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Method of obtaining pure Copper. Let the copper of commerce be dissolved in muriatic acid, and precipitate it by a polished plate of
the precipitate is pure copper.
Brass is made by fusing together lapis calaminaris (which is an ore of zinc) and copper.
Tombac is formed by melting together twelve parts of copper with three of zinc.
Gun metal consists of nine parts of copper and one of tin.
Bell metal is copper alloyed with one sixth of tin. A smaller proportion of tin is used in making church bells than clock bells, and a little zinc is added for the bells of repeating-watches, and other "small bells.
Cock metal is made with copper alloyed with zinc and lead.
The gold coins of this country are composed of eleven parts of gold and one of copper.
Standard silver contains fifteen parts of silver and one of copper.
POTTERY. Pottery, or the art of making vessels of baked earth, is of the remotest antiquity. The ancient Greeks and Etruscans particularly excelled in it. Porcelain, the most perfect of species of pottery, has been made in China from time immemorial.
Alumine and silex are the two substances of which every kind of earthen-ware is made. Clay alone shrinks and cracks; the flint gives it solidity and strength.
Common Pottery, such as coarse brown jugs, &c. are made of the ordinary clays, which are a mixture of sand and clay, coloured by oxide of iron. The clay is well ground, or kneaded, and a lump of it is put upon the centre of a wheel which is kept in motion ; then, by means of the workman's hand, or by proper tools, it is formed into the required shape. The pieces are then dried moderately, so as to bear being removed without danger ; they are then covered with a glaze, made from semi-vitreous oxide of lead, and put into a furnace, where they are baked. Some sorts are glazed by throwing sea-salt into the furnace among the different pieces of pottery. The salt is decomposed, and the vapours of it form a glazing upon the vessels ; but this, though a very simple and ingenious method, does not form a good glazing
English stone-ware is made of tobacco pipe clay. mixed with flints calcined and ground. This manufacture owes its present state of perfection to that enlightened manufacturer the late Mr. Wedgewood, who spared no pains or expense to improve the art of pottery. He first introduced a superior kind of which he called Queen's ware. of which it is made comes chiefly from the neighbourhood of Corfe Castle in Dorsetshire. It burns extremely white. The pipe clay is much beat in water; by this process the finer parts remain suspended in the water, while the coarser, sand, and other impurities, fall to the bottom. The thick liquid, consisting of water and the finer parts of the clay, is further purified by passing it through hair and lawn sieves, of different degrees of fineness. After this, the liquid is mixed (in various proportions for various wares) with another
liquor of the same density, and consisting of flints calcined, ground, and suspended in water. The mixture is then dried in a kiln ; and being afterwards beaten tó a proper temper, it becomes fit for béirig formed at the wheel into dishes, plates, bowls, &c. When this ware is to be put into the fürñáce to be haked, the several pieces of it are placed in cases made of clay, called seggars, which are piled one upon another in the dome of the furnace; a fire is then lighted, and the ware is brought to a proper temper for glazing, and in this stäte it is called biscuit. Before the glázing takes place it is coloured or painted. Formerly the painting was done by the pencil by hand. It is now commonly effected by takeri iimpressions on påper fröm engraved copper plates, and transferring them to the earthenware while the ink is wet. The ink is instantly absorbed by the biscuit; and the paper is washed off. The ink for the blue and white ware is made from oxide of cobalt. It is thèn dipped into a glaze; made by mixing together in water, tili it becomes as thick as cream; one hundred and twelve parts of white-lead; twentyfour parts of ground flint, and six parts of ground flint-glass. The ware, by being baked; acquires a strong property of imbibing moisture; and when dipped into the glaze, therefore, it greedily attracts it into its pores, and presently becomes dry. It is then exposed a second time to the fire, by which means the glaze it has imbibed is melted, and a thin glassy coat is formed upon its surface. The colour of the coat is more or less yellow, according as a greater or less proportion of lead has been used. The lead is principally instrumental in producing the glaze, as well as in giving it the yellow colours for lead, of all the substances hitherto
known, has the greatest power of promoting the vitrification of the substances with which it is mixed. The fint serves to give a consistency to the lead during the time of its vitrification, and to hinder it from becoming too fluid, and running down the sides of the ware, and thereby leaving them unglazed.
This glazing, made by means of lead, is liable to be attacked by acids, and is supposed to be productive of deleterious effects, when employed in jars used for pickling, &c.
The following composition has been recommended as a substitute.
To make this, white glass and soda, in equal portions, must be very finely pulverized, carefully sifted, and well mixed. The mixture is then exposed to a strong heat, till it is rendered very dry: It is afterwards put into vessels which have been already baked; it is then melted, and the varnish is made. It may be applied in the same manner as that in common use.
The advantage of it is, that it is safe, and can have none of those poisonous effects which arise from the decomposition of the lead varnish.
A variety of ware has been of late manufactured which the potters call lustre. The glazing is formed with platina, or gold. For the first, the platina is dissolved in the nitro-muriatic acid with heat: by adding muriate of ammonia, à yellow precipitate falls down: wash and dry it, grind this powder with a small portion of enamel, in oil of turpentine, and spread it thinly over the glazed surface of the earthen-ware. The ware is now baked in a kiln with a red heat, and the platina will assume the metallic brilliancy. The gold is managed in the same way.
The black ware owes its colour to the oxides of of iron and manganese. It has also less flint, and is more burned; its compactness renders glazing unnecessary.
Porcelain, called also China, from being first brought from China, is the most beautiful and
perfect species of earthen-ware.
Genuine or true porcelain, is a semi-vitrified. earthen-ware, intermediate between common ware and glass. It is infusible in the strongest fire excited in furnaces; it is hard, but not so brittle as glass : it is proof against sudden changes of heat and cold : it is fine grained, and dense without gloss in the fracture, and is translucent. The Chinese long excelled in the art of making porcelain, but it is now made in various parts of Europe of an equally good quality and more ornamental. The Chinese porcelain is said to be composed of two ingredients, one of which is a hard stone called petuntse, which is carefully ground to a very fine powder; and the other, called kaolin, is a white earthy substance which is intimately mixed with the ground stone.
Several compositions of mingled earths may yield a true porcelain, by being burnt; and the porcelain of various countries differ in their mixtures.
But the principal basis of any true porcelain is that kind of clay which becomes white by baking, and which, either by intermingled heterogeneous earth, or by particular additions undergoes in the fire an incipient vitrification, in which the true nature of porcelain consists. Feldspar and gypsum, if added, may give that property to infusible clay.
When porcelain is to be made, the clay is properly selected, carefully washed from impurities,