Metallizing is a process by which a coating of metal is applied to an object or surface that is not metal. It is similar to the process that is used to electroplate a metal object with a coating of another metal, but the layer of metal applied in metallizing is not as strongly bonded to the metallized object or surface as in metal-to-metal plating. The reason for this is that non-metallic objects are not as electrically conductive as metals. Strong conductivity of the object to be plated is a prerequisite for effective electroplating. Metallizing treatments may involve plating with a number of different metals.
The first attempts at metallizing objects are thought by historians to date to the first half of the 19th century, when a famous German chemist, Justus von Liebig, worked out a way to bond a thin coating of silver to glass, creating the first silvered mirror. This innovation produced mirrors of much improved quality over earlier technologies. This basic idea is still used for making mirrors, including some of the world's largest and most precise mirrors used in telescopes. With modern improvements to von Liebig's process, nearly any material can be metallized, including plastics and ceramics.
The process of metallizing is very much like standard electroplating, but the target object must first be specially prepared to increase its conductivity, so that the layer of metal will adhere. This is done through a complicated process involving acid etching the surface of the object and then dipping it in a succession of chemical baths containing metallic compounds. This results in a very thin layer of metal, usually nickel or copper, being bonded to the surface of the object. The layer of metal intended as the finish is then applied.
The use of the term metallizing can be somewhat ambiguous. A technique known as thermal spray metallizing is sometimes used to apply a protective coat of metal to existing objects or structures to prevent corrosion due to exposure to the elements. Metals such as zinc and aluminum, or a combination of metals known to be corrosion resistant, are applied by a special type of spray gun that vaporizes the metal and then applies it to a prepared surface. This type of metallizing is not the same as the process first developed by von Liebig, as it does not involve using an electric current to bond the layer of metal to the target. Rather, it is essentially applying molten metal like paint.