Glaze is a substance painted onto ceramic pieces and then melted when the piece is fired, producing a glasslike effect without running off. In addition to adding the sheen that is part of the beauty of ceramics, glaze can be manipulated in many ways to give different visual qualities to the piece, including mixing or constructing "garbage glaze". There are a few types of garbage glaze -- glaze additives featuring substances culled from drugstore products and found objects.
Many substances at the drugstore or supermarket contain minerals associated with glazes. Experimenting with different combinations of such substances, both under, over, and inside the glaze, can produce different effects, although a lot of experimentation is necessary to find something you like. Useful minerals include potassium, sodium, magnesium, calcium, and dolomite.
Firing some other object in the kiln with the glazed piece can produce lots of interesting effects. Found objects and garbage can sometimes create strange new colorations or patterns on the surface of the piece if they're placed over, under, inside, or around. Some objects will wither in the heat or dissolve into the glaze; others will remain intact and can be removed when the piece is taken out of the kiln.
Almost as important as what substances or objects you fire with the glaze is what temperature you fire it at. Temperatures of around 2150 degrees Fahrenheit produce reliable effects and do not entirely dissolve all found objects; higher temperatures may cause an entire object to become volatile and leave traces only in patterns or colors on the fired clay.