Borosilicate Glass Tube and Rod: the Vello Process

Laboratory borosilicate glass is about 80% silica, 13% boric oxide, 4% sodium or potassium oxide and 2–3% alumina, melted from boric oxide, silica sand, soda ash and alumina. It melts hotter than ordinary glass. Tube and rod are drawn continuously from the melt. In the Vello process, which L. Sanches-Vello developed in France in 1929, the glass flows down through a ring-shaped opening between the feeder's outlet nozzle and a hollow needle, and air blown through the needle keeps the tube open. The tube sags from vertical to horizontal, runs along a roller track to the drawing machine, and is cut into pieces about 1.5 m long. The nozzle runs cooler than in the older Danner process, which suits glass with volatile borates. Without the needle, the same feeder makes rod.