Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/667

Rh P T P T 643 repara- 2. Washing and Grinding the Materials. The Dorset or Poole on of clay is finely ground between mill-stones, mixed with water to the laterials. consistency of cream or slip, and then passed through fine silk sieves to strain out all grit or palpable particles.- The china stone is treated in the same way, with the additional precaution of wash ing away all the flakes of mica, which come from the decomposition of the granitic rock from which the china stone is derived. The silica is obtained from flints, which are easily ground to fine powder after being heated red-hot and thrown into water. These three substances, brought into the state of fluid slip, are repeatedly pumped up from vats and passed through the sieves ; they are then easily mixed in due proportion by being pumped into graduated vats. The water is next evaporated from the fluid mixture in large boilers heated by a complicated arrangement of flues, and the com pound is left in a soft pasty state, full of air-bubbles, which have to be got rid of by constantly turning over and beating the paste till it is quite smooth and compact, and sufficiently plastic to be thrown on the wheel. Coloured earthenware, such as that Wedg wood used to make, was prepared by the addition of various sub stances to the fluid slip. A black coloiir was given by protoxide of iron and manganese, red by red ochres or red oxide of iron, blue by oxide of cobalt, and green by protoxide of chrome. These coloured pastes are but little used now. hrow- 3. Throwing and Moulding. After sufficient kneading, the clay ig and is made up into balls of a convenient size for the thrower to mould loukl- into shape upon his wheel. The methods both of throwing and of ig. moulding are the same for porcelain as for pottery (see p. 638 above). Unfortunately in England, as at Sevres, the thrown vessels are usually finished on the lathe ; only the commonest kinds of ware escape this process, which takes away all life and spirit from the wheel -formed pottery. Consequently it is the cheapest and com monest wares that now, as a rule, have most natural beauty of form and really artistic spirit. Handles and other parts which are shaped in piece-moulds are either cast by pouring fluid slip into the plaster- moulds, or are formed by pressing and dabbing thinly-rolled pieces of soft clay into moulds made in two parts. The moulded halves of the spout or handle are fastened together while still wet, and the edges at the junction pared down and trimmed with a modelling tool. Plates, basins, and the like are formed by the combination of a mould and a shaped gauge as described above (Sevres). ilns 4. Kilns and Firing. After the vessel with its moulded handles id or spout is thoroughly dry it is ready for the first firing. The ring. usual Staffordshire biscuit-kiln is a circular building, about 18 feet in internal diameter at the base, narrowing towards the top. It is about 18 to 20 feet high, and is very carefully built of refractory fire-bricks, strengthened by rings of wrought-iron which clasp the outside. It is surrounded by eight to ten furnace openings, with flues arranged to distribute the heat equally throughout the kiln. The pottery is fired in drum-shaped &quot;saggers&quot; or boxes, made of fire clay, which are piled one above the other, as in the case of the Sevres porcelain. The fire is kept up from thirty to fifty hours, and is then allowed to die out. Several days are allowed for cooling before the kiln is opened and the saggers with their contents withdrawn. lazes. 5. Glazes. The composition of glazes for pottery varies very much according to the custom of each manufacturer. For the most part they are transparent silicates of alumina, rendered fusible by oxide of lead ; this compound is made by a mixture of Cornish china stone, flint, and white lead. The best quality of glazes have borax and some alkali added as a flux, in which case the proportion of lead is reduced. Those glazes that contain much lead are easily scratched, and can be decomposed by many acids ; thus there is always a risk of lead-poisoning if vessels coated in this way are used for cooking purposes. . The materials for the glaze are finely ground with water and made into a thin white fluid. The biscuit pottery is rapidly dipped into vats of the milky mixture, and suffi cient to form the glaze adheres to the absorbent clay in an even coating all over the surface. After being dried the potter} is ready for the second firing in the glazing kiln, which is very similar in construction to the biscuit-kiln, only, as a rule, rather smaller. It is packed in clay saggers, as in the first firing, but a stronger heat is required to fuse the finer kinds of glaze than was necessary for the baking of the raw pottery. Salt -glazing has been described above (p. 632), and is only used for the coarser sorts of ware. ecora- 6. Methods of Decoration. In the case of pottery decoration is 3n - usually applied on the biscuit-ware before it is glazed by the trans fer-printing process. The required design is engraved on copper plates ; the pigment is ground fine and mixed with a tenacious compound of oil and gums. An ordinary rolling press is used to print the engraved patterns in the oily pigment upon strips of tissue-paper, which are carefully applied and pressed face down wards on the biscuit -ware while the oil is yet wet; and so the pattern is transferred to the absorbent clay. This requires great dexterity from the difficulty there is in fitting the printed strips neatly on to curved surfaces. The paper is then washed off, and the printed ware is baked at a moderate temperature in what is called the &quot;hardening&quot; kiln, which is done before the glaze is applied, in order to drive off the oily medium with which the pig ment was mixed. The transfer process is quite fatal to all artistic beauty in the designs ; it is hard, clumsy, and mechanical, the very opposite of a rational method for the decoration of pottery, which above all things demands freedom of hand and a spirited touch. Tainted decoration which is executed by hand is now usually applied over the glaze, both because it is easier to do, not requiring so certain a touch, and also because the soft subdued colours of the under-glaze pigments do not suit the modern taste for what is bright and showy. The pigments used are necessarily oxides and salts of metals which will stand the heat of the kiln. Only those few which can stand the very high temperature of the glazing kiln can be used under the glaze. The over-glaze colours, on the other hand, only need sufficient heat to fix them on the surface of the already fired glaze ; and this is often done in a very slight and imperfect way. These colours not only lose in effect from want of the softening vitreous coat through which under-glaze colours are seen, but they are also very inferior through being unprotected, and therefore easily injured by scratches and ordinary wear. In old times the value of a protecting coat of glaze was so strongly felt that even paintings on enamel, like those on Persian pottery and Italian majolica, usually had a thin vitreous glaze added over the smooth enamel, with the double object of protecting the paint ing and increasing its soft richness of effect. The discoveries of modern chemistry have added very greatly to Pig- tile number of metallic salts which are available for the decoration ments. of pottery. Almost every possible tint can be produced for over- glaze painting. Oxides of cobalt are used for various shades of blue and grey up to black ; antimony, usually combined with lead, gives yellow ; oxides of copper give deep red or brilliant blues and greens according to the proportion of oxygen that they con tain ; oxide of chromium gives a good quiet green ; manganese gives violet and even black ; gold gives a fine ruby red ; and uranium a rich orange. The various oxides of iron give a great variety of colours, reds, yellows, and browns. Oxide of zinc is largely used, not as a pigment in itself, but in combination, to modify other colours. The oxides of iron, cobalt, and chromium give very stable colours, capable of bearing a very high temper ature, and can therefore be used for under-glaze painting ; most of the others can only be employed for over-glaze work. Over-glaze pigments cannot be used alone, but require a flux to make them combine with the glaze. Oxide of lead, borax, nitre, carbonates of potash or soda, and other substances are used for this purpose. Literature. English porcelain : Nightingale, Early English Porcelain, 1881 ; Binns, Potting in Worcester, 2d. ed., 1877 ; Owen, Ceramic Art in Bristol, &c., 1873; Porter, Manufacture of Porcelain, &c., 1832; Tiffin, Chronograph of Bow, Chelsea, and Derby Porcelain,^ 1875 ; Wallis and Bemrose, Pottery and Porce lain of Derbyshire, 1S70 ; Jewitt, Ceramic Art of Great Britain, 1877, the most complete and comprehensive work. Museums illustrative of the General History of Pottery. The Musee Ceramique Mu- of Sevres is the best and most complete for the student of all kinds of pottery, seuins In England the Jermyn Street Museum of Geology has a small but very widely-illustrative collection of pottery and the various materials used in its manufacture. The South Kensington Museum is rich in almost all kinds of mediaeval and modern pottery, but it is unfortunately very badly arranged. The British Museum possesses a very large number of specimens of prehistoric pottery, Greek vases, mediaeval English, and very choice examples of Persian and majolica wares. The other chief European museums are mostly eacli rich in some special department of ceramic art. General Literature on Pottery. Brongniart, Traite des Arts Ceramiques, 1S54 ; Jacquemart, Histoire de la Ceramique, 1873 ; Marryat, Pottery and Porcelain, 3d ed., 18(58 ; Birch, Ancient Pottery, 1873 ; Catalogue of Pottery, Museum of Geo logy, Jermyn Street, 1876 (a very useful and well-illustrated work) ; Bonneville and Jaunez, Les Arts Ceramiques, 1873 ; Brongniart and Riocreux, Musee de Sevres, 1845 ; Figuier, I^es Merveilles de I Industrie, 1873-75 ; Greslou, Eecherches sur lo Ceramique, 1864 ; Guillery, Les Arts Ceramiques, 1854 ; Jacquemart, Les Mer veilles de la Ceramique, 1866-69 ; Magnier, Manuel du Porcelainier, Fa iencier, &c., 1864 ; Mareschal, Les Faiences And ennes et Modernes, 1867 ; Maze, Recherches sur la Ceramique, 1870 ; Ris-Paquot, Histoire de la Faience Ancienne, 1873-74 ; Salvetat, Lemons de Ceramique, 1857 ; Waring, Ceramic Art in Remote Ages, 1875 ; Ziegler, Etudes Ceramiques, 1850 ; Jiinnicke, Grundriss der Keramik, Stuttgart, 1879 ; Champfleury, BibliograrJiie Ceramique, Paris, 1881 (gives a very large and complete list of works on pottery and porcelain, in all languages) ; Soden Smith, List of Works on Pottery and Porcelain in the Kensington Art Library, 1875. On Pottery and Porcelain Marks the reader may consult Earth, Porzellan- Marken und Monogramme, 1873 ; Chaffers, Handbook of Marks an-d Monograms, 1874 ; Deinmin, Guide de V Amateur de Faiences et Porcelaines, 1873 ; Graesse, Guide de I Amateur de Porcelaine, &c., 1873 ; Palliser, The China Collector s Com panion, 1874-75 ; Ris-Paquot, Dictionnaire des Marques et Monogrammes, 1874 ; Hooper and Phillips, Manual of Marks, 1876 ; Bowes, Japanese Marks, 1882. Many other lists of ceramic marks occur also in the various works mentioned under previous heads. (J. H. M.) POTTSTOWN, a borough of Montgomery county, Pennsylvania, United States, is picturesquely situated on the Schuylkill river, in a plain surrounded by hills. It is 18 miles east -south -east of Reading and 40 miles west- north-west of Philadelphia, at the junction of the Phil adelphia and Reading (main line) and the Colebrookdale Railroads, and has communication also by the Schuylkill Valley branch of the Pennsylvania Railroad. There are