Thermoluminescence dating is a complicated scientific way to work out how old pottery is.
All materials are subject to radiation given off by the decay of natural radioactive elements. This radiation is stored as energy within the crystal-like structure of pottery. When pottery is heated up or fired to make it hard, all the previously stored energy is released as a light called thermoluminescence. After heating, the build-up of radiation starts again. This means that scientists can reheat a piece of ancient pottery after it has been excavated and measure the energy that has been stored and released since it was first fired. Older pottery will have stored up more energy; and so basically the higher the measurement of thermoluminescence, the older the piece of pottery.