an oven to sinter the soot the effectively massive surface area of the soot particles permits the soot to consolidate at a temperature significantly below the temperature at which the stack 40 melts and deforms the porous layer of soot consolidates into a fine layer of silica which is relatively thin compared with the layer of soot in fact the layer of silica can be so thin that it does not appreciably increase the thickness of the cladding capillaries 41 however the layer of silica 46 u2032 is thick enough to fill and seal the joins 47 between abutting cladding capillaries 41 as illustrated in the diagram in fig6 once the joins 47 are sealed the core defect 42 of the stack is substantially isolated from other regions of the stack and the core defect may be pressurized during the fiber drawing step independently of the other regions of the stack for example the core defect region may be pressurized to a lower pressure than the holes in the cladding capillaries and the interstitial voids 48 between the cladding capillaries may be substantially evacuated in order that they collapse during fiber drawing clearly the thickness of the soot layer in the core defect can be sensitively controlled by varying the heating temperature the concentration of the silicon tetrachloride in the oxygen and the deposition time the thickness of the consolidated layer of silica is a direct function of the amount of deposited soot the skilled person is free to chose whether to form an extremely fine layer of silica for example just sufficient to seal the joins between abutting capillaries or a.