Its not an oven alone, its a kiln that smelts up the components for glass of the desired recipe, then purs the glass on a long (~half a km) basin of molten tin where it stretches and cools slowly, pushing the solidifying mass down the basin until the end. Re-Melting it is as far as i know not possible, as the melting is done before the pouring. You can melt the tin again, but not the glass.