Khan Academy: Maiolica: History, Function, and Production

The presence of tin in a glaze makes the glaze less likely to run or blur when the painted ceramic piece is heated in a kiln. Artists took advantage of this quality and used tin glazes for making pottery with finely detailed, painted decorations. The tin glaze preserves colors incredibly well over time, so maiolica is unique in that its brilliant colors may appear just as they did when they were originally painted hundreds of years ago.