The Earth's magnetic field is dynamic, with fluctuations that range from minute daily variances to massive polar field reversals that occur with a frequency on the order of one million years. Magnetic minerals in heat-treated archaeological materials have the ability to store a record of these past field changes, which can be measured using modern superconducting rock magnetometers. For example, artifacts such as fired pottery or slag from metal production contain magnetic minerals that record the Earth's field strength and direction as they are cooled from high temperatures (such as those typical in ceramic kilns), providing a fixed record of the ancient field at the time of their manufacture. Compilations of well-dated sequential field measurements can act as reference curves, which can be used to estimate the age of other heat-treated archaeological materials. This is the basis for archaeomagnetic dating, an important chronological method that is powerful even at sites that do not preserve materials suitable for radiocarbon analysis.