Method and apparatus for making optical fibres

A method of subjecting a glass preform to processing by tensile forces in a furnace to produce a glass product of predetermined shape, according to which method

• at least a part of the glass preform is introduced into the furnace through an inlet opening,
• a portion of the glass preform introduced into the furnace is heated to a temperature above the softening point of the glass,
• the heated portion of the glass preform is subjected to tensile forces in a drawing direction to process the preform into the predetermined shape,
• the portion of the preform which has been processed into the predetermined shape is drawn from the furnace through an outlet opening, and
• the heated portion of the preform and at least a part of the processed portion of the preform are flushed in the furnace with inert gas which is being fed into the furnace,
• characterized by
• maintaining the concentration of gaseous impurities in the furnace essentially on the same level as the concentration of the same impurities in the inert gas fed into the furnace,
• establishing a diffusion barrier against the inflow of undesired gaseous components from the ambient air, driven by the forces of diffusion, by generating a barrier flow of inert gas in at least one opening selected from said inlet opening and said outlet opening of the furnace, said barrier flow having a direction of flow, which is generally opposite to the direction of the diffusion.