The numbers are keys to the formula, recorded in Wedgwood's "Experiment Book." The letters indicate placement in the kiln and so the firing temperature. Jasperware clays were used for the portrait medallions and ceramic reliefs popular in the late eighteenth century. Source: Wedgewood Museum, Barlaston, Stoke-on-Trent, England.

From The creation of color in eighteenth-century Europe by Sarah Lowengard