Part 7

It is the peculiar quality of this substance to become so hard by heat that it will even strike fire with steel. The ductility of clay, and its property of thus hardening in the fire, have rendered it an article of indispensable utility to mankind in all civilized countries. It is formed into eating vessels of almost every description; plates, dishes, cups, basins, bowls, and pans for keeping provisions in. For these almost any kind of clay may be advantageously used; but it is necessary to mix it with sand, for the purpose of rendering the vessels that are made of it more firm and strong. Those that are applied to culinary, and other uses in which it is requisite for them not to be penetrable by water, are covered with a glazing. This glazing, for coarse ware, is sometimes made with lead, and sometimes by throwing a certain portion of salt into the furnace. In the formation of the better kinds of earthenware, the clay is made into a paste with water, moulded into the requisite shape upon an horizontal wheel, the inside being formed by one hand of the potter, and the outside by the other, as the wheel turns round. When the pieces have been baked, they are dipped into a glazing mixture, consisting of white lead, ground flints, and water, and are exposed a second time to the fire. The different colours of earthenware are obtained by means of various kinds of metallic oxides (21).

The coarser kinds of clay are manufactured into _bricks_ for the building of houses, and _tiles_ for the covering and paving of them. These are formed in moulds of the requisite shape, afterwards dried for some time in the sun, and finally piled in kilns, and there baked to a proper degree of hardness. The earth for bricks ought to be sufficiently fine, free from pebbles, and not too sandy, which would render them heavy and brittle; nor ought it to be entirely free from sand, as this would make them crack in drying.

Clay is a substance of inestimable value for forming the bottoms of ponds, and the bottoms and sides of canals and reservoirs, to prevent the water from draining away. It also composes, in a great measure, those tenacious earths called arable soils. What is peculiarly denominated clay land is known by its holding water, and not soon drying when wetted. Such land requires much labour from the husbandman, before it can be sufficiently pulverized, or brought to a fit state for being productive of corn or grass.

116. PIPE CLAY _is a fine and yellowish white variety of common clay_. It is very plastic, adheres strongly to the tongue; and, in a strong heat, is hardened, and rendered perfectly white.

It is of this clay that tobacco pipes are made, by the simple process of casting them in moulds, forming a hole through the stems by means of a wire, generally dipping the small end into some glazing material, and then baking them. Pipe clay is also formed into oblong pieces, dried, and employed for cleaning white woollen cloths, and for various purposes of domestic utility. It is likewise the basis of the yellow, of what is called _Queen’s ware_ pottery. This is glazed in a manner somewhat different from that of common pottery. The glazing mixture consists of a certain proportion of carbonat of lead (239), ground flint, and flint glass, worked with water to the thickness of cream. The ware, before it is glazed, is baked, and thus acquires the property of strongly imbibing moisture. It is then dipped into the above composition; exposed a second time to the fire, by which the glaze it has imbibed is melted. A thin glossy coat is thus formed upon its surface, which is more or less yellow, according to the greater or less proportion of lead that has been used.

117. _LOAM is a yellowish or brownish kind of clay; sometimes containing a considerable proportion of sand. It occurs in immense beds, and is found in almost every part of the world._

This substance, when mixed with straw or hair, to prevent it from cracking, is extensively used for the building of what are called mud cottages or houses. These are generally reared on a foundation of stone, or brickwork, to secure them from injury by the moisture of the earth. It is said to be the most advantageous practice to form the loam into bricks, and to dry these in the shade, and afterwards in the sun. The use of such bricks is of great antiquity. We are informed that the ancient city of Damascus, and even the walls of Babylon, were constructed of bricks made of loam.

118. _PORCELAIN CLAY is generally of white or reddish white colour, sometimes inclining to yellowish or grey. When dry, it absorbs moisture rapidly; and it becomes very tenacious when kneaded._

_It is known from the other clays by the fineness of its particles, its soiling the fingers much when handled, and its fine but meagre feel._

The usual distinction betwixt earthen ware and porcelain is, that the former is opaque, and the latter semi─transparent. In the manufacture of porcelain the clay is sometimes used alone, and sometimes intermixed with other earths, or with felspar (110). The earliest manufacture of porcelain is supposed to have been that in China and Japan. The quantity produced in China must formerly have been extremely great; as not only a considerable portion of the eastern parts of the world, but almost the whole of Europe, was supplied with it. In a single province it is said that nearly a million of persons were at one time employed in this manufacture.

The manufactory at Sevres, in France, has long been celebrated both for the excellence and elegance of its porcelain. There are well─known manufactories of porcelain at Meissen in Saxony, at Berlin, and in Austria; but none of these are at present superior to our own, in Worcestershire and Staffordshire.

Porcelain clay occurs chiefly in countries which abound with granite (251) and gneiss (255). It is found in small quantity in Cornwall, and other granite districts of England, as well as in those of Scotland and Ireland. But the most valuable kinds of this clay are found in China and Japan.

The mineral is not used in the state in which it is found in the earth; but is previously washed several times to free it from impurities. After the process of washing, only about fifteen parts of pure clay remain: this is the _kaolin_ of the Chinese. To form the composition of the porcelain, this clay is mixed, in certain proportions, with quartz (76), flint, gypsum (192), steatite (124), or other substances; and the mixture is sifted several times through hair sieves. It is afterwards moistened with rain water, and, in the form of a paste, is put into covered casks. Here a fermentation soon takes place, which changes its smell, colour, and consistence. Its colour passes from white into dark grey; and the matter becomes both tougher and more soft than before. The peculiar mode of preparing this mixture, and the art of rightly managing it, are secrets in most porcelain manufactories.

The next operation consists in giving to the paste thus formed the requisite shape of the vessels. This is done first by kneading it with the hands; and then by taking up certain portions of it, and turning it on a lathe, in the manner of common pottery (115), but with more care.

The third operation is the baking or firing. This is done in furnaces of a particular construction, and generally lasts from thirty─six to forty─eight hours. The state of the baking is shown by proof pieces, as they are called, which are placed in convenient situations, and can be drawn out, from time to time, for examination. The porcelain in this state, is named _biscuit porcelain_; and figures, and such other porcelain articles as are neither to be painted nor exposed to water are in the state of biscuit.

A fourth operation is covering the surface of the biscuit with a varnish or enamel. This is composed of pure white quartz (76), white porcelain, and calcined crystals of gypsum (192); and sometimes principally of felspar (110). These substances are carefully ground, then diffused through water, and formed into a paste. When used, the paste is diluted in water, so as to give it considerable fluidity; and the pieces of biscuit porcelain are separately plunged into it, in such manner as to cover their whole surface. These are then exposed to a heat sufficient to melt the enamel or covering: and in this state they constitute white porcelain.

If the porcelain is to be painted, it must again be exposed to heat in the furnace. The colours used for the painting of it are all derived from metals; and many of them, though dull when applied, acquire considerable lustre by the action of the fire. The colours are always mixed with some kind of flux, such as a mixture of glass (204), borax (208), and nitre, melted together, and afterwards ground.

Gum or oil of lavender is used for mixing up the colours. When the painting is finished, the pieces are exposed to a heat sufficient to melt the flux, and thus fix the colour.

119. _TRIPOLI is a kind of clay of yellowish grey, brown, or white colour, sometimes striped or spotted, and of an earthy texture._

_It feels harsh and dry to the touch; is soft, scarcely adheres to the tongue, and will not take a polish from the nail._

This substance obtained its name from having formerly been imported into Europe from Tripoli, on the north coast of Africa. It is, however, now found in several parts of Germany; and a granulated kind has been discovered in England.

Tripoli is used for the polishing of metals and stones. For this purpose, it is mixed with sulphur, in the proportion of two parts of tripoli to one of sulphur. These are well rubbed together on a marble slab, and are applied to the stone or metal with a piece of leather.

When tripoli is combined with red ironstone, it is used for the polishing of optical glasses. It is sometimes made into moulds, in which small metallic or glass figures and, medallions are cast; and a kind of tripoli is found near Burgos, in Spain, which is used as an ingredient in the manufacture of porcelain.

In Derbyshire,, and several parts of Staffordshire, is dug a kind of tripoli which has the name of _rotten stone_. This is considered to be a produce of limestone shale, which has undergone a decomposition by exposure to the air and moisture. It is used for most of the same purposes as tripoli.

CLAY SLATE FAMILY.

120. _CLAY SLATE, or ROOFING SLATE, is a kind of stone of foliated texture, and greyish, black, brown, green, or bluish colour._

_It breaks into splinters, does not adhere to the tongue, yields generally a clear sound when struck, and is nearly thrice as heavy as water._

Vast and extensive beds of slate occur in different parts of the world; and this mineral sometimes constitutes even a principal portion of mountains. In our own country there are many important quarries of it, particularly in Westmoreland, Yorkshire, Wales, and Derbyshire.

The uses of slate are numerous and important; but its principal use is for the roofing of houses. For this purpose it is split into thin plates or laminæ. These are fastened to the rafters by pegs driven through them; and are made to lap over each other at the edges, in such manner as to exclude the rain and other moisture. The kinds which are preferred for this purpose are such as have the smoothest surface, and split into the thinnest plates. It is requisite that slates should be damp when they are split, otherwise this cannot be done without difficulty. Hence it is generally customary to split the masses as soon as possible after they have been separated from the rock.

Slate should not be porous. If it be so, rain and snow water will pass through it, and destroy the wood─work of the house on which it is placed. Porous slate is also liable to have moss and lichens grow upon and cover it. These plants retain moisture long, and keep the surface, and even the interior of the slate, moist; so that, during the winter season, by the freezing of the moisture, the slate is apt to split and fall into pieces. To ascertain whether the slate be of requisite compactness, it should be completely dried, then weighed, and afterwards soaked for some time in water. When taken out it is to be wiped with a cloth, and again weighed. If it have not acquired any considerable increase of weight, it is a proof of its being sufficiently compact. If, on the contrary, it have absorbed much of the water, and have become considerably heavier by the immersion, it is shown to be of a porous texture. Slates that are brittle are bad. If they emit a tolerably clear sound, when struck with a hammer, it is considered a proof that they are not too brittle: if, on the contrary, the sound be dull, they are soft and shattery. A good slate ought also to resist the action of a considerable degree of heat.

The slates that are principally used in London are brought from North Wales, from quarries that are worked near Bangor. There are also extensive slate quarries near Kendal, in Westmoreland; and the Kendal slates, which are of a bluish green colour, are more highly esteemed than those from Wales. They are not of large size, but they possess great durability, and give a peculiarly neat appearance to the roofs on which they are placed. The slate quarries near Easdale, in Scotland, are so extensive as to furnish annually more than 5,000,000 in number, and to give employment to upwards of 300 men.

_French slates_ were much used in London about seventy years ago; but they have been found too small, thin, and light, to resist the winds and storms of this changeable climate.

Dark─coloured, compact, and solid slates are manufactured into _writing slates_, or _table slates_, as they are sometimes called. In the preparation of these, the slate, after it is split of proper thickness, is smoothed with an iron instrument. It is then ground with sandstone, and slightly polished with tripoli (119), and, lastly, rubbed with charcoal powder. It is cut into the requisite shape, set in a wooden frame, and is then ready for use.

For writing on these slates, pencils are used which are also made of slate. These, which are called _slate pencils_, are made of a particular kind of slate, that, on splitting, falls into long splintery fragments. It is necessary that the pencils should be considerably softer than the slate to be written upon, so that they may leave a whitish streak on its surface, without scratching it. Such is the shivery nature of the slate of which they are made, that, if it be exposed for some time to the action of the sun or frost, it is rendered useless. Hence, workmen are careful to cover it up and sprinkle it with water, as soon as it is taken from the quarry, and to preserve it in damp cellars. The pieces are afterwards split by a particular instrument, and then wrought into the requisite shape.

In some of the quarries in Derbyshire and Wales the slate is so thick as to admit of being split into large and tabular pieces. These are used for gravestones, and for slabs for dairies and cellars. Paving stones and mile─stones are also formed of them; and vessels for the salting of meat, and setting of milk in dairies. For the latter use slate is peculiarly well adapted, on account of its resistance of greasy or oily substances. But this property renders it unfit for any purpose for which it is requisite to be painted; as, the oil not entering the stone, the paint soon peels off, and leaves the stone as black as it was at first. Cut into narrow strips, slate has also been applied, in the neighbourhood of Bangor, North Wales, for the formation of fences.

When sufficiently solid for the purpose, slate is cut into inkstands, and turned into vases, and fancy articles of various kinds. And a singular circumstance has been remarked, that, if a window or door be suddenly opened, in an apartment where the workmen are turning these, they will sometimes fly in pieces; though, after the work is finished, they may be exposed to the usual changes of temperature without injury.

Pounded slate is advantageously used for cleaning iron and other works in metal. When well ground, and mixed with a certain proportion of loam, slate is made into moulds for the casting of metals in; and, when burned and coarsely ground, is used instead of sand in the making of a solid and impermeable mortar or cement, for the parts of buildings that are covered with water.

121. _BLACK CHALK, or DRAWING SLATE, is an earthy substance, of slaty texture; generally of a greyish, sometimes a bluish black colour._

_It is soft and smooth to the touch, and, in handling, stains the fingers._

To crayon painters, and other artists, black chalk is a very useful article. Considerable quantities of it are imported from France, Spain, and Italy. The best is brought from Italy. This is more free from gritty particles, more firm and compact in its texture, and in its touch much smoother than the chalk of any other country. It contains somewhat more than one─tenth part of its weight of charcoal. When prepared for use, it is cut into square pieces, which are sometimes enclosed in wooden cases, like black lead pencils. These pencils are said to become dry, hard, and unfit for use, by long keeping. To preserve them in greatest perfection, they should be kept in a moist place. Some artists prefer pencils that are made of the chalk finely ground, mixed with a certain proportion of gum water, and cast in moulds. Care should be taken not to put too much gum, as the pencils will not, in such case, leave any mark on the paper.

Drawing slate is sometimes used as a black colour for painting. For this purpose it is pounded or ground, and then mixed with oil or size, according to the kind of work for which it is required. When black chalk is strongly heated, it loses its colour, and assumes that of a reddish grey.

122. _HONE, or WHET SLATE, is a well─known kind of stone, of somewhat slaty texture, and generally of dull white, or greenish grey colour. Its surface is smooth, and feels unctuous to the touch._

These stones, when properly cut and smoothed, are of indispensable utility to carpenters, cutlers, and others, for sharpening their cutting instruments upon. Those of the finest grain are used for lancets, penknives, and razors. For this purpose their surface, when used, is covered with a small quantity of oil; by which, after a while, they are rendered considerably harder than they were at first. They ought to be kept in damp and cool places; for, if much exposed to the sun, they become too hard and dry for many purposes to which they are applied.

There is a vulgar and erroneous notion that hones are holly wood, which by lying in petrifying water, have been thereby converted into stone. The greater number of them have a fine and a coarse side. From the circumstance of their having been originally brought into this country from Turkey and the Levant, they are sometimes called _Turkey stones_. They are now found in Saxony and Bohemia, in North Wales, and near Drogheda, in Ireland.

The powder of whet slate is sometimes used, instead of emery, for the cutting and polishing of metals.

MICA FAMILY.

123. _COMMON MICA, GLIMMER, or MUSCOVY GLASS, is a mineral substance of foliated texture, which is capable of being divided into extremely thin leaves that have a sensible elasticity, and are transparent._

_The colour of mica is greenish, sometimes nearly black, reddish, brown, yellow, or silvery white, with, occasionally, a metallic lustre on the surface. Mica is so soft as easily to be scratched; and, when divided across the plates, seems rather to tear than break._

This is one of the most abundant mineral substances that is known. It not only occurs in a massive and crystallized state, but it enters into the composition of many rocks; is found filling up their fissures, or crystallized in the cavities of the veins which traverse them. In some countries, as in Siberia, it is an article of commerce, and is obtained from mines like other minerals. From these it is extracted by hammers and chisels. It is then washed, to free it from the impurities which adhere to it; split into thin leaves or pieces; and assorted into different kinds, according to their goodness, purity, and size. We are informed by the Abbé Haüy, that plates of mica a yard or more in width have been obtained from the mines in some parts of Russia.

Thin plates of mica are adopted, in many parts of Siberia and Muscovy, to supply the place of glass for windows. In the shipping of Russia it is considered preferable to glass, as the concussion produced by the firing of the guns does not shatter it. It is employed instead of window glass in Peru and New Spain; and also in Pennsylvania. Mica may be advantageously substituted for horn in lanterns, as it is not only more transparent, but is not susceptible of injury from the flame of the candle. It has, however, the inconvenience of soon becoming dirty; and of having its transparency destroyed by long exposure to the air. Mica is used for enclosing objects that are intended to be viewed by microscopes.

So plentiful is this substance in Bengal, that, for the value of five shillings, as much of it may be purchased as will yield a dozen panes, each measuring about twelve inches in length and nine in breadth, and so clear as to allow of ordinary objects being seen through them at the distance of twenty or thirty yards.

Mica, when powdered, is sold by stationers on the Continent, in place of sand, for absorbing ink in writing, but it does not dry sufficiently quick to be of much use in this respect. In Russia it is employed in different kinds of inlaid work. It is sometimes powdered, and intermixed with the glaze in particular kinds of earthen ware. The heat which melts the glaze has no effect on the mica: hence it appears, dispersed throughout the glaze, like plates or scales of silver or gold. Some artists use it in making artificial avanturines (85).

It must be observed that the best mica is of a pure pearl colour; and, when split into leaves, presents a smooth surface.

SOAPSTONE FAMILY.

124. _STEATITE, or SOAPSTONE, is a soft and unctuous substance, which has much the appearance of soap; and is generally of a white or grey colour, intermixed with greenish or yellowish shades._

_It is somewhat more than twice as heavy as water; and is distinguished from indurated talc (135) by not splitting, like that substance, into slaty fragments._

In the counties of Devon and Cornwall, and the islands in the vicinity of the Lizard Point, this mineral is found in considerable abundance. It possesses many of the same properties as fullers’ earth, and is, like that substance, employed in the scouring of woollen cloths. When mixed with water it may be formed into a paste; and, in this state, it is easily worked, like clay, for the manufacture of earthen ware. In the porcelain manufactory at Worcester considerable quantities of steatite are employed. According to Dr. Shaw, the Arabs use it in their baths, instead of soap, to soften the skin.

As it becomes hard in the fire, and does not alter its shape, this substance has been successfully adopted for imitations of engraved gems. The subjects are engraved upon it with great ease in its natural state; it is then exposed to a strong heat; afterwards polished, and then coloured by means of certain metallic solutions.