When large pieces of earthenware of any special design are required, recourse must be had to a pottery. Small vessels, plates, parts of machines, etc., can often be made in the laboratory in less time than it would take to explain to the potter what is required. For this purpose any good pipeclay may be employed. I have used a white pipe-clay dug up in the laboratory garden with complete success.
The clay should be kneaded with water and squeezed through a cloth to separate grit. It is then mixed with its own volume or thereabouts of powdered porcelain evaporating basins, broken basins being kept for this purpose. The smoothness of the resulting earthenware will depend on the fineness to which the porcelain fragments have been reduced. I have found that fragments passing a sieve of sixty threads to the inch run, do very well, though the resulting earthenware is decidedly rough.
The porcelain and clay being thoroughly incorporated by kneading, the articles are moulded, it being borne in mind that they will contract somewhat on firing. [Footnote: The contraction depends on the temperature attained as well as on the time. An allowance of one part in twelve will be suitable in the case considered.] The clay should be as stiff as is convenient to work, and after moulding must be allowed to get thoroughly dry by standing in an airy place; the drying must not be forced, especially at first, or the clay will crack.
Small articles are readily fired in a Fletcher's crucible furnace supplied with a gas blow-pipe; the furnace is heated gradually to begin with. When a dull red heat is attained, the full power of the blast may be turned on, and the furnace kept at its maximum temperature for three or four hours at least, though on an emergency shorter periods may be made to do.
The articles are supported on a bed of white sand; after firing, the crucible furnace must be allowed to cool slowly. It must be remembered that the furnace walls will get hot externally after the first few hours, consequently the furnace must be supported on bricks, to protect the bench.
The pottery when cold may be dressed on a grindstone if necessary. This amateur pottery will be found of service in making small fittings for switch-boards, commutators, and in electrical work generally.
Pottery made as described is very hard and strong, the hardness and strength depending in a great degree on the proportion of powdered porcelain added to the clay, as well, of course, as on the quality of both of these materials.
It is a good plan to knead a considerable quantity of the mixture, which may then be placed in a well-covered jar, and kept damp by the addition of a little water.
Pottery thus made does not require to be glazed, but, of course, a glaze can be obtained by any of the methods described in works on pottery manufacture. The following glaze has been recommended to me by a very competent potter:—
| Litharge | 7 parts by weight |
| Ground flint | 2 parts by weight |
| Cornish stone or felspar | 1 parts by weight |