What can old Cape bricks tell us about the earth's magnetic field? An introduction to archaeomagnetism
Vincent Hare
Tue, 13/11/2018 - 18:30
SA Astronomical Observatory auditorium
Western Cape
In archaeomagnetism, physicists team up with archaeologists to reconstruct changes in the direction and strength of the Earth’s magnetic field over timescales of hundreds to thousands of years. This is important for physicists, because reliable direct measurements only began relatively recently, and we are interested in knowing how the magnetic field has changed further back in the past. The first relatively accurate measurements of directional change were made by 17th and 18th century European seafarers; direct measurements of the intensity of Earth’s magnetic field only began with the work of C. F. Gauss in the 1840s. Ancient ceramics – including bricks – are immensely useful because they contain magnetic minerals which behave a little like the magnetic tape in a cassette disk, and preserve a record of the intensity and direction of the Earth’s magnetic field further back in time. In order to reconstruct changes in the Earth’s magnetic field, this extremely faint magnetization, which dates from the time of last firing, is measured with a combination of complex field and laboratory techniques. And, if the age of the firing is known, archaeomagnetism can be used as a very accurate dating technique. 
In this talk I’ll present an intriguing introduction to how archaeomagnetism works. I will talk about how old Cape bricks can tell us something about the magnetic field – how fast it has changed over the past three hundred years, and why it has changed. I will also be talking about other materials like pre-colonial southern African pottery, which can extend our knowledge of the changing magnetic field further back in time – to the Iron Age. The talk will also touch on the early establishment of a magnetic observatory at the Cape by Herschel, around the time when Gauss organised a “Magnetic Union” – a fascinating network of global observatories which foreshadowed the great international geophysical collaborations of the 21st century. It is thus particularly appropriate that the talk takes place in the old Astronomical observatory – a site connected to Gauss’s original Magnetic Union.   History – Science – Archaeology. This talk promises to have it all!