Fire polishing, also known as flame polishing, is a method of polishing a material, usually glass or thermoplastics, by exposing it to a flame or heat. When the surface of the material briefly melts, surface tension smooths the surface. Operator skill is critical with this method. When done properly, flame plastic polishing produces the clearest finish, especially when polishing acrylic. This method is most applicable to flat external surfaces. Flame polishing is frequently used in acrylic plastic fabrication because of its high speed compared to abrasive methods. In this application, an oxyhydrogen torch is typically used, one reason being that the flame chemistry is unlikely to contaminate the plastic.
Machines used for flame polishing make a combination of oxygen and hydrogen from water and input it into a precision flame nozzle which can output flames that range from 4500 – 6000 degrees Fahrenheit. The process is best used on acrylic or glass with curved or sloped surfaces. A specially designed machine called the hydro flame is most commonly used in this line of work. The hydro flame is a gas powered generator that uses distilled water and electricity to split water molecules to produce the flame.