In order to prevent an uncontrolled collapse of a quartz tube whose inner surface is being coated with a vitrifiable substance in making a preform or parison of an optical fiber, one or more streams of carrier gas entraining the ingredients of the coating substance through the quartz tube are supplemented by a stream of supplemental carrier gas whose flow is being continuously controlled to keep the overall mass-flow rate substantially constant. The outer tube diameter is photoelectrically monitored and incipient changes thereof are compensated by controlling a flow of additional carrier or inert gas which does not pass through the tube but joins the traversing gas flow at the tube outlet, inside a solids separator with a restricted exit aperture, to modify the pressure differential across the tube wall.