To provide a method for manufacturing a silica-coated clad tube as a precursor of an optical fiber wherein processing accompanying a homogeneous gaseous reaction product and heating are promoted because concentration on a cold surface is controlled, which method is applicable to the heating of a sol-gel tube and other substrates.
Pipe-shaped muffle furnace 10 with an open end is provided with a tube 11 or other substrate and a cylindrical shield 12 for shielding heat and gas, and a door 13. The tube 11 in the furnace 10 which is closed with the door at the open end is heated and processed with a gas stream through a feed hole 14. The heat at the door is controlled with the shield 12, by which O-ring 16 is maintained to perfection. Other example shows that a counterdiffusion of both a process gas from the tube 11 and a concentratable material toward the door is controlled with the shield 12.
MONBERG ERIC M
TREVOR DENNIS J
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