When required for use, it is again passed through the pug-mill, and the blocks are kneaded and weighed, preparatm to working up in the machine represented in Fig. 1134, which is used in the manufacture of open crucibles for metallurgical operationa. A heap of prepared clay is weighed, and inserted in a plabter mould o, which rests in, and is caused to revolve by, an iron cup p, attached to a spindle, to which, motion is comnaunicated from beneath. A gimlet-shaped tool a, fitted to a block b, can be depressed into the clay by means of a horizontal frame, balanced by weights K. The black b, together with the tool a, can be moved horizontally in the frames by means of a handle g and threaded rod. The frame can be maintained in any desired position by a catch n. When the frame is fixed, and the mould is caused to rotate, the tool a, by turning the handle g, is moved horizontally, and spreads the clay against the wall of the mould ; by this means, the form of the interior of the vessel is given by the tool, whilst that of the exterior is pro duced by pressure against the internal surface of the mould. By varying the forma of the tool and mould, variously shaped vessels may be produced. When the vessel has been fashioned, and the motion checked, the tool is moved into the centre of the vessel by turning the handle g, the frame with the tool is raised, and the mould with the vessel inside it is removed to a drying-room by means of a suitably-constructed crane. The crucibles are burnt in Baggers or muffles. In order to prevent absorption of moisture and dirt during storage, they are often coated with a waterproof paint, or with an enamel which is permanently fixed by firing.
Stone-ware.—There are two very distinct species of stone-ware, the type of the one being an ordinary glazed drain-pipe, whilst the type of the other is a vase of decorated Doulton ware. The materials and treatment of both are similar, although not identical ; and both types may generally be seen in course of manufacture at the same works. Stone-ware is always dense, refractory, and opaque ; the finer qualities resist corrosion by acids, and extreme changes of tempe rature, and, in some eases, are semi-vitreous, and capable of receiving coloured decoration. The basis of all stone-ware is the grey-coloured ball-clay from Dorsetshire and Devonshire, and espe cially those qualities containing a considerable proportion of sand. For common ware, a mixture im made of the ground ball-clay, with the pow der of burnt broken goods ; for fino and deco rative purposes, a supe rior quality of the ball clay is mixed with sand or flint and Cornish stone. The colour of the ordinary stone-ware, after burning, is buff passing into brown ; whereas that of superior stone-ware is almost white. The ma jority of stone-ware is glazed by the indirect reaction of the vapour of sodio ohloride with the constituents of the sur face of the ware. The exceptions are Bristol ware, glazed with a mix tura of felspar, borax, and plumbic oxide ; oer tain common goods, which are glazed with mixtures of the oxides or enlphides of lead and iron, or with the oxide of manganese ; wares glazed over by means of a " smear " (compare De coration) ; and Wedg wood's jasper ware, which is vitreous, and possesses a naturally orystalline surface.
By the term " stone ware," salt-glazed ware is generally understood.
Salt-glazed stone-ware is fired for biscuit, glaze, and decoration, when deooration is applied, at one time. All forms of decoration must be ap plied to the ware before burying (see Decoration).
For a description of stone-ware kilns, see pp. 1565-6. The fuel generally used is coal, and the ware is exposed to the naked flame, vrithout any protection. The difficulty of preparing colours for stone-ware decoration, which are stable enough to withstand this ordeal, can readily be understood. The heat of a stone-ware kiln is intense, and it is cnstomary to burn terra-cotta, which is made from the same materials as stone-ware, on the roof or crown of the stone-ware kiln. Terra-cotta differs from stone-ware in its oondition of solidification, whioh is less perfect, on account of the comparatively low temperature to which it is exposed. The unbelted stone-ware, preparatory to being exposed to the sodie chloride vapour in a salt-glaze kiln, is dipped in a mixture of sand and water. After the ware has been arranged, the fires are raised gradually. The salt (sodie chloride) is not introduced until 2i-4 days from first lighting, when the ware has nearly attained its highest temperature. Salt is thrown into the kiln with shovels at the fire-places, and through openings in the crown arranged for the purpose. The total charge of salt for an average-sized kiln is about 2 cwt. When half the charge has been thrown in, the fires are increased for a time, specimens of the ware are then examined, and if the inspection be satisfactory, the residue is added. The openings in the crown are now closed, and the ware is left to cool for 4-6 days. The injection of the salt causes dense white fumes of salt vapour tainted with hydrochloric acid to issue from the cone of the kiln. At Doulton's works, the fumes from all the kilns are gathered into 9,nd discharged from a chimney some 300 ft. high. By this means, all real nuisance is obviated.
The theory of salt-glazing rests upon the decomposition of salt vapour by water vapour. As the salt is volatilized, it unites with the water vapour arising from the combustion of the fuel, to form hydrochloric acid and sodic hydrate ; the latter unites with the free silica in and on the surface of the ware, to form sodic silicate. The sodic silicate renders fusible a sn3all proportion of the aluminic silicate of the body of the ware, and unites with it to produce a glass or glaze built up of the sodic and aluminic silicates. This glaze answers in composition to the glaze of Chinese and Sevres porcelain; but it is more evenly spread, and, if possible, more thoroughly incorporated. If ferric oxide be present in the body of the ware, or if, as sometimes happens, red lead be introduced into the kiln with the salt, ferric and plumbic silicates will respectively be formed, and will contribute to the fusibility of the glaze. A pure clay body is less readily glazed by the salt-glaze process than one containing free silica, alkalies, and ferric oxide; by the latter, if an appreciable quantity be present, the glaze will be tinted buff or brown. The scorched appearance, which may sometimes be observed on pieces of stone-ware, is due to the reaction of the salt vapour being in some way accidentally interrupted.