The blast is introduced into the hearth, at a small distance above the level of the upper edge of the dam, and is now generally performed by means of two _tuyeres_; in the more ancient furnaces, there was but one. The furnace being completely dried, a fire is lighted in the hearth, and fuel gradually added, until the whole is filled to the _trundle head_, which is the open and lesser base of the truncated cone that forms the body of the furnace. The blast may then be applied, slowly and gently at first, and increasing gradually, until it reach its maximum of intensity. As the blast proceeds, the charcoal gradually burns, and descends; its place is supplied at top by fresh fuel, by ore, and by the earthy matter used as a flux. This is styled _charging_ the furnaces. The earlier charges often contain no ore, but are wholly composed of charcoal and flux, and, in all cases, the proportion of ore and flux is at first small, and is gradually augmented. The charges are made as often as the mixed mass in the furnace descends sufficiently low to admit the quantity that is chosen as the proper amount. The charcoal is thrown in first, and the ore and flux are spread and mixed upon its surface. The principles which govern the amount of the charge, are as follows:-- "The volume of the charges depends upon the capacity of the furnace. If they be too large, they cool the upper part of the furnace, which will cause great inconveniences, particularly if zinc exist in the ore. On the other hand, small charges of charcoal will be cut or displaced by the ore, which will occasion a descent by sudden falls, in an oblique direction, or in a confused manner. It follows that the volume of the charge, although proportioned to the volume of the furnace, must be augmented: when the charcoal is light and susceptible of being displaced; and with the friability, the weight, and the shape of the fragments of the ore." "The heat, considered in any given horizontal section of the furnace, will be intense in proportion to the thickness of the layer of charcoal that reaches it. It follows, that the fusible ore requires smaller charges of charcoal than one that is more refractory. If the beds of charcoal and mineral are too thick, the upper part of the furnace will not be sufficiently heated. Hence it is obvious, that there must be a maximum and minimum charge for every different dimension of furnace, and for every different species of ore and fuel." _Karsten_.