Method for thermal treatment of a refractory dielectric body, such as glass, using plasma

A refractory dielectric body, such as glass, is heated with a plasma fireball at conditions which do not result in substantial removal of a surface portion of the body (i.e. ≤0.1mm from the surface of the body), yet which are sufficient to reduce both surface and bulk impurities. Typically such conditions entail ensuring that the plasma heats the body to a surface temperature in the range of 1800 to 2300 DEG C. Typically, the body is treated with the plasma in the absence of simultaneous deposition of material onto the body. The body may be solid or hollow, such as a rod or a tube, and the surface heated by the plasma may be the inside surface or the outer surface of the body. Advantageously, an isothermal, oxygen or oxygen-containing plasma is utilized. The invention is useful for reducing chlorine impurities by at least about 30% to a depth of at least about 10 mu m, with accompanying reduction of hydroxyl impurities. The invention thus provides a useful method for reducing the concentration of impurities that contribute to imperfections during the process of drawing fiber from an optical fiber preform, without requiring substantial removal of the surface of the preform.