Crucible steel is made principally from steel scrap, with which is combined cast iron if the carbon is to be increased, or muck bar if the carbon is to be lowered. All of these materials are of known chemical composition and are selected for their purity.

In the central space of the building devoted to this process is usually placed the set of crucible-steel furnaces, and along the walls on one or two sides are placed a number of bins not unlike horse stalls in a barn. In these bins are stored scraps or punchings of steel, pieces of muck bar, and chunks of pig iron, all cut or broken small enough to be readily packed in the crucibles. A highly important feature of these bins is that great care is taken not to let any metal be placed in them until samples are chemically analyzed and found suitable as crucible steel material; also each bin is reserved strictly for scrap of but one composition. It is essential that there be received no scrap which contains a greater per cent of sulphur or phosphorus than is admissible in the finished steel, because there is no practical way of removing these impurities in this process. It is a common saying in crucible-steel making that everything which goes into the pot comes out, meaning that none of the ingredients of the charge are lost by either an oxidizing or a reducing action of the furnace flame as in the other processes.

The materials for the highest grade of this steel are usually of Swedish or other iron exceptionally pure. These materials are in the forms of muck bar and pig iron mixed to give the per cent of carbon required in the finished steel. Wrought iron (mnck bar) may be melted with enough charcoal in the pot to bring it up to the degree of carbon required, or cast iron may be mixed with a little good scrap to lower the carbon.

Other materials more or less needed in making up a crucible charge are (1) charcoal free from sulphur for carburizing the steel; (2) ferro-manganese for reducing any iron oxide in the scrap of the charge (usually stirred in just before the crucible is taken from the furnace); and (3) some form of silica (sand or ground silica brick) to act as a flux for taking up slag and oxides melted from the scrap.