CIVET

( Civette , Fr.; Zibeth , Germ.) This substance approaches in smell to musk and ambergris; it has a pale yellow colour, a somewhat acrid taste, a consistence like that of honey, and a very strong aromatic odour. It is the product of two small quadrupeds of the genus viverra ( v. zibeth a and v. civetta ), of which the one inhabits Africa, the other Asia. They are reared with tenderne , especially in Aby in i a. The civet is contained in a sac, situated between the anus and the parts of generation, in either sex. The animal frees itself from an exce of this secretion by a contractile movement which it exercises upon the sac, when the civet i ues in a vermicular form, and is carefully collected. The negroes are accustomed to increase the secretion by irritating the animal; and likewise introduce a little butter, or other grease, by the natural slit in the bag, which mixes with the odoriferous substance, and increases its weight. It is employed only in perfumery. According to M. Boutron-Chalard, it contains a volatile oil, to which it owes its smell, some free ammonia, resin, fat, an extract i form matter, and mucus. It affords, by calcination, an ash, in which there are some carbonate and sulphate of potash, phosphate of lime, and oxide of iron. CLAY ( Argile , Fr.; Thon , Germ.) is a mixture of the two simple earths, alumina and silica, generally tinged with iron. Lime, magnesia, with some other colouring metallic oxides, are occasionally present in small quantities in certain natural clays. The different varieties of clay po e the following common characters :— 1. They are readily diffusible through water, and are capable of forming with it a plastic ductile ma , which may be kneaded by hand into any shape. This plasticity exists, however, in very different degrees in the different clays. 2. They concrete into a hard ma upon being dried, and a ume, upon exposure to the heat of ignition, a degree of hardne sometimes so great as to give sparks by collision with hardened steel. In this state they are no longer plastic with water, even when pulverised. Tolerably pure clays, though infusible in the furnace, become readily so by the admixture of lime, iron, manganese, . 3. All clays, even when previously freed from moisture, shrink in the fire in virtue of the reciprocal affinity of their particles; they are very absorbent of water in their dry state, and adhere strongly to the tongue. 4. Ochrey, impure clays emit a disagreeable earthy smell when breathed upon. Brongniart distributes the clays into :— 1. Fire-clays, ( argiles apyres , Fr.; feuerfeste , Germ.) 2. Fusible, ( schmelz bare , Germ.) 3. Effervescing ( brausende , Germ.), from the presence of chalk. 4. Ochrey ( ocreuses , Fr.; ockrige , Germ.) Fire-clay is found in the greatest abundance and perfection for manufacturing purposes in, 1. Slate-clay. ( Thon-schiefer , Germ.) Its colour is gray or grayish-yellow. Ma ive, dull, or glimmering from admixture of particles of mica. Fracture slaty, approaching sometimes to earthy. Fragments tabular. Soft, sectile, and easily broken. Sp. gr. = 2·6. Adheres to the tongue, and breaks down in water. It occurs along with pit coal ; which see. Slate-clay is ground, and reduced into a paste with water, for making fire-bricks; for which purpose it should be as free as po ible from lime and iron. 2. Common clay or loam. —This is an impure coarse pottery clay, mixed with iron ochre, and occasionally with mica. It has many of the external characters of plastic clay. It is soft to the touch, and forms, with water, a somewhat tenacious paste; but is in general le compact, more friable, than the plastic clays, which are more readily diffusible in water. It does not po e the property of acquiring in water that commencement of translucency which the purer clays exhibit. Although soft to the touch, the common clay wants unctuosity, properly so called. The best example of this argillaceous substance is afforded in the London clay formation, which consists chiefly of bluish or blackish clay, mostly very tough. Those of its strata which effervesce with acids partake of the nature of marl. This clay is fusible at a strong heat, in consequence of the iron and lime which it contains. It is employed in the manufacture of bricks, tiles, and coarse pottery ware. 3. Potter’s clay, or Plastic clay. —This species is compact, soft, or even unctuous to the touch, and polishes with the pre ure of the finger; it forms, with water, a tenacious, very ductile, and somewhat translucent paste. It is infusible in a porcelain kiln, but a umes in it a great degree of hardne . Werner calls it pipe-clay . Good plastic clay remains white, or if gray before, becomes white in the porcelain kiln. The geological position of the plastic clay is beneath the London clay, and above the sand which covers the chalk formation. The plastic clay of the Paris basin is described as consisting of two beds separated by a bed of sand. The lower bed is the proper plastic clay. The plastic clay of Abondant , near the forest of Dreux, analysed by Vauquelin, gave — Silica, 43·5; alumina, 33·2; lime, 0·35; iron, 1; water, 18. This clay is employed as a fire clay for making the bungs or seggars , or coarse earthenware cases, in which china ware is fired. The plastic clay of Dor sets hire and Devons hire supplies the great Staff ords hire potteries. It is gray coloured, le unctuous than that of Dreux, and consequently more friable. It becomes white in the pottery kiln, and is infusible at that heat. It causes no effervescence with nitric acid, but falls down quickly in it, and becomes higher coloured. Its refractorine allows of a harder glaze being applied to the ware formed from it without risk of the heat requisite for making the glaze flow, affecting the biscuit either in shape or colour. “Most of the plastic clays of France,” says M. Brongniart, “employed for the same ware, have the disadvantage of reddening a little in a somewhat strong heat; and hence it becomes nece ary to coat them with a soft glaze, fusible by means of exce of lead at a low heat, in order to preserve the white appearance of the biscuit. Such a glaze has a dull aspect, and cracks readily into innumerable fi ures by alternations of hot and cold water.” Hence one reason of the vast inferiority of the French stone-ware to the English. 4. Porcelain clay or Kaolin earth. —The Kaolins po e very characteristic properties. They are friable in the hand, meagre to the touch, and difficultly form a paste with water. When freed from the coarse and evidently foreign particles interspersed through them, they are absolutely infusible in the porcelain kiln, and retain their white colour unaltered. They harden with heat like other clays, and perhaps in a greater degree; but they do not acquire an equal condensation or solidity, at least when they are perfectly pure. The Kaolins in general appear to consist of alumina and silica in nearly equal proportions. Most of the Kaolin clays contain some spangles of mica which betray their origin from disintegrated granite. This origin may be regarded as one of their most distinctive features. Almost all the porcelain clays are evidently derived from the decomposition of the felspars, granites, and principally those rocks of felspar and quartz, called graphic granite. Hence, they are to be found only in primitive mountain districts, among banks or blocks of granite, forming thin seams or partings between them. In the same partings, quartz and mica occur, being relics of the granite; while some seams of Kaolin retain the external form of felspar. The most valuable Kaolins have been found :— In China and Japan. The specimens imported from these countries appear pretty white; but are more unctuous to the touch, and more micaceous than the porcelain clays of France. In Saxony. The Kaolin employed in the porcelain manufactories of that country has a slight yellow or flesh colour, which disappears in the kiln, proving as Waller i us observed, that this tint is not owing to any metallic matter. In France, at Saint-Yriex-la-Perche, about 10 leagues from Limoges. The Kaolin occurs there in a bed, or perhaps a vein of beds of granite, or rather of that felspar rock called Pe-tun-tse, which exists here in every stage of decomposition. This Kaolin is generally white, but sometimes a little yellowish with hardly any mica. It is meagre to the touch, and some beds include large grains of quartz, called pebbly by the China manufacturers. This variety, when ground, affords, without the addition of any fusible ingredient, a very transparent porcelain. Near Bayonne. A Kaolin po e ing the lamellated structure of felspar, in many places. The rock containing it is a graphic granite in every stage of decomposition. In England, in the county of Cornwall. This Kaolin or China clay is very white, and more unctuous to the touch than those upon the continent of Europe mentioned above. Like them it results from the decomposition of the felspars and granites, occurring in the middle of these rocks. Mr. Wedge wood found it to contain 60 of alumina or pure clay, and 40 of silica, in 100 parts. Pure clay, the alumina of the chemist, is absolutely infusible; but when subjected to the fire of a porcelain kiln, it contracts into about one half of its total bulk. It must, however, be heated very cautiously, otherwise it will decrepitate and fly in pieces, owing to the sudden expansion into steam of the water combined with its particles, which is retained with a considerable attractive force. It po e es little plasticity, and consequently affords a very short paste, which is apt to crack when kneaded into a cake. It is not only infusible by itself, but it will not di olve in the fusible gla es; making them merely opaque. If either lime or silica be added separately to pure clay, in any proportion, the mixture will not melt in the most violent furnace; but if alumina, lime, and silica be mixed together, the whole melts, and the more readily, the nearer the mixture approaches to the following proportions:—1 of alumina, 1 of lime, and 3 of sand. If the sand be increased to five parts, the compound becomes infusible. These interesting facts show the reciprocal action of those earths which are mixed most commonly in nature with alumina. Iron in small quantity, but in a state not precisely determined, though probably of protoxide, does not colour the clays till they are subjected to a powerful heat. There are very white clays, such as those of Montereau, which do not become red till calcined in the porcelain kiln; the oxide of iron contained in them, which colours them in that case, was previously imperceptible. It appears from this circumstance, that the clays fit for making fine white stone ware, as also the Kaolins adapted to the manufacture of porcelain, are very rare. Iron, in larger proportion, usually colours the clays green or slate-blue, before they have been heated. Such clays, exposed to the action of fire, become yellow or red according to the quantity of iron which they contain. When the iron is very abundant, it renders the clays fusible; but a little lime and silica must also be present for this effect. The earthenware made with these ferruginous clays, can bear but a moderate baking heat; it is thick, porous, and po e es the advantage merely of cheapne , and of bearing considerable alternations of temperature without breaking. Alumina and the very aluminous natural clays which po e most plasticity, are apt to crack in drying, or to lose their shape. This very serious defect for the purposes of pottery is rectified, in some measure, by adding to that earth a certain quantity of sand or silica. Thus, a compound is formed which po e es le attraction for water, and dries more equably from the openne of its body. The principal causes of the distortion of earthenware ve els, are the unequal thickne of their parts, and quicker desiccation upon one side than another. Hard burnt stone-ware ground to powder, and incorporated with clay, answers still better than sand for counteracting the great and irregular contraction which natural pottery paste is apt to experience. Such ground biscuit is called cement ; and its grains interspersed through the ware, may be regarded as so many solutions of continuity, which arrest the fi ures. The preceding observations point out the principles of those arts which employ clay for moulding by the wheel, and baking in a kiln. See Porcelain and Pottery .