A glass furnace is a specialized furnace used in glass production. Chemicals are heated together in a glass furnace to their melting point, when they fuse into molten glass. This liquid can then be formed into the shape it will retain when it cools and solidifies. The glass furnace allows glassmakers to control the temperature of the glass, so it does not break or lose its shape as it cools.
Glassmaking dates back thousands of years. The earliest glass furnaces were probably kilns built for pottery, a craft that is even older. Early glass production was pioneered by the Egyptians, and then developed by the Phoenicians and the Romans in later centuries. Much early glass was formed by the practice of glassblowing, an art that is still practiced in the 21st century.
Even early window glass was made by glassblowing; the processes were complicated, because blown glass generally takes a round shape. Some sheet glass was made by cutting and shaping glass cylinders, a process that had to be completed quickly after the glass was removed from the glass furnace. The process was imperfect, and the quality of the panes varied greatly until better industrial processes were created in the 20th century.
In glassblowing, molten glass is contained in a specially shaped glass furnace. The glassblower collects a quantity of liquid glass on a hollow rod, and then causes the glass to expand by blowing air through the unheated end of the rod. The glassblower keeps the glass at optimum temperature by working quickly and by reheating the glass in a glory hole, a special opening in the glass furnace for this purpose. Once formed, the glass is placed in another special glass furnace so it will cool slowly to room temperature without breaking, a process called annealing.
Modern industrial glass manufacturing works on much the same principles, just on a larger scale. Sheet glass or window glass is made by heating chemicals in a glass furnace, then delivering the molten glass onto the surface of a river of molten tin or other metal; this process is known as float glass, because the glass “floats” on the surface of the metal. The liquid metal has a perfectly level surface, and the glass imitates this property as it slowly cools. Pressurized nitrogen keeps the top surface of the glass level, and rollers allow for a range of thicknesses. Once the glass has annealed, it can be cut into sheets or panes by special machinery.