The term 'Glassmaker's Oven' refers to a lehr, which is an industrial oven used in the glassmaking process. It is used to slowly cool and anneal glass objects, allowing them to cool gradually and evenly without cracking or shattering. The lehr is heated to a temperature of around 500-600 degrees Celsius, and the glass objects are placed inside for a period of time before being removed and cooled.