In America, crucibles are made of a mixture of half graphite and half fire clay, carefully kneaded, and moulded compactly by hand to shape as shown in Fig. 29. After they are moulded, they are allowed to dry for several days or weeks, and those not cracked or distorted in drying are burned in a kiln.

Fig. 29.

The clay crucible is used in England, but is not so strong when hot nor so durable as the clay and graphite mixture. However, for low-carbon and alloy steels, a graphite crucible must have an inside lining of clay to keep the graphite from being absorbed by the molten steel.

A clay crucible lasts only through the two or three heats of a single day, while the graphite crucible, more expensive, lasts for ten or twelve heats and is not so easily broken in handling. The bottoms of discarded crucibles are sawed off to be used as lids for new crucibles.