Method of manufacturing a cylindrical part from glass, especially from fluorinated glass by maintaining a thin gaseous film around the cylinder

Method of manufacturing a cylindrical part from glass, comprising the steps:

• providing a body with an interior lining defining a vertical cylindrical cavity having an inside diameter which is a few tens of microns greater than the outside diameter of said cylindrical part, at least an upper portion of said interior lining being porous,
• providing a cylindrical pedestal for supporting said part as it is being formed, said pedestal having an outside diameter such that it is free to slide in said cavity,
• providing at the top of said pedestal an initial seed mass having a same composition as said glass, heating said initial seed mass until it melts,
• injecting a gas into said porous lining in order to maintain between said porous lining and said molten seed mass a layer of said gas a few tens of microns thick to thereby prevent any contact between the molten seed mass and the lining,
• feeding a powder having the same composition as said glass to the molten seed mass from the top of said cavity,
• simultaneous with the feeding, lowering said pedestal to thereby permit a lower portion of the seed mass in a lower portion of said cavity to solidify, and maintaining at least an upper portion of the seed mass in an upper portion of said cavity in a molten condition and thereby form said cylindrical part from said molten seed mass.