the poorly defined border between the Spanish and Portuguese colonies in South America. At its height, the mission was home to about 8,000 people, nearly all of them indigenous Guarani whom the Jesuits were trying to Christianize.