Abstract: A method of introducing a vapor stream into a rotating optical fiber preform tube comprising the steps of: (a) generating a vapor stream comprised of a vaporized glass forming precursor entrained in an oxidizing carrier gas; (b) flowing the vapor stream into the rotating optical fiber preform tube through a sealed rotary joint; and (c) preventing any material alteration in the composition of the vapor stream from occurring should the sealed rotary joint become leaky by flowing a stream of fluid consisting essentially of the oxidizing carrier gas over the rotary joint at a pressure greater than the pressure of the vapor stream as it flows through the rotary joint.