Plan of potteryFig.899.A, is the entrance door;B, the porter’s lodge;C, a particular warehouse;D, workshop of the plaster-moulder;E, the clay depôt;F,F, large gates, 6 feet 8 inches high;G, the winter evaporation stove;H, the shop for sifting the paste liquors;I, sheds for the paste liquor tubs;J, paste liquor pits;K, workshop for the moulder of hollow ware;L, ditto of the dish or plate moulder;M, the plate drying-stove;N, workshop of the biscuit-printers;O, ditto of the biscuit, witho′, a long window;P, passage leading to the paste liquor pits;Q, biscuit warehouse;R, place where the biscuit is cleaned as it comes out of the biscuit kilns,S S;T,T, enamel or glaze kilns;U, long passage;V, space left for supplementary workshops;X, space appointed as a depôt for the sagger fire-clay, as also for making the saggers;Z, the workshop for applying the glaze liquor to the biscuits;a, apartment for cleaning the glazed ware;b,b, pumps;c, basin;d, muffles;e, warehouse for the finished stoneware;f, that of the glazed goods;g,g, another warehouse;h, a large space for the smith’s forge, carpenter’s shop, packing room, depôt of clays, saggers, &c. The packing and loading of the goods are performed in front of thewarehouse, which has two outlets, in order to facilitate the work;i, a passage to the court or yard;l, a space for the wooden sheds for keeping hay, clay, and other miscellaneous articles,m, room for putting the biscuit into the saggers;m′, a long window;n, workshop with lathes and fly-wheels;o, drying-room;p, room for mounting or furnishing the pieces;q, repairing room;r, drying room of the goods roughly turned;s, rough turning or blocking-out room;t, room for beating the paste or dough;u, counting-house.The declared value of the earthenware exported in 1836, was 837,774l.; in 1837, 558,682l.There are from 33,000 to 35,000 tons of clay exported annually from Poole, in Dorsetshire, to the English and Scotch potteries. A good deal of clay is also sent from Devonshire and Cornwall.The Spanishalcarazzas, or cooling vessels, are made porous, to favour the exudation of water through them, and maintain a constantly moist evaporating surface. Lasteyrie says, that granular sea salt is an ingredient of the paste of the Spanish alcarazzas; which being expelled partly by the heat of the baking, and partly by the subsequent watery percolation, leaves the body very open. The biscuit should be charged with a considerable proportion of sand, and very moderately fired.OF PORCELAIN.Porcelain is a kind of pottery ware whose paste is fine grained, compact, very hard, and faintly translucid; and whose biscuit softens slightly in the kiln. Its ordinary whiteness cannot form a definite character, since there are porcelain pastes variously coloured. There are two species of porcelain, very different in their nature, the essential properties of which it is of consequence to establish; the one is calledhard, and the othertender; important distinctions, the neglect of which has introduced great confusion into many treatises on this elegant manufacture.Hardporcelain is essentially composed, first, of a natural clay containing some silica, infusible, and preserving its whiteness in a strong heat; this is almost always a true kaolin; secondly, of a flux, consisting of silica and lime, composing a quartzose felspar rock, calledpe-tun-tse. The glaze of this porcelain, likewise earthy, admits of no metallic substance or alkali.Tenderporcelain, styled also vitreous porcelain, has no relation with the preceding in its composition; it always consists of a vitreous frit, rendered opaque and less fusible by the addition of a calcareous or marly clay. Its glaze is an artificial glass or crystal, into which silica, alkalis, and lead enter.This porcelain has a more vitreous biscuit, more transparent, a little less hard, and less fragile, but much more fusible than that of the hard porcelain. Its glaze is more glossy, more transparent, a little less white, much tenderer, and more fusible.The biscuit of the hard porcelain made at the French national manufactory of Sèvres is generally composed of a kaolin clay, and of a decomposed felspar rock; analogous to the china clay of Cornwall, and Cornish stone. Both of the above French materials come from Saint Yriex-la-perche, near Limoges.After many experiments, the following composition has been adopted for theservice pasteof the royal manufactory of Sèvres; that is, for all the ware which is to be glazed: silica, 59; alumina, 35·2; potash, 2·2; lime, 3·3. The conditions of such a compound are pretty nearly fulfilled by taking from 63 to 70 of the washed kaolin or china clay, 22 to 15 of the felspar; nearly 10 of flint powder, and about 5 of chalk. The glaze is composed solely of solid felspar, calcined, crushed, and then ground fine at the mill. This rock pretty uniformly consists of silica 73, alumina 16·2, potash 8·4, and water 0·6.The kaolin is washed at the pit, and sent in this state to Sèvres, under the name ofdecanted earth. At the manufactory it is washed and elutriated with care; and its slip is passed through fine sieves. This forms the plastic, infusible, and opaque ingredient to which the substance must be added which gives it a certain degree of fusibility and semi-transparency. The felspar rock used for this purpose, should contain neither dark mica nor iron, either as an oxide or sulphuret. It is calcined to make it crushable, under stamp-pestles driven by machinery, then ground fine in hornstone mills, as represented infigs.897,898,899, and900.This pulverulent matter being diffused through water, is mixed in certain proportions, regulated by its quality, with the argillaceousslip. The mixture is deprived of the chief part of its water in shallow plaster pans without heat; and the resulting paste is set aside to ripen, in damp cellars, for many months.When wanted for use, it is placed in hemispherical pans of plaster, which absorb the redundant moisture; after which it is divided into small lumps, and completely dried. It is next pulverized, moistened a little, and laid on a floor, and trodden upon by a workman marching over it with bare feet in every direction; the parings and fragments of soft moulded articles being intermixed, which improve the plasticity of the whole. When sufficiently tramped, it is made up into masses of the size of a man’s head, and kept damp till required.The dough is now in a state fit for the potter’s lathe; but it is much less plastic than stoneware paste, and is more difficult to fashion into the various articles; and hence one cause of the higher price of porcelain.The round plates and dishes are shaped on plaster moulds; but sometimes the paste is laid on as a crust, and at others it is turned into shape on the lathe. When a crust is to be made, a moistened sheep-skin is spread on a marble table; and over this the dough is extended with a rolling pin, supported on two guide-rules. The crust is then transferred over the plaster mould, by lifting it upon the skin; for it wants tenacity to bear raising by itself. When the piece is to be fashioned on the lathe, a lump of the dough is thrown on the centre of the horizontal wooden disc, and turned into form as directed in treating of stoneware, only it must be left much thicker than in its finished state. After it dries to a certain degree on the plaster mould, the workman replaces it on the lathe, by moistening it on its base with a wet sponge, and finishes its form with an iron tool. A good workman at Sèvres makes no more than from 15 to 20 porcelain plates in a day; whereas an English potter, with two boys, makes from 1000 to 1200 plates of stoneware in the same time. The pieces which are not round, are shaped in plaster moulds, and finished by hand. When the articles are very large, as wash-hand basins, salads, &c., a flat cake is spread above a skin on the marble slab, which is then applied to the mould with the sponge, as for plates; and they are finished by hand.The projecting pieces, such as handles, beaks, spouts, and ornaments, are moulded and adjusted separately; and are cemented to the bodies of china-ware with slip, or porcelain dough thinned with water. In fact, the mechanical processes with porcelain and the finer stoneware are substantially the same; only they require more time and greater nicety. The least defect in the fabrication, the smallest bit added, an unequal pressure, the cracks of the moulds, although well repaired, and seemingly effaced in the clay shape, re-appear after it is baked. The articles should be allowed to dry very slowly; if hurried but a little, they are liable to be spoiled. When quite dry, they are taken to the kiln.The kiln for hard porcelain at Sèvres, is a kind of tower in two flats, constructed offire-bricks; and resembles, in other respects, the stoneware kiln already figured and described. The fuel is young aspin wood, very dry, and cleft very small; it is put into the apertures of the four outside furnaces or fire-mouths, which discharge their flame into the inside of the kiln; each floor being closed in above, by a dome pierced with holes. The whole is covered in by a roof with an open passage, placed at a proper distance from the uppermost dome. There is, therefore, no chimney proper so called. SeeStone, artificial.The raw pieces are put into the upper floor of the kiln; where they receive a heat of about the 60th degree of Wedgewood’s pyrometer, and a commencement of baking, which, without altering their shape, or causing a perceptible shrinking of their bulk, makes them completely dry, and gives them sufficient solidity to bear handling. By this preliminary baking, the clay loses its property of forming a paste with water; and the pieces become fit for receiving the glazing coat, as they may be dipped in water without risk of breakage.The glaze of hard porcelain is a felspar rock: this being ground to a very fine powder, is worked into a paste with water mingled with a little vinegar. All the articles are dipped into this milky liquid for an instant; and as they are very porous, they absorb the water greedily, whereby a layer of the felspar glaze is deposited on their surface, in a nearly dry state, as soon as they are lifted out. Glaze-pap is afterwards applied with a hair brush to the projecting edges, or any points where it had not taken; and the powder is then removed from the part on which the article is to stand, lest it should get fixed to its support in the fire. After these operations it is replaced in the kiln, to be completely baked.The articles are put into saggers, like those of fine stoneware; and this operation is one of the most delicate and expensive in the manufacture of porcelain. The saggers are made of the plastic or potter’s clay of Abondant, to which about a third part of cement of broken saggers has been added.As the porcelain pieces soften somewhat in the fire, they cannot be set above each other, even were they free from glaze; for the same reason, they cannot be baked on tripods, several of them being in one case, as is done with stoneware. Every piece of porcelain requires a sagger for itself. They must, moreover, be placed on a perfectly flat surface, because in softening they would be apt to conform to the irregularities of a rough one. When therefore any piece, a soup plate for example, is to besaggered, there is laid on the bottom of the case a perfectly true disc or round cake of stoneware, made of the sagger material, and it is secured in its place on three small props of a clay-lute, consisting of potter’s clay mixed with a great deal of sand. When the cake is carefully levelled, it is moistened, and dusted over with sand, or coated with a film of fire-clay slip, and the porcelain is carefully set on it. The sand or fire-clay hinders it from sticking to the cake. Several small articles may be set on the same cake, provided they do not touch one another.The saggers containing the pieces thus arranged, are piled up in the kiln over each other, in the columnar form, till the whole space be occupied; leaving very moderate intervals between the columns to favour the draught of the fires. The whole being arranged with these precautions, and several others, too minute to be specified here, the door of the kiln is built up with 3 rows of bricks, leaving merely an opening 8 inches square, through which there is access to a sagger with the nearest side cut off. In this sagger are put fragments of porcelain intended to be withdrawn from time to time, in order to judge of the progress of the baking. These are called time-pieces or watches (montres). This opening into the watches is closed by a stopper of stoneware.The firing begins by throwing into the furnace-mouths some pretty large pieces of white wood, and the heat is maintained for about 15 hours, gradually raising it by the addition of a larger quantity of the wood, till at the end of that period the kiln has a cherry-red colour within. The heat is now greatly increased by the operation termedcovering the fire. Instead of throwing billets vertically into the four furnaces, there is placed horizontally on the openings of these furnaces, aspin wood of a sound texture, cleft small, laid in a sloping position. The brisk and long flame which it yields dips into the tunnels, penetrates the kiln, and circulates round the sagger-piles. The heat augments rapidly, and, at the end of 13 or 15 hours of this firing, the interior of the kiln is so white, that the watches can hardly be distinguished. The draught, indeed, is so rapid at this time, that one may place his hand on the slope of the wood without feeling incommoded by the heat. Every thing is consumed, no small charcoal remains, smoke is no longer produced, and even the wood-ash is dissipated. It is obvious that the kiln and the saggers must be composed of a very refractory clay, in order to resist such a fire. The heat in the Sèvres kilns mounts so high as the 134th degree of Wedgewood.At the end of 15 or 20 hours of the great fire; that is, after from 30 to 36 hours’ firing, the porcelain is baked; as is ascertained by taking out and examining thewatches. The kiln is suffered to cool during 3 or 4 days, and is then opened and discharged. The sand strewed on the cakes to prevent the adhesion of the articles to them, gets attached to their sole, and is removed by friction with a hard sandstone; an operation which one woman can perform for a whole kiln in less than 10 days; and is the last applied to hard porcelain, unless it needs to be returned into the hot kiln to have some defects repaired.The materials of fine porcelain are very rare; and there would be no advantage in making a gray-white porcelain with coarser and somewhat cheaper materials, for the other sources of expense above detailed, and which are of most consequence, would still exist; while the porcelain, losing much of its brightness, would lose the main part of its value.Its pap or dough, which requires tedious grinding and manipulation, is also more difficult to work into shapes, in the ratio of 80 to 1, compared to fine stoneware. Each porcelain plate requires a separate sagger; so that 12 occupy in the kiln a space sufficient for at least 38 stoneware plates. The temperature of a hard porcelain kiln being very high, involves a proportionate consumption of fuel and waste of saggers. With 40steres(cubic metres) of wood, 12,000 stoneware plates may be completely fired, both in the biscuit and glaze kilns; while the same quantity of wood would bake at most only 1000 plates of porcelain.To these causes of high price, which are constant and essential, we ought to add the numerous accidents to which porcelain is exposed at every step of its preparation, and particularly in the kiln; these accidents damage upwards of one-third of the pieces, and frequently more, when articles of singular form and large dimensions are adventured.The best English porcelain is made from a mixture of the Cornish kaolin (called china clay), ground flints, ground Cornish stone, and calcined bones in powder, or bone-ash, besides some other materials, according to the fancy of the manufacturers. A liquid pap is made with these materials, compounded in certain proportions, and diluted with water. The fluid part is then withdrawn by the absorbent action of dry stucco basins or pans. The dough, brought to a proper stiffness, and perfectly worked and kneaded on the principles detailed above, is fashioned on the lathe, by the hands of modellers, or by pressure in moulds. The pieces are then baked to the state of biscuit in a kiln, being enclosed, of course, in saggers.This biscuit has the aspect of white sugar, and being very porous, must receive a vitreous coating. The glaze consists of ground felspar or Cornish stone. Into this, diffused in water, along with a little flint-powder and potash, the biscuit ware is dipped, as already described, under stoneware. The pieces are then fired in the glaze-kiln, care being taken, before putting them into their saggers, to remove the glaze powder from their bottom parts, to prevent their adhesion to the fire-clay vessel.TENDER PORCELAIN.Tender porcelain, or soft china-ware, is made with a vitreous frit, rendered less fusible and opaque by an addition of white marl or bone-ash. The frit is, therefore, first prepared. This, at Sèvres, is a composition, made with some nitre, a little sea salt, Alicant barilla, alum, gypsum, and much siliceous sand or ground flints. That mixture is subjected to an incipient pasty fusion in a furnace, where it is stirred about to blend the materials well; and thus a very white spongy frit is obtained. It is pulverized, and to every three parts of it, one of the white marl of Argenteuil is added; and when the whole are well ground, and intimately mixed, the paste of tender porcelain is formed.As this paste has no tenacity, it cannot bear working till a mucilage of gum or black soap be added, which gives it a kind of plasticity, though even then it will not bear the lathe. Hence it must be fashioned in the press, between two moulds of plaster. The pieces are left thicker than they should be; and when dried, are finished on the lathe with iron tools.In this state they are baked, without any glaze being applied; but as this porcelain softens far more during the baking than the hard porcelain, it needs to be supported on every side. This is done by baking on earthen moulds all such pieces as can be treated in this way, namely plates, saucers, &c. The pieces are reversed on these moulds, and undergo their shrinkage without losing their form. Beneath other articles, supports of a like paste are laid, which suffer in baking the same contraction as the articles, and of course can serve only once. In this operation saggers are used, in which the pieces and their supports are fired.The kiln for the tender porcelain at Sèvres is absolutely similar to that for the common stoneware; but it has two floors; and while the biscuit is baked in the lower story, the glaze is fused in the upper one; which causes considerable economy of fuel. The glaze of soft porcelain is a species of glass or crystal prepared on purpose. It is composed of flint, siliceous sand, a little potash or soda, and about two-fifth partsof lead oxide. This mixture is melted in crucibles or pots beneath the kiln. The resulting glass is ground fine, and diffused through water mixed with a little vinegar to the consistence of cream. All the pieces of biscuit are covered with this glazy matter, by pouring this slip over them, since their substance is not absorbent enough to take it on by immersion.The pieces are encased once more each in a separate sagger, but without any supports; for the heat of the upper floor of the kiln, though adequate to melt the glaze, is not strong enough to soften the biscuit. But as this first vitreous coat is not very equal, a second one is applied, and the pieces are returned to the kiln for the third time. SeeStone, artificial, for a view of this kiln.The manufacture of soft porcelain is longer and more difficult than that of hard; its biscuit is dearer, although the raw materials may be found every where; and it furnishes also more refuse. Many of the pieces split asunder, receive fissures, or become deformed in the biscuit-kiln, in spite of the supports; and this vitreous porcelain, moreover, is always yellower, more transparent, and incapable of bearing rapid transitions of temperature, so that even the heat of boiling water frequently cracks it. It possesses some advantages as to painting, and may be made so gaudy and brilliant in its decorations, as to captivate the vulgar eye.DESCRIPTION OF THE PORCELAIN MILL.Porcelain millFig. 900 enlarged(192 kB)1. The following figures of a felspar and flint mill are taken from plans of apparatus lately constructed by Mr. Hall of Dartford, and erected by him in the royal manufactory of Sèvres. There are two similar sets of apparatus,fig.900., which may be employed together or in succession; composed each of an elevated tubA, and of three successive vats of receptionA′, and two behind it, whose top edges are upon a lower level than the bottom of the casksA,A, to allow of the liquid running out of them with a sufficient slope. A proper charge of kaolin is first put into the caskA, then water is gradually run into it by the gutter adapted to the stopcocka, after which the mixture is agitated powerfully in every direction by hand with the stirring-bar, which is hung within a hole in the ceiling, and has at its upper end a small tin-plate funnel to prevent dirt or rust from dropping down into the clay. The stirrer may be raised or lowered so as to touch any part of the cask. The semi-fluid mass is left to settle for a few minutes, and then the finer argillaceous pap is run off by the stopcocka′, placed a little above the gritty deposit, into the zinc pipe which conveys it into one of the tubsA′; but as this semi-liquid matter may still contain some granular substances, it must be passed through a sieve before it is admitted into the tub. There is, therefore, at the spot upon the tub where the zinc pipe terminates, a wire-cloth sieve, of an extremely close texture, to receive the liquid paste. This sieve is shaken upon its support, in order to make it discharge the washed argillaceous kaolin. After the clay has subsided, the water is drawn off from its surface by a zinc syphon. The vatsA′ have covers, to protect their contents from dust. In the pottery factories of England, the agitation is produced by machinery, instead of the hand. A vertical shaft, with horizontal or oblique paddles, is made to revolve in the vats for this purpose.Small triturating wheelThe small triturating millis represented infig.901.There are three similar grinding-tubs on the same line. The details of the construction are shown infigs.902,903, where it is seen to consist principally of a revolving millstoneB(fig.902.), of a fast or sleeper millstoneB′, and of a vatC, hooped with iron, with its top raised above the upper millstone. The lower block of hornstone rests upon a very firm basis,b′; it is surrounded immediately by the strong wooden circlec, which slopes out funnel-wise above, in order to throw back the earthy matters as they are pushed up by the attritionof the stones. That piece is hollowed out, partially to admit the keyC, opposite to which is the faucet and spigotc′, for emptying the tub. When one operation is completed, the keyCis lifted out by means of a peg put into the holes at its top; the spigot is then drawn, and the thin paste is run out into vats. The upper grindstone,Bd, like the lower one, is about two feet in diameter, and must be cut in a peculiar manner. At first there is scooped out a hollowing in the form of a sector, denoted byd e f,fig.903.; the arcd fis about one-sixth of the circumference, so that the vacuity of the turning grindstone is one-sixth of its surface; moreover, the stone must be channelled, in order to grind or crush the hard gritty substances. For this purpose, a wedge-shaped grooved e g, about an inch and a quarter deep, is made on its under face, whereby the stone, as it turns in the direction indicated by the arrow, acts with this inclined plane upon all the particles in its course, crushing them and forcing them in between the stones, till they be triturated to an impalpable powder. When the grindstone wears unequally on its lower surface, it is useful to trace upon it little furrows, proceeding from the centre to the circumference, like those shown by the dotted linese′e′′. It must, moreover, be indented with rough points by the hammer.Details of millThe turning hornstone-block is set in motion by the vertical shaftH, which is fixed by the clamp-iron crossIto the top of the stone. When the stone is new, its thickness is about 14 inches, and it is made to answer for grinding till it be reduced to about 8 inches, by lowering the clampIupon the shaft, so that it may continue to keep its hold of the stone. The manner in which the grindstones are turned, is obvious from inspection offig.901., where the horizontal axisL, which receives its impulsion from the great water-wheel, turns the prolonged shaftL′, or leaves it at rest, according as the clutchl,l′, is locked or opened. This second shaft bears the three bevel wheelsM,M,M. These work in three corresponding bevel wheelsM′M′M′, made fast respectively to the three vertical shafts of the millstones, which pass through the cast-iron guide tubesM′′M′′. These are fixed in a truly vertical position by the collar-barm′′,m′,fig.902.In this figure we see atmhow the strong cross-bar of cast iron is made fast to the wooden beams which support all the upper mechanism of the mill-work. The bearingm′is disposed in an analogous manner; but it is supported against two cast-iron columns, shown atL′′L′′, infig.901.The guide tubesM′′ are bored smooth for a small distance from each of their extremities, and their interjacent calibre is wider, so that the vertical shafts touch only at two places. It is obvious, that whenever the shaftL′ is set a-going, it necessarily turns the wheelsMandM′, and their guide tubesM′′; but the vertical shaft may remain either at rest, or revolve, according to the position of the lever click or catchK, at the top, which is made to slide upon the shaft, and can let fall a finger into a vertical groove cut in the surface of that shaft. The clamp-fork of the click is thus made to catch upon the horizontal bevel-wheelM′, or to release it, according as the leverKis lowered or lifted up. Thus each millstone may be thrown out of or into geer at pleasure.Safety clutchThese stones make upon an average 11 or 12 turns in a minute, corresponding to 3 revolutions of the water-wheel, which moves through a space of 3 feet 4 inches in the second, its outer circumference being 66 feet. The weight of the upper stone, with its iron mountings, is about 6 cwt., when new. The charge of each mill in dry material is 2 cwt.; and the water may be estimated at from one-half to the whole of this weight; whence the total load may be reckoned to be at least 3 cwt.; the stone, by displacement of the magma, loses fully 400 pounds of its weight, and weighs therefore in reality only 2 cwt. It is charged in successive portions, but it is discharged all at once. When the grinding of the siliceous or felspar matters is nearly complete, a remarkablephenomenon occurs; the substance precipitates to the bottom, and assumes in a few seconds so strong a degree of cohesion, that it is hardly possible to restore it again to the pasty or magma state; hence if a millstone turns too slowly, or if it be accidentally stopped for a few minutes, the upper stone gets so firmly cemented to the under one, that it is difficult to separate them. It has been discovered, but without knowing why, that a little vinegar added to the water of the magma almost infallibly prevents that sudden stiffening of the deposit and stoppage of the stones. If the mills come to be set fast in this way, the shafts or geering would be certainly broken, were not some safety provision to be made in the machinery against such accidents. Mr. Hall’s contrivance to obviate the above danger is highly ingenious. The clutchl,l′,fig.901., is not a locking crab, fixed in the common way, upon the shaftL; but it is composed, as shown infigs.904,905,906,907, of a hoopu, fixed upon the shaft by means of a key, of a collarv, and of a flat ring or washerx, with four projections, which are fitted to the collarv, by four boltsy.Fig.905.represents the collarvseen in front; that is, by the face which carries the clutch teeth; andfig.906.represents its other face, which receives the flat ringx,fig.907., in four notches corresponding to the four projections of the washer-ring. Since the ringuis fixed upon the shaftL, and necessarily turns with it, it has the two other pieces at its disposal, namely the collarv, and the washerx, because they are always connected with it by the four boltsy, so as to turn with the ringu, when the resistance they encounter upon the shaftL′ is not too great, and to remain at rest, letting the ringuturn by itself, when that resistance increases to a certain pitch. To give this degree of friction, we need only interpose the leather washersz,z′,fig.904.; and now as the collarcoupling-box,v, slides pretty freely upon the ringu, it is obvious that by tightening more or less the screw boltsy, these washers will become as it were a lateral brake, to tighten more or less the bearing of the ringu, to which they are applied: by regulating this pressure, every thing may be easily adjusted. When the resistance becomes too great, the leather washers, pressed upon one side by the collarv, of the washerx, and rubbed upon the other side by the prominence of the ringu, get heated to such a degree, that they are apt to become carbonized, and require replacement.This safety clutch may be recommended to the notice of mechanicians, as susceptible of beneficial application in a variety of circumstances.GREAT PORCELAIN MILL.The large felspar and kaolin mill, made by Mr. Hall, for Sèvres, has a flat bed of hornstone, in one block, laid at the bottom of a great tub, hooped strongly with iron. In most of the English potteries, however, that bed consists of several flat pieces of chert or hornstone, laid level with each other. There is, as usual, a spigot and faucet at the side, for drawing off the liquid paste. The whole system of the mechanism is very substantial, and is supported by wooden beams.The following is the manner of turning the upper blocks. Infig.900.the main horizontal shaftPbears at one of its extremities a toothed wheel, usually mounted upon the periphery of the great water-wheel (fig.908.shows this toothed wheel by a dotted line) at its other end;Pcarries the fixed portionpof a coupling-box, similar to the one just described as belonging to the little mill. On the prolongation ofP, there is a second shaftP′, which bears the movable portion of that box, and an upright bevel wheelP′′. Lastly, infigs.900.and908.there is shown the vertical shaftQ, which carries at its upper end a large horizontal cast-iron wheelQ′, not seen in this view, because it is sunk within the upper surface of the turning hornstone, like the clampd,f, infig.902.At the lower end of the shaftQ, there is the bevel wheelQ′′, which receives motion from the wheelP′′,fig.900.The shaftPalways revolves with the water-wheel; but transmits its motion to theshaftP′ only when the latter is thrown into geer with the coupling-boxp′, by means of its forked lever. Then the bevel wheelP′ turns round with the shaftP′, and communicates its rotation to the bevel wheelQ′′, which transmits it to the shaftQ, and to the large cast-iron wheel, which is sunk into the upper surface of the revolving hornstone.Porcelain millThe shaftQis supported and centred by a simple and solid adjustment; at its lower part, it rests in a stepR, which is supported upon a cast-iron archQ′, seen in profile infig.900.; its base is solidly fixed by four strong bolts. Four set screws aboveR,fig.900., serve to set the shaftQtruly perpendicular: thus supported, and held securely at its lower end, in the step atR,figs.900.and908., it is embraced near the upper end by a brass bush or collar, composed of two pieces, which may be drawn closer together by means of a screw. This collar is set into the summit of a great truncated cone of cast-iron, which rises within the tub through two-thirds of the thickness of the hornstone bed; having its base firmly fixed by bolts to the bottom of the tub, and having a brass collet to secure its top. The iron cone is cased in wood. When all these pieces are well adjusted and properly screwed up, the shaftQrevolves without the least vacillation, and carries round with it the large iron wheelQ′, cast in one piece, and which consists of an outer rim, three arms or radii, and a strong central nave, made fast by a key to the top of the shaftQ, and resting upon a shoulder nicely turned to receive it. Upon each of the three arms, there are adjusted, with bolts, three upright substantial bars of oak, which descend vertically through the body of the revolving mill to within a small distance of the bed-stone; and upon each of the three arcs of that wheel-ring, comprised between its three strong arms, there are adjusted, in like manner, five similar uprights, which fit into hollows cut in the periphery of the moving stone. They ought to be cut to a level at their lower part, to suit the slope of the bottom of the tubo,figs.900.and908., so as to glide past it pretty closely, without touching.The speed of this large mill is eight revolutions in the minute. The turning hornstone describes a mean circumference of 1411⁄3inches (its diameter being 45 inches), and of course moves through about 100 feet per second. The tubO, is 52 inches wide at bottom, 56 at the surface of the sleeper block (which is 16 inches thick), and 64 at top, inside measure. It sometimes happens that the millstone throws the pasty mixture out of the vessel, though its top is 6 inches under the lip of the tubo; an inconvenience which can be obviated only by making the pap a little thicker; that, is by allowing only from 25 to 30 per cent. of water; then its density becomes nearly equal to 2·00, while that of the millstones themselves is only 2·7; whence, supposing them to weigh only 2 cwt., there would remain an effective weight of less than1⁄2cwt. for pressing upon the bottom and grinding the granular particles. This weight appears to be somewhat too small to do much work in a short time; and therefore it would be better to increase the quantity of water, and put covers of some convenient form over the tubs. It is estimated that this mill will grind nearly 5 cwt. of hard kaolin or felspar gravel, in 24 hours, into a proper pap.To the preceding methodical account of the porcelain manufacture, I shall now subjoin some practical details relative to certain styles of work, with comparisons between the methods pursued in this country and upon the Continent, but chiefly by our jealous rivals the French.The blue printed ware of England has been hitherto a hopeless object of emulation in France. M. Alexandre Brongniart, membre de l’Institut, and director of theManufacture Royal de Sèvres, characterizes the French imitations of theFayence fine, ou Anglaise, in the following terms: “Les défauts de cette poterie, qui tiennent à sa nature, sont de ne pouvoir aller sur le feu pour les usages domestiques, et d’avoir un vernis tendre, qui se laisse aisément entamer par les instruments d’acier et de fer. Mais lorsque cette poterie est mal fabriquée, ou fabriquée avec une économie mal entendue, ses défauts deviennent bien plus graves; son vernis jaunâtre et tendre tressaille souvent; il se laisse entamer ou user avec la plus grande facilité par les instruments de fer, ou par l’usage ordinaire. Les fissures que ce tressaillement ou ces rayures ouvrent dans le vernis permettent aux matières grasses de pénétrer dans le biscuit, que dans les poteries affectées de ce défaut, a presque toujours une texture lâche; les pièces se salissent, s’empuantissent, et se brisent même avec la plus grande facilité.”[42][42]Dict. Technologique, tom. xvii., article Poteries, p. 253.What a glaze, to be scratched or grooved with soft iron; to fly off in scales, so as to let grease soak into the biscuit or body of the ware; to become foul, stink, and break with the utmost ease! The refuse crockery of the coarsest pottery works in the United Kingdom would hardly deserve such censure.In the minutes of evidence of theEnquête Ministérielle, published in 1835, MM. de Saint Cricq and Lebeuf, large manufacturers of pottery-ware at Creil and Montereau, give a very gratifying account of the English stoneware manufacture. They declare that the English possess magnificent mines of potter’s clay, many leagues in extent; while those of theFrench are mere patches orpots. Besides, England, they say, having upwards of 200 potteries, can constantly employ a great many public flint-mills, and thereby obtain that indispensable material of the best quality, and at the lowest rate. “The mill erected by M. Brongniart, at Sèvres, does its work at twice the price of the English mills. The fuel costs in England one-fourth of what it does in France. The expense of a kiln-round, in the latter country, is 200 francs; while in the former it is not more than 60.” After a two-months tour among the English potteries, these gentlemen made the following additional observations to their first official statement:—“The clay, which goes by water carriage from the counties of Devon and Dorset, into Staffordshire, to supply more than 200 potteries, clustered together, is delivered to them at a cost of 4 francs (3s.2d.) the 100 kilogrammes (2 cwt.); at Creil, it costs 4f.50c., and at Montereau, only 2f.40c.There appears, therefore, to be no essential difference in the price of the clay; but the quality of the English is much superior, being incontestably whiter, purer, more homogeneous, and not turning red at a high heat, like the French.” The grinding of the flints costs the English potter 41⁄2d.per 100 kilos., and the French 6d.; but as that of the latter is in general ground dry, it is a coarser article. The kaolin, or china clay, is imported from Cornwall for the use of many French potteries; but the transport of merchandise is so ill managed in France, that while 2 cwts. cost in Staffordshire only 8f.75c.(about 7s.1d.), they cost 12f.at Creil, and 13f. 50c.at Montereau. The white lead and massicot, so much employed for glazes, are 62 per cent. dearer to the French potters than the English. As no French mill has succeeded in making unsized paper fit for printing upon stoneware, our potters are under the necessity of fetching it from England; and, under favour of our own custom-house, are allowed to import it at a duty of 165f.per 100 kilogrammes, or about 8d.per pound English. No large stock of materials need be kept by the English, because every article may be had when wanted from its appropriate wholesale dealers; but the case is quite different with the French, whose stocks, even in small works, can never safely be less in value than 150,000f.or 200,000f.; constituting a loss to them, in interest upon their capital, of from 7,500f.to 10,000f.per annum. The capital sunk in buildings is far less in England than in France, in consequence of the different styles of erecting stoneware factories in the two countries. M. de Saint Cricq informs us, that Mr. Clewes, of Shelton, rents his works for 10,000f.(380l.) per annum; while the similar ones of Creil and Montereau, in France, have cost each a capital outlay of from 500,000f.to 600,000f., and in which the products are not more than one half of Mr. Clewes’. “This forms a balance against us,” says M. St. C., “of about 20,000f.per annum; or nearly 800l.sterling. Finally, we have the most formidable rival to our potteries in the extreme dexterity of the English artisans. An enormous fabrication permits the manufacturers to employ the same workmen during the whole year upon the same piece; thus I have seen at Shelton a furnisher, for sixpence, turn off 100 pieces, which cost at Creil and Montereau 30 sous (1s.21⁄2d.); yet the English workman earns 18f.75c.a week, while the French never earns more than 15f.I have likewise seen an English moulder expert enough to make 25 waterpots a day, which, at the rate of 2d.a piece, bring him 4s.2d.of daily wages; while the French moulder, at daily wages also of 4s.2d., turns out of his hands only 7, or at most 8 pots. In regard to hollow wares, the English may be fairly allowed to have an advantage over us, in the cost of labour, of 100 per cent.; which they derive from the circumstance, that there are in Staffordshire 60,000 operatives, men, women, and children, entirely dedicated to the stoneware manufacture; concentrating all their energies within a space of 10 square leagues. Hence a most auspicious choice of good practical potters, which cannot be found in France.”M. Saint Amans, a French gentleman, who spent some years in Staffordshire, and has lately erected a large pottery in France, says the English surpass all other nations in manufacturing a peculiar stoneware, remarkable for its lightness, strength, and elegance; as also in printing blue figures upon it of every tint, equal to that of the Chinese, by processes of singular facility and promptitude. After the biscuit is taken out of the kiln, the fresh impression of the engraving is transferred to it from thin unsized paper, previously immersed in strong soap water; the ink for this purpose being a compound of arseniate of cobalt with a flux, ground up with properly boiled linseed oil. The copper-plates are formed by the graving tool with deeper or shallower lines according to the variable depth of shades in the design. The cobalt pigment, on melting, spreads so as to give the soft effect of water-colour drawing. The paper being still moist, is readily applied to the slightly rough and adhesive surface of the biscuit, and may be rubbed on more closely by a dossil of flannel. The piece is then dipped in a tub of water, whereby the paper gets soft, and may be easily removed, leaving upon the pottery the pigment of the engraved impression. After being gently dried, the piece is dipped into the glaze mixture, and put into the enamel oven.Composition of the Earthy Mixtures.The basis of the English stoneware is, as formerly stated, a bluish clay, brought from Dorsetshire and Devonshire, which lies at the depth of from 25 to 30 feet beneath the surface. It is composed of about 24 parts of alumina, and 76 of silica, with some other ingredients in very small proportions. This clay is very refractory in high heats, a property which, joined to its whiteness when burned, renders it peculiarly valuable for pottery. It is also the basis of all the yellow biscuit-ware calledcream colour, and in general of what is called theprinting body; as also for the semi-vitrified porcelain of Wedgewood’s invention, and of the tender porcelain.The constituents of the stoneware are, that clay, the powder of calcined flints, and of the decomposed felspar called Cornish stone. The proportions are varied by the different manufacturers. The following are those generally adopted in one of the principal establishments of Staffordshire:—Forcream colour,Silex or ground flints20parts.Clay100Cornish stone2Composition of the Paste for receiving the Printing Body under the Glaze.For this purpose the proportions of the flint and the felspar must be increased. The substances are mixed separately with water into the consistence of a thick cream, which weighs per pint, for the flints 32 ounces, and for the Cornish stone 28. The china clay of Cornwall is added to the same mixture of flint and felspar, when a finer pottery or porcelain is required. That clay cream weighs 24 ounces per pint. These 24 ounces in weight are reduced to one-third of their bulk by evaporation. The pint of dry Cornish clay weighs 17 ounces, and in its first pasty state 24, as just stated. The dry flint powder weighs 141⁄2ounces per pint; which when made into a cream weighs 32 ounces. To 40 measures of Devonshire clay-cream there are added,13measures offlint liquor.12—Cornish clay ditto.1—Cornish stone ditto.The whole are well mixed by proper agitation, half dried in thetroughsof the slip-kiln, and then subjected to the machine for cutting up the clay into junks. The above paste, when baked, is very white, hard, sonorous, and susceptible of receiving all sorts of impressions from the paper engravings. When the silica is mixed with the alumina in the above proportions, it forms a compact ware, and the impression remains fixed between the biscuit and the glaze, without communicating to either any portion of the tint of the metallic colour employed in the engraver’s press. The felspar gives strength to the biscuit, and renders it sonorous after being baked; while the china clay has the double advantage of imparting an agreeable whiteness and great closeness of grain.
Plan of pottery
Fig.899.A, is the entrance door;B, the porter’s lodge;C, a particular warehouse;D, workshop of the plaster-moulder;E, the clay depôt;F,F, large gates, 6 feet 8 inches high;G, the winter evaporation stove;H, the shop for sifting the paste liquors;I, sheds for the paste liquor tubs;J, paste liquor pits;K, workshop for the moulder of hollow ware;L, ditto of the dish or plate moulder;M, the plate drying-stove;N, workshop of the biscuit-printers;O, ditto of the biscuit, witho′, a long window;P, passage leading to the paste liquor pits;Q, biscuit warehouse;R, place where the biscuit is cleaned as it comes out of the biscuit kilns,S S;T,T, enamel or glaze kilns;U, long passage;V, space left for supplementary workshops;X, space appointed as a depôt for the sagger fire-clay, as also for making the saggers;Z, the workshop for applying the glaze liquor to the biscuits;a, apartment for cleaning the glazed ware;b,b, pumps;c, basin;d, muffles;e, warehouse for the finished stoneware;f, that of the glazed goods;g,g, another warehouse;h, a large space for the smith’s forge, carpenter’s shop, packing room, depôt of clays, saggers, &c. The packing and loading of the goods are performed in front of thewarehouse, which has two outlets, in order to facilitate the work;i, a passage to the court or yard;l, a space for the wooden sheds for keeping hay, clay, and other miscellaneous articles,m, room for putting the biscuit into the saggers;m′, a long window;n, workshop with lathes and fly-wheels;o, drying-room;p, room for mounting or furnishing the pieces;q, repairing room;r, drying room of the goods roughly turned;s, rough turning or blocking-out room;t, room for beating the paste or dough;u, counting-house.
The declared value of the earthenware exported in 1836, was 837,774l.; in 1837, 558,682l.
There are from 33,000 to 35,000 tons of clay exported annually from Poole, in Dorsetshire, to the English and Scotch potteries. A good deal of clay is also sent from Devonshire and Cornwall.
The Spanishalcarazzas, or cooling vessels, are made porous, to favour the exudation of water through them, and maintain a constantly moist evaporating surface. Lasteyrie says, that granular sea salt is an ingredient of the paste of the Spanish alcarazzas; which being expelled partly by the heat of the baking, and partly by the subsequent watery percolation, leaves the body very open. The biscuit should be charged with a considerable proportion of sand, and very moderately fired.
OF PORCELAIN.
Porcelain is a kind of pottery ware whose paste is fine grained, compact, very hard, and faintly translucid; and whose biscuit softens slightly in the kiln. Its ordinary whiteness cannot form a definite character, since there are porcelain pastes variously coloured. There are two species of porcelain, very different in their nature, the essential properties of which it is of consequence to establish; the one is calledhard, and the othertender; important distinctions, the neglect of which has introduced great confusion into many treatises on this elegant manufacture.
Hardporcelain is essentially composed, first, of a natural clay containing some silica, infusible, and preserving its whiteness in a strong heat; this is almost always a true kaolin; secondly, of a flux, consisting of silica and lime, composing a quartzose felspar rock, calledpe-tun-tse. The glaze of this porcelain, likewise earthy, admits of no metallic substance or alkali.
Tenderporcelain, styled also vitreous porcelain, has no relation with the preceding in its composition; it always consists of a vitreous frit, rendered opaque and less fusible by the addition of a calcareous or marly clay. Its glaze is an artificial glass or crystal, into which silica, alkalis, and lead enter.
This porcelain has a more vitreous biscuit, more transparent, a little less hard, and less fragile, but much more fusible than that of the hard porcelain. Its glaze is more glossy, more transparent, a little less white, much tenderer, and more fusible.
The biscuit of the hard porcelain made at the French national manufactory of Sèvres is generally composed of a kaolin clay, and of a decomposed felspar rock; analogous to the china clay of Cornwall, and Cornish stone. Both of the above French materials come from Saint Yriex-la-perche, near Limoges.
After many experiments, the following composition has been adopted for theservice pasteof the royal manufactory of Sèvres; that is, for all the ware which is to be glazed: silica, 59; alumina, 35·2; potash, 2·2; lime, 3·3. The conditions of such a compound are pretty nearly fulfilled by taking from 63 to 70 of the washed kaolin or china clay, 22 to 15 of the felspar; nearly 10 of flint powder, and about 5 of chalk. The glaze is composed solely of solid felspar, calcined, crushed, and then ground fine at the mill. This rock pretty uniformly consists of silica 73, alumina 16·2, potash 8·4, and water 0·6.
The kaolin is washed at the pit, and sent in this state to Sèvres, under the name ofdecanted earth. At the manufactory it is washed and elutriated with care; and its slip is passed through fine sieves. This forms the plastic, infusible, and opaque ingredient to which the substance must be added which gives it a certain degree of fusibility and semi-transparency. The felspar rock used for this purpose, should contain neither dark mica nor iron, either as an oxide or sulphuret. It is calcined to make it crushable, under stamp-pestles driven by machinery, then ground fine in hornstone mills, as represented infigs.897,898,899, and900.This pulverulent matter being diffused through water, is mixed in certain proportions, regulated by its quality, with the argillaceousslip. The mixture is deprived of the chief part of its water in shallow plaster pans without heat; and the resulting paste is set aside to ripen, in damp cellars, for many months.
When wanted for use, it is placed in hemispherical pans of plaster, which absorb the redundant moisture; after which it is divided into small lumps, and completely dried. It is next pulverized, moistened a little, and laid on a floor, and trodden upon by a workman marching over it with bare feet in every direction; the parings and fragments of soft moulded articles being intermixed, which improve the plasticity of the whole. When sufficiently tramped, it is made up into masses of the size of a man’s head, and kept damp till required.
The dough is now in a state fit for the potter’s lathe; but it is much less plastic than stoneware paste, and is more difficult to fashion into the various articles; and hence one cause of the higher price of porcelain.
The round plates and dishes are shaped on plaster moulds; but sometimes the paste is laid on as a crust, and at others it is turned into shape on the lathe. When a crust is to be made, a moistened sheep-skin is spread on a marble table; and over this the dough is extended with a rolling pin, supported on two guide-rules. The crust is then transferred over the plaster mould, by lifting it upon the skin; for it wants tenacity to bear raising by itself. When the piece is to be fashioned on the lathe, a lump of the dough is thrown on the centre of the horizontal wooden disc, and turned into form as directed in treating of stoneware, only it must be left much thicker than in its finished state. After it dries to a certain degree on the plaster mould, the workman replaces it on the lathe, by moistening it on its base with a wet sponge, and finishes its form with an iron tool. A good workman at Sèvres makes no more than from 15 to 20 porcelain plates in a day; whereas an English potter, with two boys, makes from 1000 to 1200 plates of stoneware in the same time. The pieces which are not round, are shaped in plaster moulds, and finished by hand. When the articles are very large, as wash-hand basins, salads, &c., a flat cake is spread above a skin on the marble slab, which is then applied to the mould with the sponge, as for plates; and they are finished by hand.
The projecting pieces, such as handles, beaks, spouts, and ornaments, are moulded and adjusted separately; and are cemented to the bodies of china-ware with slip, or porcelain dough thinned with water. In fact, the mechanical processes with porcelain and the finer stoneware are substantially the same; only they require more time and greater nicety. The least defect in the fabrication, the smallest bit added, an unequal pressure, the cracks of the moulds, although well repaired, and seemingly effaced in the clay shape, re-appear after it is baked. The articles should be allowed to dry very slowly; if hurried but a little, they are liable to be spoiled. When quite dry, they are taken to the kiln.
The kiln for hard porcelain at Sèvres, is a kind of tower in two flats, constructed offire-bricks; and resembles, in other respects, the stoneware kiln already figured and described. The fuel is young aspin wood, very dry, and cleft very small; it is put into the apertures of the four outside furnaces or fire-mouths, which discharge their flame into the inside of the kiln; each floor being closed in above, by a dome pierced with holes. The whole is covered in by a roof with an open passage, placed at a proper distance from the uppermost dome. There is, therefore, no chimney proper so called. SeeStone, artificial.
The raw pieces are put into the upper floor of the kiln; where they receive a heat of about the 60th degree of Wedgewood’s pyrometer, and a commencement of baking, which, without altering their shape, or causing a perceptible shrinking of their bulk, makes them completely dry, and gives them sufficient solidity to bear handling. By this preliminary baking, the clay loses its property of forming a paste with water; and the pieces become fit for receiving the glazing coat, as they may be dipped in water without risk of breakage.
The glaze of hard porcelain is a felspar rock: this being ground to a very fine powder, is worked into a paste with water mingled with a little vinegar. All the articles are dipped into this milky liquid for an instant; and as they are very porous, they absorb the water greedily, whereby a layer of the felspar glaze is deposited on their surface, in a nearly dry state, as soon as they are lifted out. Glaze-pap is afterwards applied with a hair brush to the projecting edges, or any points where it had not taken; and the powder is then removed from the part on which the article is to stand, lest it should get fixed to its support in the fire. After these operations it is replaced in the kiln, to be completely baked.
The articles are put into saggers, like those of fine stoneware; and this operation is one of the most delicate and expensive in the manufacture of porcelain. The saggers are made of the plastic or potter’s clay of Abondant, to which about a third part of cement of broken saggers has been added.
As the porcelain pieces soften somewhat in the fire, they cannot be set above each other, even were they free from glaze; for the same reason, they cannot be baked on tripods, several of them being in one case, as is done with stoneware. Every piece of porcelain requires a sagger for itself. They must, moreover, be placed on a perfectly flat surface, because in softening they would be apt to conform to the irregularities of a rough one. When therefore any piece, a soup plate for example, is to besaggered, there is laid on the bottom of the case a perfectly true disc or round cake of stoneware, made of the sagger material, and it is secured in its place on three small props of a clay-lute, consisting of potter’s clay mixed with a great deal of sand. When the cake is carefully levelled, it is moistened, and dusted over with sand, or coated with a film of fire-clay slip, and the porcelain is carefully set on it. The sand or fire-clay hinders it from sticking to the cake. Several small articles may be set on the same cake, provided they do not touch one another.
The saggers containing the pieces thus arranged, are piled up in the kiln over each other, in the columnar form, till the whole space be occupied; leaving very moderate intervals between the columns to favour the draught of the fires. The whole being arranged with these precautions, and several others, too minute to be specified here, the door of the kiln is built up with 3 rows of bricks, leaving merely an opening 8 inches square, through which there is access to a sagger with the nearest side cut off. In this sagger are put fragments of porcelain intended to be withdrawn from time to time, in order to judge of the progress of the baking. These are called time-pieces or watches (montres). This opening into the watches is closed by a stopper of stoneware.
The firing begins by throwing into the furnace-mouths some pretty large pieces of white wood, and the heat is maintained for about 15 hours, gradually raising it by the addition of a larger quantity of the wood, till at the end of that period the kiln has a cherry-red colour within. The heat is now greatly increased by the operation termedcovering the fire. Instead of throwing billets vertically into the four furnaces, there is placed horizontally on the openings of these furnaces, aspin wood of a sound texture, cleft small, laid in a sloping position. The brisk and long flame which it yields dips into the tunnels, penetrates the kiln, and circulates round the sagger-piles. The heat augments rapidly, and, at the end of 13 or 15 hours of this firing, the interior of the kiln is so white, that the watches can hardly be distinguished. The draught, indeed, is so rapid at this time, that one may place his hand on the slope of the wood without feeling incommoded by the heat. Every thing is consumed, no small charcoal remains, smoke is no longer produced, and even the wood-ash is dissipated. It is obvious that the kiln and the saggers must be composed of a very refractory clay, in order to resist such a fire. The heat in the Sèvres kilns mounts so high as the 134th degree of Wedgewood.
At the end of 15 or 20 hours of the great fire; that is, after from 30 to 36 hours’ firing, the porcelain is baked; as is ascertained by taking out and examining thewatches. The kiln is suffered to cool during 3 or 4 days, and is then opened and discharged. The sand strewed on the cakes to prevent the adhesion of the articles to them, gets attached to their sole, and is removed by friction with a hard sandstone; an operation which one woman can perform for a whole kiln in less than 10 days; and is the last applied to hard porcelain, unless it needs to be returned into the hot kiln to have some defects repaired.
The materials of fine porcelain are very rare; and there would be no advantage in making a gray-white porcelain with coarser and somewhat cheaper materials, for the other sources of expense above detailed, and which are of most consequence, would still exist; while the porcelain, losing much of its brightness, would lose the main part of its value.
Its pap or dough, which requires tedious grinding and manipulation, is also more difficult to work into shapes, in the ratio of 80 to 1, compared to fine stoneware. Each porcelain plate requires a separate sagger; so that 12 occupy in the kiln a space sufficient for at least 38 stoneware plates. The temperature of a hard porcelain kiln being very high, involves a proportionate consumption of fuel and waste of saggers. With 40steres(cubic metres) of wood, 12,000 stoneware plates may be completely fired, both in the biscuit and glaze kilns; while the same quantity of wood would bake at most only 1000 plates of porcelain.
To these causes of high price, which are constant and essential, we ought to add the numerous accidents to which porcelain is exposed at every step of its preparation, and particularly in the kiln; these accidents damage upwards of one-third of the pieces, and frequently more, when articles of singular form and large dimensions are adventured.
The best English porcelain is made from a mixture of the Cornish kaolin (called china clay), ground flints, ground Cornish stone, and calcined bones in powder, or bone-ash, besides some other materials, according to the fancy of the manufacturers. A liquid pap is made with these materials, compounded in certain proportions, and diluted with water. The fluid part is then withdrawn by the absorbent action of dry stucco basins or pans. The dough, brought to a proper stiffness, and perfectly worked and kneaded on the principles detailed above, is fashioned on the lathe, by the hands of modellers, or by pressure in moulds. The pieces are then baked to the state of biscuit in a kiln, being enclosed, of course, in saggers.
This biscuit has the aspect of white sugar, and being very porous, must receive a vitreous coating. The glaze consists of ground felspar or Cornish stone. Into this, diffused in water, along with a little flint-powder and potash, the biscuit ware is dipped, as already described, under stoneware. The pieces are then fired in the glaze-kiln, care being taken, before putting them into their saggers, to remove the glaze powder from their bottom parts, to prevent their adhesion to the fire-clay vessel.
TENDER PORCELAIN.
Tender porcelain, or soft china-ware, is made with a vitreous frit, rendered less fusible and opaque by an addition of white marl or bone-ash. The frit is, therefore, first prepared. This, at Sèvres, is a composition, made with some nitre, a little sea salt, Alicant barilla, alum, gypsum, and much siliceous sand or ground flints. That mixture is subjected to an incipient pasty fusion in a furnace, where it is stirred about to blend the materials well; and thus a very white spongy frit is obtained. It is pulverized, and to every three parts of it, one of the white marl of Argenteuil is added; and when the whole are well ground, and intimately mixed, the paste of tender porcelain is formed.
As this paste has no tenacity, it cannot bear working till a mucilage of gum or black soap be added, which gives it a kind of plasticity, though even then it will not bear the lathe. Hence it must be fashioned in the press, between two moulds of plaster. The pieces are left thicker than they should be; and when dried, are finished on the lathe with iron tools.
In this state they are baked, without any glaze being applied; but as this porcelain softens far more during the baking than the hard porcelain, it needs to be supported on every side. This is done by baking on earthen moulds all such pieces as can be treated in this way, namely plates, saucers, &c. The pieces are reversed on these moulds, and undergo their shrinkage without losing their form. Beneath other articles, supports of a like paste are laid, which suffer in baking the same contraction as the articles, and of course can serve only once. In this operation saggers are used, in which the pieces and their supports are fired.
The kiln for the tender porcelain at Sèvres is absolutely similar to that for the common stoneware; but it has two floors; and while the biscuit is baked in the lower story, the glaze is fused in the upper one; which causes considerable economy of fuel. The glaze of soft porcelain is a species of glass or crystal prepared on purpose. It is composed of flint, siliceous sand, a little potash or soda, and about two-fifth partsof lead oxide. This mixture is melted in crucibles or pots beneath the kiln. The resulting glass is ground fine, and diffused through water mixed with a little vinegar to the consistence of cream. All the pieces of biscuit are covered with this glazy matter, by pouring this slip over them, since their substance is not absorbent enough to take it on by immersion.
The pieces are encased once more each in a separate sagger, but without any supports; for the heat of the upper floor of the kiln, though adequate to melt the glaze, is not strong enough to soften the biscuit. But as this first vitreous coat is not very equal, a second one is applied, and the pieces are returned to the kiln for the third time. SeeStone, artificial, for a view of this kiln.
The manufacture of soft porcelain is longer and more difficult than that of hard; its biscuit is dearer, although the raw materials may be found every where; and it furnishes also more refuse. Many of the pieces split asunder, receive fissures, or become deformed in the biscuit-kiln, in spite of the supports; and this vitreous porcelain, moreover, is always yellower, more transparent, and incapable of bearing rapid transitions of temperature, so that even the heat of boiling water frequently cracks it. It possesses some advantages as to painting, and may be made so gaudy and brilliant in its decorations, as to captivate the vulgar eye.
DESCRIPTION OF THE PORCELAIN MILL.
Porcelain millFig. 900 enlarged(192 kB)
Fig. 900 enlarged(192 kB)
1. The following figures of a felspar and flint mill are taken from plans of apparatus lately constructed by Mr. Hall of Dartford, and erected by him in the royal manufactory of Sèvres. There are two similar sets of apparatus,fig.900., which may be employed together or in succession; composed each of an elevated tubA, and of three successive vats of receptionA′, and two behind it, whose top edges are upon a lower level than the bottom of the casksA,A, to allow of the liquid running out of them with a sufficient slope. A proper charge of kaolin is first put into the caskA, then water is gradually run into it by the gutter adapted to the stopcocka, after which the mixture is agitated powerfully in every direction by hand with the stirring-bar, which is hung within a hole in the ceiling, and has at its upper end a small tin-plate funnel to prevent dirt or rust from dropping down into the clay. The stirrer may be raised or lowered so as to touch any part of the cask. The semi-fluid mass is left to settle for a few minutes, and then the finer argillaceous pap is run off by the stopcocka′, placed a little above the gritty deposit, into the zinc pipe which conveys it into one of the tubsA′; but as this semi-liquid matter may still contain some granular substances, it must be passed through a sieve before it is admitted into the tub. There is, therefore, at the spot upon the tub where the zinc pipe terminates, a wire-cloth sieve, of an extremely close texture, to receive the liquid paste. This sieve is shaken upon its support, in order to make it discharge the washed argillaceous kaolin. After the clay has subsided, the water is drawn off from its surface by a zinc syphon. The vatsA′ have covers, to protect their contents from dust. In the pottery factories of England, the agitation is produced by machinery, instead of the hand. A vertical shaft, with horizontal or oblique paddles, is made to revolve in the vats for this purpose.
Small triturating wheel
The small triturating millis represented infig.901.There are three similar grinding-tubs on the same line. The details of the construction are shown infigs.902,903, where it is seen to consist principally of a revolving millstoneB(fig.902.), of a fast or sleeper millstoneB′, and of a vatC, hooped with iron, with its top raised above the upper millstone. The lower block of hornstone rests upon a very firm basis,b′; it is surrounded immediately by the strong wooden circlec, which slopes out funnel-wise above, in order to throw back the earthy matters as they are pushed up by the attritionof the stones. That piece is hollowed out, partially to admit the keyC, opposite to which is the faucet and spigotc′, for emptying the tub. When one operation is completed, the keyCis lifted out by means of a peg put into the holes at its top; the spigot is then drawn, and the thin paste is run out into vats. The upper grindstone,Bd, like the lower one, is about two feet in diameter, and must be cut in a peculiar manner. At first there is scooped out a hollowing in the form of a sector, denoted byd e f,fig.903.; the arcd fis about one-sixth of the circumference, so that the vacuity of the turning grindstone is one-sixth of its surface; moreover, the stone must be channelled, in order to grind or crush the hard gritty substances. For this purpose, a wedge-shaped grooved e g, about an inch and a quarter deep, is made on its under face, whereby the stone, as it turns in the direction indicated by the arrow, acts with this inclined plane upon all the particles in its course, crushing them and forcing them in between the stones, till they be triturated to an impalpable powder. When the grindstone wears unequally on its lower surface, it is useful to trace upon it little furrows, proceeding from the centre to the circumference, like those shown by the dotted linese′e′′. It must, moreover, be indented with rough points by the hammer.
Details of mill
The turning hornstone-block is set in motion by the vertical shaftH, which is fixed by the clamp-iron crossIto the top of the stone. When the stone is new, its thickness is about 14 inches, and it is made to answer for grinding till it be reduced to about 8 inches, by lowering the clampIupon the shaft, so that it may continue to keep its hold of the stone. The manner in which the grindstones are turned, is obvious from inspection offig.901., where the horizontal axisL, which receives its impulsion from the great water-wheel, turns the prolonged shaftL′, or leaves it at rest, according as the clutchl,l′, is locked or opened. This second shaft bears the three bevel wheelsM,M,M. These work in three corresponding bevel wheelsM′M′M′, made fast respectively to the three vertical shafts of the millstones, which pass through the cast-iron guide tubesM′′M′′. These are fixed in a truly vertical position by the collar-barm′′,m′,fig.902.In this figure we see atmhow the strong cross-bar of cast iron is made fast to the wooden beams which support all the upper mechanism of the mill-work. The bearingm′is disposed in an analogous manner; but it is supported against two cast-iron columns, shown atL′′L′′, infig.901.The guide tubesM′′ are bored smooth for a small distance from each of their extremities, and their interjacent calibre is wider, so that the vertical shafts touch only at two places. It is obvious, that whenever the shaftL′ is set a-going, it necessarily turns the wheelsMandM′, and their guide tubesM′′; but the vertical shaft may remain either at rest, or revolve, according to the position of the lever click or catchK, at the top, which is made to slide upon the shaft, and can let fall a finger into a vertical groove cut in the surface of that shaft. The clamp-fork of the click is thus made to catch upon the horizontal bevel-wheelM′, or to release it, according as the leverKis lowered or lifted up. Thus each millstone may be thrown out of or into geer at pleasure.
Safety clutch
These stones make upon an average 11 or 12 turns in a minute, corresponding to 3 revolutions of the water-wheel, which moves through a space of 3 feet 4 inches in the second, its outer circumference being 66 feet. The weight of the upper stone, with its iron mountings, is about 6 cwt., when new. The charge of each mill in dry material is 2 cwt.; and the water may be estimated at from one-half to the whole of this weight; whence the total load may be reckoned to be at least 3 cwt.; the stone, by displacement of the magma, loses fully 400 pounds of its weight, and weighs therefore in reality only 2 cwt. It is charged in successive portions, but it is discharged all at once. When the grinding of the siliceous or felspar matters is nearly complete, a remarkablephenomenon occurs; the substance precipitates to the bottom, and assumes in a few seconds so strong a degree of cohesion, that it is hardly possible to restore it again to the pasty or magma state; hence if a millstone turns too slowly, or if it be accidentally stopped for a few minutes, the upper stone gets so firmly cemented to the under one, that it is difficult to separate them. It has been discovered, but without knowing why, that a little vinegar added to the water of the magma almost infallibly prevents that sudden stiffening of the deposit and stoppage of the stones. If the mills come to be set fast in this way, the shafts or geering would be certainly broken, were not some safety provision to be made in the machinery against such accidents. Mr. Hall’s contrivance to obviate the above danger is highly ingenious. The clutchl,l′,fig.901., is not a locking crab, fixed in the common way, upon the shaftL; but it is composed, as shown infigs.904,905,906,907, of a hoopu, fixed upon the shaft by means of a key, of a collarv, and of a flat ring or washerx, with four projections, which are fitted to the collarv, by four boltsy.Fig.905.represents the collarvseen in front; that is, by the face which carries the clutch teeth; andfig.906.represents its other face, which receives the flat ringx,fig.907., in four notches corresponding to the four projections of the washer-ring. Since the ringuis fixed upon the shaftL, and necessarily turns with it, it has the two other pieces at its disposal, namely the collarv, and the washerx, because they are always connected with it by the four boltsy, so as to turn with the ringu, when the resistance they encounter upon the shaftL′ is not too great, and to remain at rest, letting the ringuturn by itself, when that resistance increases to a certain pitch. To give this degree of friction, we need only interpose the leather washersz,z′,fig.904.; and now as the collarcoupling-box,v, slides pretty freely upon the ringu, it is obvious that by tightening more or less the screw boltsy, these washers will become as it were a lateral brake, to tighten more or less the bearing of the ringu, to which they are applied: by regulating this pressure, every thing may be easily adjusted. When the resistance becomes too great, the leather washers, pressed upon one side by the collarv, of the washerx, and rubbed upon the other side by the prominence of the ringu, get heated to such a degree, that they are apt to become carbonized, and require replacement.
This safety clutch may be recommended to the notice of mechanicians, as susceptible of beneficial application in a variety of circumstances.
GREAT PORCELAIN MILL.
The large felspar and kaolin mill, made by Mr. Hall, for Sèvres, has a flat bed of hornstone, in one block, laid at the bottom of a great tub, hooped strongly with iron. In most of the English potteries, however, that bed consists of several flat pieces of chert or hornstone, laid level with each other. There is, as usual, a spigot and faucet at the side, for drawing off the liquid paste. The whole system of the mechanism is very substantial, and is supported by wooden beams.
The following is the manner of turning the upper blocks. Infig.900.the main horizontal shaftPbears at one of its extremities a toothed wheel, usually mounted upon the periphery of the great water-wheel (fig.908.shows this toothed wheel by a dotted line) at its other end;Pcarries the fixed portionpof a coupling-box, similar to the one just described as belonging to the little mill. On the prolongation ofP, there is a second shaftP′, which bears the movable portion of that box, and an upright bevel wheelP′′. Lastly, infigs.900.and908.there is shown the vertical shaftQ, which carries at its upper end a large horizontal cast-iron wheelQ′, not seen in this view, because it is sunk within the upper surface of the turning hornstone, like the clampd,f, infig.902.At the lower end of the shaftQ, there is the bevel wheelQ′′, which receives motion from the wheelP′′,fig.900.
The shaftPalways revolves with the water-wheel; but transmits its motion to theshaftP′ only when the latter is thrown into geer with the coupling-boxp′, by means of its forked lever. Then the bevel wheelP′ turns round with the shaftP′, and communicates its rotation to the bevel wheelQ′′, which transmits it to the shaftQ, and to the large cast-iron wheel, which is sunk into the upper surface of the revolving hornstone.
Porcelain mill
The shaftQis supported and centred by a simple and solid adjustment; at its lower part, it rests in a stepR, which is supported upon a cast-iron archQ′, seen in profile infig.900.; its base is solidly fixed by four strong bolts. Four set screws aboveR,fig.900., serve to set the shaftQtruly perpendicular: thus supported, and held securely at its lower end, in the step atR,figs.900.and908., it is embraced near the upper end by a brass bush or collar, composed of two pieces, which may be drawn closer together by means of a screw. This collar is set into the summit of a great truncated cone of cast-iron, which rises within the tub through two-thirds of the thickness of the hornstone bed; having its base firmly fixed by bolts to the bottom of the tub, and having a brass collet to secure its top. The iron cone is cased in wood. When all these pieces are well adjusted and properly screwed up, the shaftQrevolves without the least vacillation, and carries round with it the large iron wheelQ′, cast in one piece, and which consists of an outer rim, three arms or radii, and a strong central nave, made fast by a key to the top of the shaftQ, and resting upon a shoulder nicely turned to receive it. Upon each of the three arms, there are adjusted, with bolts, three upright substantial bars of oak, which descend vertically through the body of the revolving mill to within a small distance of the bed-stone; and upon each of the three arcs of that wheel-ring, comprised between its three strong arms, there are adjusted, in like manner, five similar uprights, which fit into hollows cut in the periphery of the moving stone. They ought to be cut to a level at their lower part, to suit the slope of the bottom of the tubo,figs.900.and908., so as to glide past it pretty closely, without touching.
The speed of this large mill is eight revolutions in the minute. The turning hornstone describes a mean circumference of 1411⁄3inches (its diameter being 45 inches), and of course moves through about 100 feet per second. The tubO, is 52 inches wide at bottom, 56 at the surface of the sleeper block (which is 16 inches thick), and 64 at top, inside measure. It sometimes happens that the millstone throws the pasty mixture out of the vessel, though its top is 6 inches under the lip of the tubo; an inconvenience which can be obviated only by making the pap a little thicker; that, is by allowing only from 25 to 30 per cent. of water; then its density becomes nearly equal to 2·00, while that of the millstones themselves is only 2·7; whence, supposing them to weigh only 2 cwt., there would remain an effective weight of less than1⁄2cwt. for pressing upon the bottom and grinding the granular particles. This weight appears to be somewhat too small to do much work in a short time; and therefore it would be better to increase the quantity of water, and put covers of some convenient form over the tubs. It is estimated that this mill will grind nearly 5 cwt. of hard kaolin or felspar gravel, in 24 hours, into a proper pap.
To the preceding methodical account of the porcelain manufacture, I shall now subjoin some practical details relative to certain styles of work, with comparisons between the methods pursued in this country and upon the Continent, but chiefly by our jealous rivals the French.
The blue printed ware of England has been hitherto a hopeless object of emulation in France. M. Alexandre Brongniart, membre de l’Institut, and director of theManufacture Royal de Sèvres, characterizes the French imitations of theFayence fine, ou Anglaise, in the following terms: “Les défauts de cette poterie, qui tiennent à sa nature, sont de ne pouvoir aller sur le feu pour les usages domestiques, et d’avoir un vernis tendre, qui se laisse aisément entamer par les instruments d’acier et de fer. Mais lorsque cette poterie est mal fabriquée, ou fabriquée avec une économie mal entendue, ses défauts deviennent bien plus graves; son vernis jaunâtre et tendre tressaille souvent; il se laisse entamer ou user avec la plus grande facilité par les instruments de fer, ou par l’usage ordinaire. Les fissures que ce tressaillement ou ces rayures ouvrent dans le vernis permettent aux matières grasses de pénétrer dans le biscuit, que dans les poteries affectées de ce défaut, a presque toujours une texture lâche; les pièces se salissent, s’empuantissent, et se brisent même avec la plus grande facilité.”[42]
[42]Dict. Technologique, tom. xvii., article Poteries, p. 253.
[42]Dict. Technologique, tom. xvii., article Poteries, p. 253.
What a glaze, to be scratched or grooved with soft iron; to fly off in scales, so as to let grease soak into the biscuit or body of the ware; to become foul, stink, and break with the utmost ease! The refuse crockery of the coarsest pottery works in the United Kingdom would hardly deserve such censure.
In the minutes of evidence of theEnquête Ministérielle, published in 1835, MM. de Saint Cricq and Lebeuf, large manufacturers of pottery-ware at Creil and Montereau, give a very gratifying account of the English stoneware manufacture. They declare that the English possess magnificent mines of potter’s clay, many leagues in extent; while those of theFrench are mere patches orpots. Besides, England, they say, having upwards of 200 potteries, can constantly employ a great many public flint-mills, and thereby obtain that indispensable material of the best quality, and at the lowest rate. “The mill erected by M. Brongniart, at Sèvres, does its work at twice the price of the English mills. The fuel costs in England one-fourth of what it does in France. The expense of a kiln-round, in the latter country, is 200 francs; while in the former it is not more than 60.” After a two-months tour among the English potteries, these gentlemen made the following additional observations to their first official statement:—
“The clay, which goes by water carriage from the counties of Devon and Dorset, into Staffordshire, to supply more than 200 potteries, clustered together, is delivered to them at a cost of 4 francs (3s.2d.) the 100 kilogrammes (2 cwt.); at Creil, it costs 4f.50c., and at Montereau, only 2f.40c.There appears, therefore, to be no essential difference in the price of the clay; but the quality of the English is much superior, being incontestably whiter, purer, more homogeneous, and not turning red at a high heat, like the French.” The grinding of the flints costs the English potter 41⁄2d.per 100 kilos., and the French 6d.; but as that of the latter is in general ground dry, it is a coarser article. The kaolin, or china clay, is imported from Cornwall for the use of many French potteries; but the transport of merchandise is so ill managed in France, that while 2 cwts. cost in Staffordshire only 8f.75c.(about 7s.1d.), they cost 12f.at Creil, and 13f. 50c.at Montereau. The white lead and massicot, so much employed for glazes, are 62 per cent. dearer to the French potters than the English. As no French mill has succeeded in making unsized paper fit for printing upon stoneware, our potters are under the necessity of fetching it from England; and, under favour of our own custom-house, are allowed to import it at a duty of 165f.per 100 kilogrammes, or about 8d.per pound English. No large stock of materials need be kept by the English, because every article may be had when wanted from its appropriate wholesale dealers; but the case is quite different with the French, whose stocks, even in small works, can never safely be less in value than 150,000f.or 200,000f.; constituting a loss to them, in interest upon their capital, of from 7,500f.to 10,000f.per annum. The capital sunk in buildings is far less in England than in France, in consequence of the different styles of erecting stoneware factories in the two countries. M. de Saint Cricq informs us, that Mr. Clewes, of Shelton, rents his works for 10,000f.(380l.) per annum; while the similar ones of Creil and Montereau, in France, have cost each a capital outlay of from 500,000f.to 600,000f., and in which the products are not more than one half of Mr. Clewes’. “This forms a balance against us,” says M. St. C., “of about 20,000f.per annum; or nearly 800l.sterling. Finally, we have the most formidable rival to our potteries in the extreme dexterity of the English artisans. An enormous fabrication permits the manufacturers to employ the same workmen during the whole year upon the same piece; thus I have seen at Shelton a furnisher, for sixpence, turn off 100 pieces, which cost at Creil and Montereau 30 sous (1s.21⁄2d.); yet the English workman earns 18f.75c.a week, while the French never earns more than 15f.I have likewise seen an English moulder expert enough to make 25 waterpots a day, which, at the rate of 2d.a piece, bring him 4s.2d.of daily wages; while the French moulder, at daily wages also of 4s.2d., turns out of his hands only 7, or at most 8 pots. In regard to hollow wares, the English may be fairly allowed to have an advantage over us, in the cost of labour, of 100 per cent.; which they derive from the circumstance, that there are in Staffordshire 60,000 operatives, men, women, and children, entirely dedicated to the stoneware manufacture; concentrating all their energies within a space of 10 square leagues. Hence a most auspicious choice of good practical potters, which cannot be found in France.”
M. Saint Amans, a French gentleman, who spent some years in Staffordshire, and has lately erected a large pottery in France, says the English surpass all other nations in manufacturing a peculiar stoneware, remarkable for its lightness, strength, and elegance; as also in printing blue figures upon it of every tint, equal to that of the Chinese, by processes of singular facility and promptitude. After the biscuit is taken out of the kiln, the fresh impression of the engraving is transferred to it from thin unsized paper, previously immersed in strong soap water; the ink for this purpose being a compound of arseniate of cobalt with a flux, ground up with properly boiled linseed oil. The copper-plates are formed by the graving tool with deeper or shallower lines according to the variable depth of shades in the design. The cobalt pigment, on melting, spreads so as to give the soft effect of water-colour drawing. The paper being still moist, is readily applied to the slightly rough and adhesive surface of the biscuit, and may be rubbed on more closely by a dossil of flannel. The piece is then dipped in a tub of water, whereby the paper gets soft, and may be easily removed, leaving upon the pottery the pigment of the engraved impression. After being gently dried, the piece is dipped into the glaze mixture, and put into the enamel oven.
Composition of the Earthy Mixtures.
The basis of the English stoneware is, as formerly stated, a bluish clay, brought from Dorsetshire and Devonshire, which lies at the depth of from 25 to 30 feet beneath the surface. It is composed of about 24 parts of alumina, and 76 of silica, with some other ingredients in very small proportions. This clay is very refractory in high heats, a property which, joined to its whiteness when burned, renders it peculiarly valuable for pottery. It is also the basis of all the yellow biscuit-ware calledcream colour, and in general of what is called theprinting body; as also for the semi-vitrified porcelain of Wedgewood’s invention, and of the tender porcelain.
The constituents of the stoneware are, that clay, the powder of calcined flints, and of the decomposed felspar called Cornish stone. The proportions are varied by the different manufacturers. The following are those generally adopted in one of the principal establishments of Staffordshire:—
Composition of the Paste for receiving the Printing Body under the Glaze.
For this purpose the proportions of the flint and the felspar must be increased. The substances are mixed separately with water into the consistence of a thick cream, which weighs per pint, for the flints 32 ounces, and for the Cornish stone 28. The china clay of Cornwall is added to the same mixture of flint and felspar, when a finer pottery or porcelain is required. That clay cream weighs 24 ounces per pint. These 24 ounces in weight are reduced to one-third of their bulk by evaporation. The pint of dry Cornish clay weighs 17 ounces, and in its first pasty state 24, as just stated. The dry flint powder weighs 141⁄2ounces per pint; which when made into a cream weighs 32 ounces. To 40 measures of Devonshire clay-cream there are added,
The whole are well mixed by proper agitation, half dried in thetroughsof the slip-kiln, and then subjected to the machine for cutting up the clay into junks. The above paste, when baked, is very white, hard, sonorous, and susceptible of receiving all sorts of impressions from the paper engravings. When the silica is mixed with the alumina in the above proportions, it forms a compact ware, and the impression remains fixed between the biscuit and the glaze, without communicating to either any portion of the tint of the metallic colour employed in the engraver’s press. The felspar gives strength to the biscuit, and renders it sonorous after being baked; while the china clay has the double advantage of imparting an agreeable whiteness and great closeness of grain.