A commonly used technique for the mass production of glass optical fiber, IVD begins with heating silica and germanium dopant to the point of vaporization. As the glass vapor cools, it is deposited as layers of soot on the inside of a rotating hollow glass cylinder, which typically remains as the outside cladding of the end product.The first layers deposited are cladding of relatively pure silica.The final layer is germanium-doped silica, which forms the core. If the end product is to be a step-index fiber, there is an abrupt change in the chemical composition between the core and cladding. If the end product is to be a graded-index fiber, there will be many graded layers of silica of slightly different chemical compositions deposited on the cylinder wall to yield slightly and successively less pure layers of cladding surrounding the fiber axis.The composition of the glass layers in a graded-index fiber is much like the arrangement of the annular rings of a tree. Once the deposition process is complete, the entire glass cylinder is sintered and collapsed into a preform cylinder, which is cooled and stored.The tip of the preform cylinder is reheated to a temperature of 2,500 degrees in a drawing tower. The resulting gob of molten glass is carefully drawn by gravity, in a process known as broomsticking, into a fiber as long as 20 kilometers. As the fibers cool, an acrylate coating is applied to protect the raw glass from physical damage. As is the case with all of these techniques, OVD takes place in a vacuum environment, as it is the exposure to oxygen that makes glass so brittle. Outside vapor deposition (IVD) is a similar process, with the soot deposited on the outside of a rotating ceramic bait rod that is slipped out of the glass cylinder prior to the formation of the preform cylinder. See also cladding, core, dopant, graded-index fiber, IVD, sintering, and step-index fiber.