Crucibles, Plumba′go

Syn. Graphite c. , Blacklead c. , Blue pots . From graphite, ground and sifted, mixed with sufficient refractory clay to render it plastic. They are shaped by hand on an ordinary potter’s wheel, or by moulds of metal like that figured above under the head of Crucibles, Earthen . Prop., . Good blacklead crucibles, even when of the largest size, support the greatest and most sudden alternations of temperature without cracking, and may be used after repeated heating and cooling. Their surface, within as well as without, may be made very smooth, so that particles of melted metal will not hang about the sides. They are now almost universally used for melting the precious metals.