This technique is usually used on igneous rocks like basaltic lava flows that ‘lock in’ local magnetic fields when the magnetite goes through the curie temperature. An oriented core sample is taken with a diamond bit, then cut into pieces that are mounted in a special freely spinning gimbal mounted in three axis magnetometer. The sample is spun and an average magnetization vector is determined and worked back to the original sample orientation in space. Here they are doing it with furnace slag.
Yup. That’s the technique that provided the evidence for plate tectonics, still better known (and more accurately known) as continental drift. But it’s been used on pottery and such as well, the results providing consternation at times.
thanks for the explanation.