or some other gaseous medium in those locations where the glass lies against the mould parts the glass will cool down comparatively strongly and a thin layer of glass 21 of comparatively high viscosity will be formed on both mould parts 3 and 5 the glass which has not yet cooled down so strongly is pressed down and up through an interspacing 23 present between the layers 21 in the direction of the arrows 25 27 respectively the compression force p required for this increases in proportion as the interspacing 23 becomes narrower at a certain moment the pressure in the glass in the vicinity 29 of the cone tip to be formed will become greater than the gas pressure prevailing in the escape space 11 from that moment the glass will flow into the escape space 11 after moulding the second mould part 5 is displaced in a direction opposite to the arrow 19 the moulded cone is subsequently pressed from the first mould part 3 by the gas pressure the escape space 11 is funnel shaped to facilitate stripping of the moulded cone from the first mould part 3 the gas transmitting filter 13 is provided with channels 17 whose cross sectional dimensions 0 05 0 3 mm are so chosen that the glass cannot penetrate these channels owing to its viscosity when it is pressed against the gas transmitting filter 13 in practice the aim will be to prevent this penetration by.