enables slower cooling, or above ground in open air, depending on the foundry's traditions. The raw materials of copper and tin are melted in a furnace until they become liquid at a temperature of approximately . Often scrap bronze from old bells is added, especially if the bell being cast is a replacement for an existing bell, which is in effect being recycled. The liquid metal is skimmed to remove impurities, then poured into the mould, using either a tilting ladle suspended from a crane, or else a system of brick channels constructed in the casting pit, which allows the metal to flow directly from the furnace into the individual moulds. As the metal enters the mould, holes in the top of the mantle ensure that gases are able to escape, otherwise there would be a risk the bell would be porous and susceptible to cracking. Porosity can also develop if the mould is damp, or is not at the proper temperature, or the metal when poured is not hot enough. The casting is allowed to cool for up to several days and large bells can take over a week to cool. Small bells, those under , can be removed from the moulds the following day. Cooling After the bell and equipment have cooled, the mould, containing the newly cast bell, is raised from the pit by the projecting trunnions of the bell case. The core plate is unclamped and the core broken out. The bell is then carefully extracted from the case. At this stage, any remaining loam adhered to the bell is brushed away and flash (excess metal), which may have formed below the bell's rim—owing to mould contraction in the presence of hot metal—is trimmed off. This completes the casting process. Tuning Bells are cast with defined profiles which were perfected in the early 20th century to ensure they can be harmonically tuned by the removal of small amounts of metal to adjust their harmonics. For a carillion or an English ring of full circle bells, the strike note of each bell must accord with the diatonic scale of the others, and to produce that the harmonics of each bell must be tuned to harmonise with its strike note. As a bell's strike note is affected slightly by its harmonics this can be an iterative process. An initial assessment is made to arrive at an average pitch