The workshops, where glass objects and vessels are made, are called glasshouses. In the Middle Ages, glasshouses perhaps only had a small roof on top of the furnace, but otherwise they are still very similar today to what they have been like in the centuries past. The metal was heated in the large furnace, so that it became soft and could be manipulated. There was also a smaller annealing furnace, heated to about 500C, where the glass objects were allowed to cool slowly. If glass is cooled quickly, strong internal stresses will be created, and they can make the glass break very easily in use.
