A pot in common use for a variety of chemical purposes. It is generally made of clay, and is designed to withstand a strong heat. The best crucibles for this purpose are the Hessian crucibles, composed, according to Pott, of a mixture of very refractory clay with sand. The vessels are not turned upon a potter's wheel, but the earth is kneaded into a very stiff" mass, and the form given by ramming it in an iron mould. A composition consisting of two parts of Stourbridge clay, and one part of the hardest coke, well ground and tempered together, has been employed with excellent results by Mr. Anstey, of Somers Town. The following description of his process is an extract from his account published in the Transactions of the Society of Arts: -
"Take two parts of fine ground raw Stourbridge clay, and one part of the hardest gas coke, previously pulverized, and sifted through a sieve of one-eighth of an inch mesh. Mix the ingredients together with the proper quantity of water, and tread the mass well (if the coke is ground fine the pots are very apt to crack). The pot is moulded by hand on a wooden block, as shown in the engraving in the next page, in which a is the bench; b b two uprights supporting a cross-board c; d the wooden block on which the pots are moulded, supported on a spindle e which turns in a hole in the bench; fa gauge to regulate the thickness of the melting pot, as shown in the dotted lines; g a cap of linen or cotton, placed wet on the core before the clay is put on - its use is to prevent the clay from sticking partially to the core while it is taking off; the cap adheres to the pot only while wet, and may be renewed without trouble or hazard (to the pot) when dry; h a wooden bat to assist in moulding the pot; when moulded, they are carefully dried at a gentle heat.
A pot dried as above, when wanted for use, is first warmed by the fire-side, and is then laid in the furnace with the mouth downwards (the red cokes being previously damped with cold ones in order to lessen the heat); more coke is then thrown in till the pot is covered, and it is then brought up gradually to a red heat. The pot is then turned and fixed in a proper position in the furnace, without being allowed to cool, and is then charged with cold iron, so that the metal, when melted, shall have its surface a little below the mouth of the pot. The iron is melted in about an hour and a half, and no flux or addition of any kind is made use of. A pot will last for fourteen or even eighteen successive meltings, provided it is not allowed to cool in the intervals; but if it cools, it probably cracks. These pots will bear a greater heat than others without softening, and will, consequently, deliver the metal in a more fluid state than the best Birmingham pots will." Crucibles are also sometimes made of porcelain, plumbago, iron, silver, and platina.