A commonly used technique for the mass production of glass optical fiber, OVD begins with heating silica and germanium to the point of vaporization. As the glass vapor cools, it is deposited as layers of soot on the outside of a rotating hollow ceramic bait rod to create a glass cylinder. The first layer is the core material of germanium-doped silica. On top of the core material, many layers of slightly purer silica soot, i.e., silica with lower levels of dopants, are deposited to form the cladding. If the end product is to be a step-index fiber, there is an abrupt change in the chemical com position 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 core to yield slightly and successively purer 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.When the deposition process is complete, the bait rod is slipped out of the glass cylinder, which is then 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. Inside vapor deposition (IVD) is a similar process, with the soot deposited on the inside of a rotating glass tube that becomes the outside cladding. See also cladding, core, graded-index fiber, IVD, sinter, and step-index fiber.