Typically, a crucible is defined as a container in which metals are melted, usually at temperatures above 550 °C. These crucibles are typically made of graphite with clay as a binder material. These crucibles are extremely durable and resist temperatures to over 1650 °C. A crucible is lowered into a furnace and, after the melting; the liquid metal is removed from the furnace and slowly poured into the mold. Some old steel making furnaces (usually electrically powered) had an embedded crucible and was tilted as the metal was poured out.

Crucibles are also called pots, and used for melting small amounts of various materials, but more particularly for the manufacture of crucible general steel. For this industry crucibles are made of a high quality of clay mixed with a little powdered coke (clay crucible or white pot), or of a mixture of clay and graphite (graphite, plumbago, or blacklead crucible).

Graphite crucibles can be made to contain a heavier charge, and also last a greater number of heats. They are made of a mixture of Ceylon graphite, German clay and pure sand, the final composition being approximately: The clay is dried, ground, made into a paste with water, and the sand and graphite thoroughly mixed in, after which the mass is allowed to remain for a few days in a damp place to season or temper, i.e., be in a better condition for working. The amount of crucible material prepared at one time is called a batch. A lump of the proper size is cut off, kneaded slightly to insure its uniformity, and put inside a mold which is placed on a potter's wheel, and the mass spun up (by revolving the wheel) to fill the mold. The proper thickness of the wall is obtained by means of an arm or profile iron which descends and shapes the inside of the crucible. The excess at the top of the mold is sliced off and the mold removed. Spinning up gives better results than simple pressing because it causes the flakes or plates (in which natural graphite occurs) to take a tangential direction and intermesh, thereby binding the material together.

Artificial graphite is rarely if ever used, as it does not occur in these plates. The crucibles are now dried, first for about 24 hours, at about 20° to 25° C. (70° to 80° F.), after which they are smoothed up; and then for about three weeks at a temperature high enough to drive off the hygroscopic moisture. They are then heated (annealed or burned) in an oven (annealing oven) for about three days at a temperature of about 825° C. (2500° F.) to drive off all the combined water. The crucibles are stacked up in a number of tiers, and, as they are still very tender, they are placed in loose-fitting clay molds (seggars or saggars) which keep them from being crushed, and also prevent excessive oxidation. When crucibles of different sizes are being made they are usually nested, i.e., the smaller are placed inside the larger. The slight oxidation of carbon on the surface, which always occurs, gives the crucibles, originally black, a brownish color (the color of the clay). The covers are made and treated in a similar manner.

Clay crucibles were chiefly in England, and are manufactured from a high grade of fireclay (Burton, Stourbridge, etc.), usually mixed with about 5% of good ground coke. The mixing is done very carefully, frequently by treading the mass with the bare feet on the treading floor. A lump is then placed in a flask or mold, and a plunger having the shape of the interior is forced down, being centered by a pin passing through a hole in the bottom. The flask is removed and the top of the crucible forced inward, by means of another conical mold, to give it a shape like a barrel. After drying for a few days in the pot house (where they are made), the crucibles are further dried at a somewhat higher temperature near the flues of the melting furnaces. The hole left in the bottom is closed when the crucible is set in the furnace for use by throwing in a little sand which frits the crucible to the clay stand on which it rests.

These processes have not been used in the United States since before World War II. The modern general steel making processes is highly computerized and fields an army of robotic machinery.



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