the furnace installation, known as "anderson's cupola". burned coke and used a steam-powered fan to produce a forced draft. from the time of lighting, around an hour was required to bring of pig iron to its melting point of – this amount could fill thirty 8-inch shells. after filling, the shells were left for a few minutes before firing, which allowed the metal in the filling hole to solidify and seal the hole. the shells remained effective even if an hour elapsed between filling and firing as, by this time, the filling would have solidified and the shell casing heated, making them equivalent to conventional heated shot. this included shells that had failed to break up on impact and had remained embedded in the timbers of the target.