After the regenerators have brought the furnace up to a high heat, the charged crucibles are lowered one by one into the melting holes. The chimney draft prevents flame from coming out of the melting-hole openings while the crucibles are being lowered in, but these openings should not be left open longer than absolutely necessary.

The path of the gases through the regenerators is changed about every 20 minutes to keep the crucibles from eating away on the hot side. In about an hour the melter looks into the pots to see if the melting has begun, using a pair of blue glasses framed in a small board to protect his eyes. His experience enables him to make such other inspections as will keep him in thorough touch with the progress of melting. In from 3 to 6 hours the charge is thoroughly melted. It is slightly stirred with an iron rod to lift any lumps of muck bar possibly at the bottom, and to mix the charge thoroughly. The seal of the lid on the crucible is broken by such inspections, but a new joint is soon made by the heat of the furnace.

After becoming molten, the temperature of the metal will increase, and soon the occluded gases begin to boil out. This step lasts about 20 minutes and is called "killing" or "melting to a dead heat." It requires skill to determine when this has gone far enough. At the proper time, the crucible is lifted from the furnace and is set into a holder shown in Fig. 31 to be poured directly into a small mould, or into a hot ladle in which the contents of several crucibles are assembled if a large mould is to be poured.

If a crucible is taken from the furnace before the metal is "dead" it will pour "fiery," throwing off sparks and showing some agitation due to the escape of gasses, but if kept too long in the furnace, the metal will pour quietly, and the moulded ingots will be solid, but will be brittle and weak. The cause of this is uncertain, but is possibly to the absorption of an excess of silicon from the crucible walls at a very high heat.

In pouring from the crucible into the mould, which is washed inside with lime-wash to prevent the steel from sticking, great care must be exercised to keep the metal from striking the side of the mould as this would chill a film of it and cause a lamination in the ingot.

Fig. 31. - Ladle Shanks.

It requires a large crucible plant, expert skill and quick handling to assemble enough crucible steel to make successfully a large casting. The Krupp works in Germany make crucible-steel ingots large enough for high-powered guns and armor plate.