an arrangement for collapsing tubular formations into optical preforms includes a control arrangement which controls the pressure build - up at the downstream end of the tubular formation during all of but the last one of the collapsing increments to control the flow of a gaseous medium through the interior of the tubular formation from an upstream end to and beyond the downstream end of the tubular formation . the magnitude of the back - up pressure is controlled by a set point controller in dependence on the actual value of the back - up pressure , the set point controller being operative for counteracting any deviation from the desired pressure . the set point controller controls the flow - through cross - section of a control valve that admits a pressurized inert gas to a region situated downstream of the downstream end of the tubular formation and also downstream of the point at which the pressure build - up is being measured .