Abstract: The blank is made from a tube of silica with a layer of other glass coated on its inner surface. The said other glass is doped with P.sub.2 O.sub.5 for example to increase its refractive index. The blank is then reduced in diameter from a tube to a rod by heating a region of the tube while it rotates in a glassmaker's lathe. The heated region is moved back and forth along the length of the tube until it is finally reduced to a rod. One effect of the moving heated zone is to evaporate some of the doping material, and the vapour tends to move along the tube ahead of the heated zone until it is eventually shifted out of the ends of the tube. To minimize this loss the back and the forth movement of the heated zone includes a sudden step forward at the end of each pass before returning in the opposite direction.

Abstract: A reactive gaseous mixture which reacts in a localized heating zone to form a glass deposit on the inner wall of a tube is made to flow along the tube, and in the heating zone it is channelled around a cylindrical element which occupies much of the bore of the tube. The glass deposit is used for making glass fibres for telecommunications.