What’s causing the purple discolouration in my fired lustre?

Lustres are a very thin (molecular) layer of metallic components deposited in the surface of the glaze. Quite often, the inexperienced user can deposit minute traces of lustre on the glaze layer by accident. Sometimes a mistake can be made and an attempt made to remove it by wiping it away. Sadly, when the ware is fired the minute trace will show as a purple discolouration of even fingerprint. These mistakes can only be removed by etching away the glaze surface with acid, normally hydrofluoric acid. Such acids can be purchased as proprietary gold removers from suppliers.