512

FELSPARS-ALUMINOUS ROCKS.

39

The varieties of felspar (M,O, 3 SiÐ ̧”. Al1⁄2Ð ̧, 3 SiÐ ̧) are likewise double silicates of aluminum with potassium, sodium, lithium, or calcium. Potash-felspar, the adularia or orthoclase of mineralogists, and the petuntze of the Chinese potters, is sufficiently hard to scratch glass; it is used as a glaze in the manufacture of the finest kinds of porcelain. Felspar requires the most intense heat of the porcelain furnace for its fusion, when it forms a white milky glass. Soda-felspar, from its usual white colour, has received the name of albite. The felspar containing lithia constitutes petalite. Common, or potash-felspar, crystallizes in oblique rhombic prisms. Labradorite is a double silicate of aluminum, analogous to felspar, but it contains calcium instead of the alkaline metals: it crystallizes in doubly oblique prisms belonging to the sixth system. These minerals, by disintegration, yield the porcelain clay, or kaolin.

Felspar not only forms the regularly crystallized minerals just mentioned, but it occurs mingled with quartz and other crystallized minerals: it is indeed one of the most abundant constituents of many of the older rocks. Granite, for example, is a rock consisting of intermixed crystals of quartz, felspar, and mica. When it contains hornblende instead of mica, the term syenite is given 'to it. Gneiss contains the same components as granite, but it has a more stratified appearance, as the mica occurs more in layers. Porphyry consists chiefly of compact felspar, with crystals of felspar disseminated through it; it is often red or green, and takes a fine polish. Basalt is a dark-coloured volcanic rock, consisting of compact felspar containing crystals of augite. When the place of the felspathic constituent is supplied by labradorite (or lime-felspar), the basalt is called dolerite. Trap, or greenstone, is a very tough, compact, igneous rock, of a dark-greenish or brownish-black colour; it is composed of an intimate mixture of felspar and hornblende. If it contain soda-felspar (albite), the rock is known under the name of diorite. Trachyte is a volcanic rock also consisting chiefly of felspar, less compact than either porphyry or basalt. The porous pumice-stone of volcanic districts is probably altered felspar; it contains a much smaller proportion of alkali than the crystallized mineral. Melted pumice constitutes obsidian, or volcanic glass.

Garnet, which commonly crystallizes in rhombic dodecahedra, and idocrase, which crystallizes in square prisms, are basic double silicates of calcium and aluminum, in which part of the lime is displaced by other protoxides, and the alumina by sesquioxide of iron [3 (CaMgFeMn)"O, 2 Sie,. (AlFe)",,, Sie,]. In pyrope,

MICA-VARIETIES OF SLATE-POTTERY-WARE.

513

which is a species of garnet found in Bohemia, the colouring matter is partly sesquioxide of chromium. These minerals have a hardness greater than that of quartz.

The different forms of mica are also double silicates of aluminum, which contain in addition a small quantity of water and some alkaline fluoride. Uniaxal mica consists chiefly of silicate of magnesium and silicate of aluminum, 2 [2 (MgKFe)O,SiO2] (AlFe),,, 2 Sie. In biaxal mica (KFe), 3 Sie,. 3 [(AlFe),3, Sie,], on the other hand, silicate of potassium predominates, Lepidolite is a variety of biaxal mica in which silicate of lithium takes the place of silicate of potassium.

2

Another important double silicate of aluminum and magnesium constitutes chlorite [4 (MgFe), SiO,. (AlFe),,, Sie,. 3 H2→], which occurs both massive and in crystals with a granular fracture; it is of a green colour. In the massive form of chlorite slate it occurs as one of the primitive rocks which is widely distributed. There are many varieties of slate. Roofing slate is an argillaceous rock which splits readily into thin laminæ. Mica slate, as its name implies, contains particles of mica, to which it owes its glistening appearance. Hornblende slate contains hornblende in place of mica, and has little lustre.

(671) Porcelain and Pottery Ware.-In the preparation of earthenware the material employed is required to possess a plasticity equal to that of red-hot glass, and yet to be capable of being rendered by heat sufficiently firm and hard to resist the mechanical violence necessarily inflicted on it by daily use.

:

The basis of earthenware, porcelain, and china, is silicate of aluminum it possesses the plasticity required, and when heated assumes a great degree of hardness. Pure silicate of aluminum, however, contracts greatly and unequally on drying: the utensils made from it would consequently be liable to crack during their desiccation; in order, therefore, to diminish the amount of this contraction, an addition of some indifferent powder, such as ground flint, is made; while to compensate for the loss of tenacity thus occasioned, and which is particularly experienced in the use of the fine clays employed for porcelain, some fusible material is added, which, at the temperature required for firing, undergoes vitrification, and greatly assists in binding the mass together. According to the greater or less proportion of these fusible materials, the ware is more or less semi-transparent, and more or less subject, like glass, to fly on the application of sudden changes of temperature.

II.

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514

VARIETIES OF EARTHENWARE-PORCELAIN, OR CHINA.

The articles which have passed once through the kiln, and have thus acquired firmness, are rough and uneven, and the coarser kinds of ware are very porous. It is usual, after the first firing, in order to give smoothness and uniformity to the surface, as well as to render the body of the ware impermeable to moisture, to cover it with a kind of flux or glaze, which melts at a lower temperature than the material composing the ware itself; and in order to melt the glaze the articles are a second time passed through the kiln.

The materials employed in the fabrication of porcelain and earthenware are, clays of various degrees of purity and fineness, ground felspar, calcined flints or sand, burnt bones, chalk, and carbonate of sodium or of potassium; they do not, therefore, differ very greatly from those which are employed in glass-making except in the great preponderance of silicate of aluminum. The varieties of pottery or earthenware are numerous: the following include those which are of most importance :

1.-Porcelain, or China.-This is the finest and most valuable description of ware: it is distinguished from ordinary earthenware by the composition of the paste from which it is formed. The materials are selected with great care, in order that they may give a colourless mass after firing. Porcelain consists mainly of two classes of materials, one of which, the clay, is plastic, and is infusible at the temperature employed to fire it; the other (chiefly silicate of calcium and potassium) softens and becomes vitrified, forming a kind of cement which binds the clay firmly together, and thus produces a translucent mass, which when broken appears to be of a uniform texture throughout, and is impervious to liquids. Much judgment is required in the due proportioning of the fusible and infusible materials.

The celebrated Sèvres porcelain resembles the original Chinese ware, of which indeed it is an imitation. Regnault states the composition of the paste used at Sèvres for ornamental purposes to be the following:-Washed kaolin, 62 parts; Bougival chalk, 4; Aumont sand, 17; quartzose felspar, 17. These ingredients are carefully levigated and then thoroughly incorporated. however, the composition of the kaolin varies, the proportion of the other materials is necessarily varied likewise, so as to obtain a porcelain of uniform composition.

As,

In order to give a smooth surface to the ware, a glaze similar in composition to the fusible material is used. The glaze employed at Sèvres consists of a mixture of felspar and quartz. It is transparent, and rather more fusible than the body of the ware

STONEWARE-FINE EARTHENWARE.

515

but becomes thoroughly incorporated with it, and from its similarity in composition it expands and contracts by heat uniformly with the paste which it covers; hence it is not liable to crack and split in all directions in the manner which is so commonly observed in the glaze of the more ordinary kinds of earthenware.

The china of Berlin and Meissen is very similar in composition to that of Sèvres: these constitute what is termed hard, or true, porcelain.

English porcelain contains, in addition to the Cornish clay and felspar or flint, a large proportion of burnt bones; the glaze, which is transparent, usually contains both borax and oxide of lead to increase its fusibility. English porcelain is softer than the Chinese, French, or German porcelain, and constitutes one variety of what the French term porcelaine tendre, the manufacture of which in France is now rarely practised.

2.-Stoneware is a species of porcelain in which the body of the ware is more or less coloured, less care being taken with regard to the purity of the material. It generally contains more oxide of iron, and consequently is somewhat more fusible than the best porcelain, and is usually salt-glazed in a manner shortly to be described. Wedgwood-ware is a fine description of stone

ware.

3.-Fine Earthenware.-Articles of this description are very extensively manufactured in the Staffordshire Potteries, and constitute the ordinary table-service of this country. The Devonshire and Dorsetshire clays are those chiefly made use of; they are mixed with a large proportion of ground flints, and yield an infusible paste which burns nearly white. The body of the ware is not fused in the firing, but it is rendered impervious to liquids by means of a fusible lead glaze.

4.-Common earthenware is made of an inferior and more fusible description of clay: both this kind of ware and the foregoing one crack easily on the sudden application of heat.

5. The coarsest description of clay goods are bricks, tiles, flowerpots, and similar articles.

6.-Articles which are required to stand a high temperature, such as fire-bricks for lining furnaces, muffles, pots for the fusion of glass, crucibles for melting steel, and the Hessian crucibles so largely in demand in the laboratory, are made of a pure, infusible siliceous clay, the shrinking of which during drying is diminished by the addition either of burnt clay of the same description, or of, what amounts to the same thing, broken pots of the same material, which are reduced to a fine powder and incorporated

516

PORCELAIN-FIRE-WARE.

with the paste. Good fire-ware is nearly white: if coloured, the presence of oxide of iron would be indicated, and this would render it fusible.

The following table gives the composition of some of the more important varieties of china and pottery ware :

C. Cowper Wilson. Cowper. Laurent. Laurent. Salvetat. Salvetat. Berthier.

(672) Manufacture of Porcelain.-For the finer kinds of porcelain much care is taken to ensure the purity and minute subdivision of the constituents, as well as their intimate admixture. The clay is first ground between horizontal stones under water; it is next levigated in water, to allow the coarser particles to subside while the lighter ones remain in suspension. The finer suspended particles are then formed into a mixture of the consistence of thin cream, a wine pint of this being made to weigh 24 or 26 ounces: in this state the cream or pulp is mixed with the ground felspar, flint, or other material. Suppose, for example, that the pulp is to be mixed with ground flints; the flints are heated to redness, suddenly quenched in cold water, and then reduced by stamping and grinding them under water to an impalpable powder; this also is suspended in water, a wine pint of the mixture being made to weigh 32 ounces. The two ingredients are easily mixed in the necessary proportions by taking a given measure of each pulp and thoroughly incorporating them. The mixture thus obtained is technically termed slip. The slip is well agitated and allowed to subside; the deposit is drained (carefully mixing it from time to time), and dried, until it has acquired sufficient consistence to allow of its being wrought by the potter. Much labour is afterwards bestowed in working this clay in such a manner as to render it of uniform composition throughout, and to preserve it free from air-bubbles. The mixture is found to be greatly improved in quality by being allowed to remain for some