The reason for the superiority of crucible steel over steel of like composition from other processes is not always apparent, but is no doubt due in greater part to the following conditions, viz.:

(1) The stock is carefully selected, and has had the advantage of more or less refining by having been previously produced in a steel-making process.

(2) The covering of the crucible enables the elements of the charge to be better controlled in melting by avoiding losses due to the action of the flame.

(3) The melting of the charge distributes the carbon and other elements equally throughout the mass, and sets free the slag and oxides which the scrap metals contained, allowing them to float to the surface.

(4) The boiling out of occluded gases before pouring gives a more compact steel.

(5) Before a crucible-steel billet is rolled, it is thoroughly hammered, while hot, under a steam hammer to make it compact and dense. This is called tilting because of the old method of hammering newly made iron under a tilting machine.

About the only important chemical changes are (1) the reducing of iron oxide by the introduction of ferro, when this is found necessary, and (2) the absorption of carbon by the metal. Incidentally the metal will absorb some manganese from the charge, silicon from the crucible walls, and sulphur and phosphorus from the slag, if these elements are present in the crucible.