Process for producing double helical glass tube, light-emitting tube for fluorescent lamp, and fluorescent lamp

A manufacturing method of an arc tube, comprising:

• a deforming step of softening and bending a straight glass tube at a central portion thereof and deforming the glass tube into a double spiral shape;
• a processing step, following the deforming step, of processing the double spiral shaped glass tube so that a bump forming section in the central portion has a greater tube thickness than other sections in the central portion, the bump forming section being a section at which a bump is to be formed;
• and
• a forming step, following the processing step, of forming the bump by expanding the bump forming section.

A manufacturing method of a fluorescent lamp, comprising:

• a deforming step of softening and bending a straight glass tube at a central portion thereof and deforming the glass tube into a double spiral shape;
• a processing step, following the deforming step, of processing the double spiral shaped glass tube so that a bump forming section in the central portion has a greater tube thickness than other sections in the central portion, the bump forming section being a section at which a bump is to be formed;
• and
• a forming step, following the processing step, of forming the bump by expanding the bump forming section.