Pottery

to a glass at a lower temperature than that required to burn the body. Much kitchen ware is of this type.

Majolica and Fayence.-These two terms are often rather loosely used. The former includes wares with a colored clay body, of porous character, which is covered by an opaque enamel consisting of an artificial mixture whose melting point is at the same temperature as that required to burn the body. The latter has also a colored clay body, but is covered with a clear glaze, through which the decoration shows. Both terms are of Italian derivation.

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Stoneware.-Pottery hard vitrified body, usually of bluish color, but never white. It is covered with an opaque or transparent glaze of natural or artificial character, and may be further decorated either by rehef or incised designs. Common jugs, crocks, and many cooking utensils belong to this group.

Whiteware or White Earthenware. This includes those products having a white, or nearly white, porous body, usually covered with a glaze, There are several trade varieties known as C. C. ware, white graniteware or iron-stone china, semi-vitreous ware, semi-porcelain, and china. Some of these differ in name only. Theoretically they differ in the whiteness and degree of vitrification of the body, the C. C. ware having the poorest color, and greatest porosity. The body, which is too hard to be scratched with a knife, is covered with the glaze (an artificial mixture), and the decoration is applied either over or under the latter.

Porcelain.-This is a white vitrified ware, made of a mixture of the best grades of clay obtainable, ground quartz, and feldspar or calcined bone. It is glazed with an artificial mixture, of higher melting point than the whiteware glaze. In spar porcelain the body is fluxed with feldspar and shows a bluish color by transmitted light, while in boneporcelain the fluxing material is calcined bone, and the body shows a yellowish tint. Many ornamental wares, and the best grades of table ware are to be grouped here.

Belleck or egg-shell porcelain is a porcelain of extreme thinness, fluxed with a high percentage of feldspar, and named after the locality, Belleck, Ireland, where it was originally made.

Parian ware, so-called from its resemblance to Parian marble, is an unglazed porcelain.

Raw Materials.-There is considerable difference in the character of the clays used for the different grades of pottery, the

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lowest being usually made from one clay, while the higher ones are composed of a mixture of clays to which other ground mineral substances are added, for the purpose of improving the body. The characters of the raw materials used are briefly as follows:

Common Earthenware Clays. These are usually impure surface clays, free from grit and of sufficient plasticity to permit their being turned on a wheel. If too sandy, they can be improved by washing. Such clays burn to a red or cream-colored body at a low temperature. Earthenware clays are widely distributed.

Stoneware Clays.-Stoneware is usually made from a refractory or semi-refractory clay, but many small potteries use a lower grade. It is highly plastic, of good bonding power, free from grit, and must hold its shape in burning at such a temperature as is required to melt the glaze used. Most stoneware is now made from a mixture of clays so as to develop a body of the proper quality both before and after burning. An Akron, O., clay analyzed: SiO2, 64.26; Al2O3, 22.95; Fe2O3, 1.28; CaO, .45; MgO, .37; K2O, 1.96; H2O, 6.74; moisture, 2.05. Large quantities are obtained in Ohio, New Jersey, and Indiana.

Yellow and Rockingham Ware Clays are of semi-refractory character and good plasticity, but present no peculiarities otherwise.

Whiteware and Porcelain Clays. Two distinct types of clay are employed-viz., kaolin and ball clay. The kaolin is a white-burning clay, of residual character and high refractoriness, but is usually deficient in plasticity. Most kaolins require washing before shipment to market. The ball clay, which is also of a white-burning character, is of sedimentary origin, highly plastic, and may even burn to a vitrified body at Seger cone 9. It therefore serves as a bonding element, but cannot replace the kaolin, as it does not burn to as white a color. The two as marketed do not differ greatly chemically, as the following analyses will show:

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from deposits along the Appalachians a great deal is imported from England, and is often superior to the domestic material. Ball clay is found in Tennessee, Florida, and New Jersey, but large quantities are also brought from England.

In addition to these, feldspar or calcined bone is added to the body for fluxing purposes, and Cornish stone (a partly decomposed granite) from Cornwall, Eng., serves a similar use. Quartz is added to counteract the shrinkage of the body in burning.

Glaze or Slip Clays are those containing a high percentage of fluxing impurities and of such texture that at a low cone it melts to a greenish or brown glass, thus forming a natural glaze. It must be fine-grained, free from lumps, and melt at about cone 5.

That dug near Albany, N. Y., forms a brown glaze and has been used by stoneware potters in all parts of the United States. Its analysis is: SiO2, 55.60; Al2O3, 14.80; Fe2O3, 5.80; CaO, 5.70; Mgo, 2.48; K2O, 3.23; Na2O, 1.07; MnO, .14; P2O5, .15; H2O, 5.18; CO2 and moisture, 4.94. The use of slip clays is decreasing each year, because an artificial glaze is now usually preferred.

Methods of Manufacture.The steps common to all grades of ware are: preparation, tempering, moulding, drying, and burning. The care of preparation increases as the grade of the ware rises, and in glazed wares a second burning is also necessary if the body has to be burned before the glaze is applied.

Preparation. For the lower grades such as common earthenware or even stoneware the clay is used as mined, but for the higher ones washing is frequently necessary in order to remove sandy impurities.

In using a mixture of materials, as for whiteware or porcelain, these are commonly mixed with water in a vat with revolving stirrers, termed a blunger. From this the clay is run through a screen to remove coarse grains, under magnets to separate iron particles, and then into a filter press to squeeze out the water.

The tempering or kneading of the clay is now commonly performed in a pugmill, consisting of a vertical box containing a revolving shaft provided with knives. Although this kneads and mixes the mass quite thoroughly, it receives further kneading by hand (termed wedging).

The properly tempered clay may be moulded by one of the following methods:

Turning. This is done by the potter taking a lump of tempered clay, placing it on a rapidly revolving disc (potters' wheel),

Plates and saucers are made by a modification of this method, with the aid of a plaster form, whose surface has the shape as the interior or upper surface of the plate to be formed. A lump of tempered clay is then pounded into a flat cake, and laid on the mold. As the latter revolves the potter shapes the other side or bottom of the plate by pressing a wooden template of the proper profile against it.

The

Pressing is a method required for making ewers and vessels of oval or elliptical section. mold used is a sectional one, whose inner surface conforms to the outer surface of the object to be molded. A slab of clay is laid in each section, carefully pressed in, the mold put together and all seams smoothed with a wet sponge. In drying the ware shrinks away from the mold and is easily removed. Clock-cases, lamps, water-pitchers, etc., are made in this manner.

Casting consists in pouring a clay slip into a plaster mold, which absorbs some of the water and causes a thin layer of the ciay to adhere to the interior surface of the mold. When this layer is sufficiently thick the mold is inverted and the remaining slip poured out. The mold is removed in a few hours. This method is extensively employed for making thin-walled objects of porcelain or even whiteware. It is more often practised in Europe than in the United States.

Drying has usually to be carried on with care, especially if the ware is of complicated shape. The ware commonly receives a preliminary drying in an open room, and is then removed to the heated green wareroom.

Subsequent Steps.-The method of treatment up to this point is similar for nearly all grades, except for the blunging of white

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ware and porcelain mixtures. Common earthenware, such as flower pots, is generally burned at a low heat, the burning being done in rectangular or circular up-draft kilns. The completion of the burning is often judged by Seger cones. These are testpieces composed of feldspar, clay, lime, and silica, the lower ones containing also iron and boric acid. The cones are placed in the kiln where they can be watched through a peep-hole, and as each one reaches its melting point it bends over. There is a graded series ranging from cone 022 up to cone 39. Cone 05, for example, melts at 1050°c., cone 1 at 1150° c., and cone 9 at 1210° C. Common earthenware is burned at about the melting point of cone 010. Decoration can be done by incised designs, relief work, or by the application of an easily fusible glaze.

Yellow and Rockingham ware mixtures are first burned to develop the body, which is glazed and then fired a second time to develop it. The ware has to be placed in saggers to protect it from the flames and dirt of the fuel. These protective receptacles are oval or cylindrical in shape, with a flat bottom; they are made of fire clay. The saggers filled with unburned ware are set one on top of the other, so that the bottom of one forms a cover for the one below it, the joint between the two being closed by a strip of 'wad-clay.'

Yellow and Rockingham ware glazes are artificial mixtures, melting at a lower heat than is required for the body, so that while the first or biscuit' burn may range from cones 2 to 8 the second or glost burn is as low as from cones 07 to 03.

In stoneware the body and glaze are developed in one firing, and several types of glaze may be used. The first is a natural glaze or slip clay which melts to a brown glass at a temperature at which the ware is nearly vitrified. A second is the salt-glaze, applied more often to sewer-pipe than to stoneware. For this the wares are placed unprotected in the kiln, and as soon as the proper heat is reached, salt is put into the fires. The vapors passing through the kiln give up their sodium, which fluxing with the silica of the clay form a glaze on the surface of the ware. All clays will not take a salt glaze.

A third type are Bristol glazes, artificial mixtures of fluxes, kaolins, ball clay, and flint; they can be produced in a variety of colors, but white due to tin or zinc is a common one, and much used

now.

Stoneware is burned in either up-draft or down-draft kilns at

Pottery

from cone 6 to 8 if fire clays are used.

For burning whiteware and porcelain, the product is always placed in saggers; the kilns are usually of the circular up-draft type having a diameter of from 10-18 feet. Down-draft kilns are much less used for burning in the United States than they are in Europe.

Whiteware is commonly burned first at cone 8 to 9, then glazed and burned in the glost kiln at cones 2 to 6.

Porcelain usually receives its biscuit burn at cone 2, and the glost burn probably at cones 11 to 13. Foreign porcelains are often burned at a still higher cone.

Since the color of ferrous iron is less noticeable than ferric iron the fires should be reducing, during at least a part of the burning, and the cooling is then done as rapidly as possible to prevent the iron from oxydizing. It is also usual to counteract the yellowish tone imparted to the ware by iron, by adding some cobalt compound to the mixture.

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The glazes for whiteware and porcelain are complex pounds of an artificial character, being a mixture of acids and bases combined according to a definite formula, and in such proportions that they will melt to a glass at the temperature reached in burning. A glaze thus produced must agree with the body in its shrinkage and coefficient of expansion, in order to prevent such defects as crazing, shivering, peeling, etc. Whiteware glazes which commonly owe their easy fusibility to borax and lead, are usually fritted first. That is, the ingredients of the glaze after mixing are melted either in a frit kiln or a sagger, broken up and ground wet, together with certain added materials. Porcelain glazes contain no lead, and require, therefore, a higher heat for melting.

The ground glaze is mixed with water to a cream-like consistency, and the biscuit ware is dipped into it. Various metallic oxides are used as coloring agents in the glaze or other decoration, for color effects. Thus cobalt for blue, chromium for green, chromate of iron for brown, etc. A good glaze should have a smooth bright surface, and be free from cracks, bubbles, pinholes, etc.

Whiteware and porcelain are often elaborately decorated, either under or over the glaze, but the style of decoration most often seen is printwork. This is done by printing a copper plate design on special paper, and applying this to the surface of the ware. After being allowed to

Pottery

stand for a few hours the paper is washed off, but the ink of the design is retained on the surface of the ware. The colors are then fixed by firing in a muffle kiln at a dull red heat. The printwork

is sometimes filled in 'and elaborated by brushwork, or, on better grades of ware the entire design may be hand-painted. The more delicate colors as well as gold have to be applied over the glaze, as they would be destroyed by the firing necessary to mature the latter.

History.-Probably the earliest home of the ceramic art is to be sought for in Egypt, the manufacture of porcelain being known to have existed there as far back as the 16th century B.C.

The use and development of the art in Assyria probably synchronizes with its rise and development in Babylon and Phoenicia, and the very beautiful results which have rendered Greek pottery famous are probably a heritage from these earlier times. The history of the art in Greece shows a remarkable and rapid development from the crude productions of prehistoric times to the exquisite beauty of the work of the 4th century B.C., and what is commonly known as Etruscan pottery was in reality Greek. The practise of the art in Roman times led to no extraordinary development, although the manufacture of Samian or Aretine ware (so-called from Arezzo, the home of the industry), in the early part of the 1st century, spread into Germany and Gaul, and later into Britain. In the 16th and 17th centuries the art flourished in its most perfect state in Italy, France, and Germany, and the delft ware of the Dutch is world renowned.

In England the pottery industry did not reach a noteworthy state of development until the beginning of the last century, following which the work of Josiah Wedgewood attracted considerable attention.

Among the products of European countries several stand out prominently for their beauty and artistic merit. Thus we have the gray, blue, and white delft of Holland (now much imitated), the famous porcelain of Sevres and Limoges in France, the Dresden and Royal Berlin porcelain of Germany, and the Royal Copenhagen, and Rörstrand_porcelains of Denmark and Sweden respectively.

In the United States the pottery industry has advanced with phenomenal strides, but only in the last fifty years. Leaving the aboriginal and modern Indian pottery out of consideration, pottery was probably made in Virginia, previous even to 1649, and

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from that time on small potteries (chiefly for making earthenware) were started at different points. No serious attempts were made to produce whiteware, however, until 1791. Since then the industry has developed gradually in different parts of the United States until in 1905 the total value of the pottery manufactured in the country amounted to $27,918,894.

Stoneware for domestic purposes is manufactured in enormous quantities in Ohio and Indiana, largely from local materials which are so abundant in those two states. Yellow and Rockingham are also extensively made in Ohio. Most of the whiteware and porcelain produced in this country is for table and toilet purposes, and while there are not a few factories scattered over the states east of the Mississippi, the two great pottery centres are Trenton, N. J., and East Liverpool, Ohio. The history of the former dates back to 1852 and that of the latter to 1839, when small potteries were established at both localities. In 1905 these two centres produced $6,196,833 and $5,986,601 worth of pottery respectively. Although in the earlier years of their development the potters of these two towns devoted considerable time to the production of artistic ware, they have for the most part narrowed down to the making of table, toilet, and sanitary ware. Not a little Belleek porcelain is, however, made at Trenton.

Some attention is now given to the development of art-ware by a number of potteries. Of these the Rookwood Pottery of Cincinnati has achieved great success, and its ware has won an international reputation. The original Rookwood consisted of a stoneware body, decorated with slip glaze colors, chiefly shades of brown and green. While this is still made, their latest production, I which has attracted much attention, is the 'velum' ware, in which lighter hand-painted decorations are covered with a transparent matte glaze. This has been conceded to mark a notable achievement in ceramic art.

Many art potteries have recently been specializing in the development of opaque matte glazes of green, blue, and other colors, such as are seen in the Grueby, Teco, and Van Briggle ware.

Quite decorative and artistic in their character also are the Aurelian, Louwelsa, Eocean, and Sicardo wares of the Weller pottery at Zanesville, O.; likewise the copper red Rozane pottery of the Roseville pottery.

Crystalline glazes, productive of such beautiful effects, and which attracted so much admira

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tion for the French, German, Danish, and Swedish exhibits at the Paris Exposition in 1900, have been successfully produced by not a few American potters.

It will be seen from what has been said above that while the manufacture of pottery has been greatly increased and the product improved in an incredibly short period of time, that high grade art-wares are not yet produced to the same extent as they are abroad, where the industry is much older, and generation after generation of laborers and artists have been trained up to the work. It is not a case of lack of raw materials.

Bibliography.-W. C. Prime's Pottery and Porcelain of all Times and Nations (1879); E. A. Barber's Pottery and Porcelain of the United States (1893); E. Bourry's Treatise on Ceramic Industries (1901); H. Riès's Clays, Their Occurrence, Properties, and Uses (1906); K. Langenbeck's The Chemistry of Pottery (1895); W. P. Jervis's Encyclopedia of Ceramics (1905); id., Foreign and Domestic Pottery Marks (1898); Transactions of American Ceramic Society.

Potting. Snft-wnnded plants of rapid growth, such as coleus, geraniums, fuchsias, and begonias, thrive most satisfactorily when the soil is loose rather than hard about the roots. Ferns should have it moderately firm; and hard-wooded stock, azaleas, ericas, acacias, and the like, should be potted firmly. In repotting plants, more especially those of slow growth, the ball of soil and roots should never be sunk to any great extent below the original level, and it is always preferable to pot a plant twice, or even three times, rather than place it in a pot too large.

Potto, a West-African name for the galago lemurs (Galagina), one of which is especially known as the asigwantibo. They bear the general characteristics and habits of lemurs. The same name is given to certain more or less closely related animals in the Malayan region.

Pottstown, bor., Montgomery co., Pa., 35 m. N.w. of Philadelphia, on the Schuylkill R., and on the Phila. and Read., and the Pa. R. Rs. It has important manufactures, those of iron and steel being especially large. Other products are agricultural implements, silk, and hosiery. The census of manufactures in 1905 returned 77 industrial establishments, with $6,419,713 capital, and products valued at $8,144,723. The place was settled in 1752 and incorporated as a borough in 1815. The limits were extended in 1888. Pop. (1900) 13,696.