Similar to a kiln, a muffle furnace sits atop a control unit that introduces heat electronically at extremely high temperatures within an enclosed, insulated area. The appliance is typically small and may be operated on a tabletop. Historically, a muffle furnace was an oven capable of reaching high temperatures with a heating chamber that was isolated from the fuel source and the by-products created. Fuel sources in the past might have included wood or liquid fuel. Muffle furnaces are typically used in craft, industrial, and scientific laboratory settings.
Embedded heating elements usually line all surfaces of the specially insulated ceramic or concrete interior of the heating chamber. The maximum output temperatures vary, with some models achieving temperatures up to 2,200 degrees Fahrenheit (1,204 degrees Celsius). Many bench-top muffle furnaces heat by means of conduction. As the internal heating elements increase in temperature, the heat circulates throughout the chamber and is absorbed by the inserted object. Some models also have convection capabilities, circulating the heat by means of fans.
Gases and combustion by-products generally escape through a small vent located on the top of the oven. Some interior chambers may contain no shelving while other models come equipped with up to two shelves for multiple items. The largest muffle furnaces contain chambers measuring up to 1.6 cubic feet (0.04 cubic meters). The exterior of the muffle furnace is generally stainless steel, and the largest available models measure 9 cubic feet (0.25 cubic meters).
Many models have a 0.38 inch (0.96 cm) monitoring port on the rear of the oven for viewing. The heavily insulated door of the muffle furnace may open using conventional hinges or may operate on sliding hinges. Most manufacturers design these furnaces with safety mechanisms, which cut the power once the door opens. While certain models receive electricity through a three-pronged power cord, many require permanent wiring by an electrician for safe, continuous operation. Muffle furnaces can have either analog or digital temperature monitoring.
Control settings may be operated manually. A programmable muffle furnace, however, allows the technician to preset heat up and combustion times. The ovens accomplish dozens of operations including heating, ramping, sintering, and soaking. As a ceramics or craft tool, the muffle furnace can fire or harden enamel glaze on pottery, braze and solder objects together, and melt and fuse glass.
Scientific environments might use a laboratory muffle furnace for nutritional analysis by calculating the amount of fat, carbohydrate, protein, and water content in a particular food. Labs may also identify organic and inorganic materials by the rate of combustion and the by-products obtained afterward. Industrial uses might include determining the combustion temperatures of specific materials.