How do iron oxides in clay affect the colour of brick?

In the production of brick and tile, iron oxides are the primary agents responsible for the characteristic red, brown, and black colorations found in many clays and shales.21

The final color of a brick is heavily influenced by the chemical composition of the raw clay and the conditions within the kiln during firing. While iron oxides naturally produce red ranges in ordinary clays, manufacturers can manipulate these colors by regulating the oxidizing conditions during the firing process. Through a technique known as flashing, the atmosphere in the kiln is adjusted to produce browns, purples, and blacks. It is important to note that this flashing process typically alters only the surface color of the brick, while the interior body retains its original hue.2