POT

POT

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Many of the impure coloured natural claysare of themfelves fufficiently mixed with otherearths for the potter’s ufe without addition, butthe v/hite and finer clays moftly require dilutionwith filex in one form or other, which can bedone to a very confiderable degree withouttaking away the plafliicity requifite for working.

The molt material circumftances required tobe confidered in feiecting the materials forpottery appear to be chiefly plafticity, Ihrinkage,folidity, and compadtnefs after drying; colour,and fufibility.

The plafticity feems to be Amply owing tothe quantity of day ufed, that is to fay, relative-ly to the original plafticity of the clay itfelf,for this quality is not the fame in all claysequally aluminous, but is much lefs in propor-tion in the indurated clays, and is increafed byexpofure to air.

The texture, including the qualities of hard-nefs and compadtnefs, depends partly on themixture of filiceous ingredient with the clay,and partly on the heat employed in the burningof the pottery. The purer natural clays arealmoft infufible in any furnace-heat, theirhardnefs is nearly progreffive with the intenfityof the fire, but they have the eflential defedtsof drying very flowly, of Ihrinking very con-fiderably, and of becoming rifty or full ofminute cracks when dried, fo as on this accountto be porous. It is therefore neceffary to mixthem intimately with any other earth of qualitiesoppofite to thofe of clay, that is, which abforbsbut little water and readily parts with it,(qualities diredtly oppofite to plafticity) andwhich dries compadl and clofe.

The colour of the earths ufed is alfo ofeflential importance in the finer pottery, inwhich the great defideratum is to find a claywhich after burning remains perfedtly white.The appearance before burning cannot alwaysbe depended on, for though in general thewhiteft clays before burning are thofe thatremain white afterwards, it is only in a fewdiftridts where clays are found that remain ab-folutely white. Thus there is found at the footof a range of high hills that directly overlookthe Staffbrdlhire potteries, a ftratum of whiteclay to appearance fully equal if not fuperior tothe belt Devonfhire clays, but it cannot beemployed for the fine pottery, as it acquires inburning a yellowilh cream colour, which no artcan corredt. The colour is fuppofed to dependon iron.

The fufibility,..of clays and other potteryearths is a fubjedt of extreme importance as it

is on this that the diftinction between Potteryand Porcelain chiefly depends.

Under the article Glafs we have treated atconfiderable length on the fufibility of earthymixtures, to which therefore we {hall refer ourreaders. Porcelain may be defined to be afpecies of pottery ware compofed of an earthymixture which refills compleat fufion in a veryconfiderable heat, but has been brought by alefs heat than its melting point to a ftateof incipient fufion, and thereby acquired ex-treme hardnefs, fonoroufnefs, femitranfparency,and a femi-conchoidal fplintery fradture, ap-proaching to the vitreous which is compleatlyconchoidal. This laft is quite a diftindtivecharacter between porcelain and pottery, forthe fradture of the latter is limply granular. Itappears probable therefore that no chemicalaction takes place in any pottery mixture till itarrives at the ftate of porcelain. The moltperfect and beautiful porcelains of Japan andChina are compofed (as appears from the bellteftimony) of one earth in which filex predomi-nates, and which melts in a ftrong fire, and ofanother which is infufible per fe , and by anunion of thefe alone (as it appears) a porcelainis produced which fcarcely vitrifies at theutmoft furnace heat which art can produce.This fubftance has the united excellencies ofbeing very hard, of a beautiful femi-tranfpa-rence, very white where not artificially colour-ed, very tough and cohefive, fo that it hasftrength enough for the purpofes for which it isdefigned when made very thin, and it bearsfudden heating and cooling without cracking.

Of the beautiful European porcelains whichhave been made in imitation of the oriental, itIhould feem that none of them precifely unitesall its diftinguilhing qualities. Earthy mixtureshave been made equally ftrong, tough, andinfufible, and as truly porcellanous when burnt,but thefe have not exactly equalled the beltJapanefe in delicate whitenefs and luftre. Butas the latter are the molt eflential qualities,that of infufibility has been in general facrificed,(which indeed is of no confequence for any ofthe common ufes of porcelain) and thereforethofe that come up to the oriental in beauty anddelicacy, of which there are feveral manufac-tures in different parts of Europe , for the moltpart foften and melt down in the molt intenfeheat of a wind-furnace, at which the trueNankin and Japanefe undergo no change.

The manufacture of the ordinary pottery ison the whole very Ample, where the due felec-tion of materials is made, but it is the more