THE NEW-YORK EXHIBITION ILLUSTRATED.

portion of the petuntze was incorporated with it, the fusible felspar at onceimparted new properties to the biscuit, rendering it semi-transparent, and very-strong and sonorous. Thus porcelain differs from common earthenware, by be-ing somewhat assimilated to the nature of glass. The felspar fuses in the heat ofthe furnace, owing to the potash or soda it contains, and the fused portion socompletely permeates the whole mass that we may conceive of the clay as beingsaturated with glass, until it has the peculiarity of semi-transparency belonging toit, as paper is penetrated and made translucent by oil or varnish. Under themicroscope the two ingredients are distinguished in the milky mass, the flux orglassy portion seeming to be permeated by white crystalline needles.

Porcelain is also distinguished from common earthenware by its glaze, whichis formed of pulverized felspar and has therefore the same composition and hard-ness with the body of the biscuit. This glaze it will be remarked has no lead inits composition, and thus differs in a most important particular from the glazingof opaque ware. The latter may be removed from the body which it covers bymechanical or chemical means, and often chips off and cracks. No such accidentcan happen to the porcelain glaze, which, owing to its being of the same con-stitution with the body of the ware, is completely one with it, and suffers nofracture from any inequality of expansion or contraction between the two mate-rials. It is almost needless to say that porcelain is not liable in the slightest de-gree to the evils arising from the porosity of common earthenware.

The classification of porcelain and other ceramic manufactures, is convenientlymade, as in London, into a hard porcelain, T> statuary, c tender porcelain, d stone-ware glazed and unglazed, e earthenware,/terra-cotta porcelain, g ornamental ordecorated.

The mass or body of the ware for hard porcelain is composed as follows:

At Sevres (near Paris).

At Meissen (Saxony),

Called Dresden Ware.

At Berlin (Prussia).

FOR VESSELS (1853.)

Kaolin,.48.

Sand (separated from the

Kaolin above), ... 48.Lime (chalk),.4.

FOR PLATES AND TABLE WARE.Kaolin (from 3 localities

mingled),.72.

Felspar,.26.

Broken Porcelain, , . . 2.

FOR PLATES, DISHES, ETC.

Kaolin,.76.

Felspar,.24.

FOR ORNAMENTAL PURPOSES.

Kaolin,.37.

Quartz, .37.

Lime,.17^.

Broken Porcelain, ... 8J-.

FOR ORNAMENTAL USE.

Kaolin from Marl, ... 25.

“ “ Beidersee, . 50.

Felspar,.15.

It is plain from a glance at this table that hard porcelain is entirely composed oftwo minerals, both derived from granitic rocks, both very infusible, and forming ai compound after the burning, of the most unchangeable nature. The kaolin is rarelyfound in beds or masses of sufficient purity or uniformity, to admit its being used,without preparation by washing and subsidence in water to separate the undecompo-sed portions of felspar, quartz, &c. The clay mass for porcelain must be perfectlyhomogeneous, and very finely ground. This condition is met by first washingthe crude material in a series of tubs placed one above the other; and providedwith means to keep up agitation by stirring at intervals, between which thecoarser matters fall to the bottom, and the finely divided particles of clay flow onwith water into the lower vessels, where they subside. The coarser particles,consisting of fragments of felspar and quartz, are then ground under mill-stonesto an impalpable powder, and the same process is repeated upon the felspar andbroken porcelain, if it is used. The term slip is given by the manufacturers tothe soft creamy mixture of the finely ground materials with water, having aboutthe consistence of thick cream. This is passed through fine sieves to separate anyfloating organic matters. It is easy to see, that according as the crude kaolin isclay-like, or sandy and coarse, that the process of washing will furnish more orless of the felspathio and quartzose materials which it is required to add to thekaolin, and hence the practice of every porcelain establishment in the mixing ofits several ingredients, must be adapted to the local peculiarities of its own sour-ces of supply for raw material. It is indispensable that the several constituentsshould be mingled in exact proportions by weight. To effect this with ease,it is usual to employ measured quantities of the slip of each constituent,separately ground, the value of an unit of each having been accurately deter-mined by analysis, and. the standard requires the best porcelain to contain in100 parts ; Silica, 58; Alumina, 34.5 ; Lime, 4.5 ; and Potash 3.

The paste must always present the same constitution in 100 parts; but as it isa mechanical mixture, it is still liable to separation by gravity into layers of un-equal value, in the process of drying. To avoid this, various expedients havebeen adopted. In France the slip is put in sacks of strong linen cloth,previously soaked in hot linseed oil (which, strange to say, renders themmore permeable to water and more durable), and is then submitted to anhydraulic press, as in the expression of flaxseed oil. The paste, by whatevermode dried, is always unevenly so, and is trod out under feet of men,beaten by stamps, and thrown forcibly in balls against an iron plate. Finally,when it is made as homogeneous as possible, the paste made into blocks orballs of moderate size, is stored away in a damD cellar for many months,

during which time it undergoes a sort of fermentation, exhales the fetid odorof sulphuretted hydrogen, and blackens on the outside. The beneficial effect ofthis long storage is universally conceded, but the cause has not been wellexplained. The clay becomes more plastic, as one good result, and of courseworks better on the wheel. Porcelain clay, as prepared for use, is never asplastic as potters’ clay, and the labor required to produce the same results withit in the wheel, is reckoned as ten times greater than that required when onlythe pure plastic clay is used. Six hundred plates a day, is a common day’s workfor a potter in Staffordshire ware, while sixty to eighty are the utmost whichcan be fashioned in porcelain clay.

All simple circular and cylindrical vessels can be formed upon the wheel bythe hands of the potter, aided by a few simple tools; but when the forms arecomplicated, and especially when they are angular or elliptical, it is requisite toform the paste in moulds of gypsum. The vessels formed on the wheel, afterbeing air-dried, are turned in a lathe in a manner similar to that in use for turningwood.

For the moulds, the clay is prepared by rolling it out upon a table with arolling-pin into an even sheet, much as a cook prepares pastry. The sheet is madomanageable by being spread upon a cloth, and in this way it can be convenientlytaken to the mould and adapted to its surface by the pressure of a moist sponge.

The handles, etc., are moulded separately, and attached to the roughened surfaceby a little slip, care being taken that the parts joined have a similar degree of humid-ity. ' Open or reticulated work, as in baskets, &c., is produced by cutting away thoopen space with a knife. Most of the ornamental surfaces are produced in mouldsor in the lathe, but stamps may be employed to impress the surface in an ornamentalmanner, adhesion being prevented between the stamp and the clay by the use ofod of turpentine. The beautiful lace-work so gracefully disposed upon the statu-ettes of Oopeland and Minton, is produced by dipping real lace into the porcelainslip, thus inclosing the fibre, which in the heat of the furnace is complete^ de-stroyed, leaving its perfect counterpart in biscuit.

When the several articles are completely formed, they are set away to dryupon shelves in a shady place free from currents of air; no means has been dis-covered by which the process of air-drying can be hastened without serious injuiyto the porcelain. When this tedious process is complete, the work is readyto go into the furnace for the first firing.

The porcelain kiln, like all furnaces used for the potters’ art, is a reverberatoryfurnace, but its form and structure are peculiar. It is a circular structure of threestories, separated from each other by low domes of masonry, through which fluesadmit air for the draught of the furnace and distribution of heat. The structure endsin a conical funnel over the third story, acting as the chimney, where the productsof combustion are discharged. The fires are built in four external furnaces equidis-tant, and at the base of the lower story, and these are so constructed that the coldair is made to pass downward through the fire, which is fed with dry wood,(usually pine or poplar), split small and introduced vertically, so that it burns onlyat its lower extremities, the ash-pit being closed and the air thus made to passin through the interstices of the fuel. The effect of the disposition of the fire is toprevent the access of cold air to the kiln, to produce a more perfect combustion,and of course more intense and uniform heat. When the furnaces are well served,the combustion is so complete and the current of air so steady and strong, that nota pound of ashes is found in the furnace or kiln, although the quantity of woodconsumed would produce over two hundred pounds, burned in the usual manner.A most serious evil is avoided by this absence of ashes, namely, the destruc-tion of the seggars or protecting cases in which the ware is packed, whichwould easily fuse and run together by reason of the combination of the alkali ofthe ash with the silica of the vessels, endangering the safety of the whole contentsof the kiln. The heat passes into the kiln from the furnaces through severalsmall square openings, and then inpinges against thick protecting plates of fineclay, which spread and diffuse it equally as possible through the whole of thekiln. The lower story of the kiln receives by far the most intense heat, andtherein are placed the things requiring to be baked at the highest temperature, inother words, the ware to be glazed, while the biscuit is fired in the second storyof the kiln,' which receives its heat entirely from the first; and lastly, in thethird story, the protecting cases or seggars are baked.

Porcelain, and all delicate ceramic wares, requires to be protected fromcontact with the ashes and dust of the furnace, by external cases of fire-proof sandy clay, made in a cylindrical form, and of such diameter andheight as will receive the various vessels, the bottom of one seggar or pro-tecting case, forming the top of the one below. The success of a porcelain man-ufactory may be said to depend upon its being possible to obtain a supply of therequisite material near at hand for forming the seggars, whose destruction in eachfiring is unavoidably considerable, even with the best care and materials, in-volving of course a large increase of cost chargeable upon the finished ware.Openings are left through the door of each story of the kiln, by means of which,trial pieces (called watchmen), can be inserted and withdrawn from time totime, as tests of the progress of the heat. For some hours (8 or 10), after thefires are kindled, the ash-pit is kept open and the heat kept purposely low, that.tlse