This video shows the glassblowers using thermal shock to crack 45 litre borosilicate bottles in half. Both sections are required intact as they will be glued back together after housing enlarged polycystic kidneys for the Anatomy Department Museum.
It was calculated 200 revolutions at 1 revolution/second of applied heat was required, based on a diameter of 406mm and an estimated wall thickness of 10 -12mm (actually 5mm).
Greg Kerr scores the bottle with a diamond pen, two intense lpg/oxygen flames are held at a tangent to the surface score. Much to our surprise the first bottle cracks off at 118 seconds into the well placed box of cardboard. The second bottle follows the planned method; after 130 revolutions, lamps are removed, 5 seconds is allowed for the surface temperature to equalise and a damp cloth is applied to thermally shock the bottle. A perfect outcome for both sections of both bottles. Thanks to John Penno for his assistance and advice.