Cracked Crucible
Crucibles are sometimes cracked from too much heat when the gas is first lighted. When the gas is turned on full at first, the metal in the bottom of the pot is melted before the metal in the upper part gets very hot. This metal in the bottom expands and powerful pressure is exerted on the walls of the well and lower part of the crucible. To eliminate this danger the gas should be turned on about half force for about twenty minutes, or until the metal becomes warm and expanded, after which the gas may be turned on full force.
Ordinarily it takes about one hour and a half to melt the metal and have it ready for use. The small cracks which are made in the bottom of the crucible when first heating the pot will usually be closed by the heat of the burner after the pressure is released.
To remove an old crucible: Dip out as much metal as possible, turn out the fire, remove the four pot jacket cover screws, take off the cover, dig out part of the old asbestos, release the screw that clamps the left-hand crucible lug to the jacket, lift out the crucible, clean all of the old asbestos out of the inside of the jacket.
To put in a new crucible: The pot jacket should be lined with the asbestos about an inch thick on the inside, except in the front part where the burners are located. Place the crucible in position. Care should be taken to see that it fits firmly in its proper position. Pack the asbestos around the crucible, tamping it down with a stick, and filling in all the spaces. Cover the well of the crucible with a rag while packing, so that none of the asbestos will get into the well. On completion of the packing, place the pot jacket cover in position, and fasten it down firmly with the four pot jacket screws, which extend down through the cover, into the pot jacket.