Abstract: A blow-pipe 1 with an upper end 2 shaped to retain a working amount of molten glass is vertically movable from beneath the surface of molten glass 9 in a furnace to an upper position within orifice 10a of cover plate 10 and just below a cylindrical guide tube 11. Upward movement brings the blow-pipe generally into a hotter environment, expands the internal air and thus provides an initial bubble at the blow-pipe end. This is progressively expanded into the guide tube, preferably by two-stage air supply at different pressures, to produce a constrained glass bubble, of a few microns thickness.