Popular Science Monthly/Volume 21/July 1882/Porcelain and the Art of its Production
|PORCELAIN AND THE ART OF ITS PRODUCTION.|
ADMINISTRATOR OF THE NATIONAL MANUFACTORY AT SÈVRES.
THE uses of porcelain have so multiplied, the employment of that material has become so general, that few persons recollect the time, not yet far back, when it was considered an object of luxury, and only delf was within the reach of all. In this paper I shall consider, first, the nature of porcelain and the history of its discovery; next, the principal points in its manufacture; and, lastly, the different methods of decorating it.
It is generally understood that porcelain is, as a rule, the resultant, of the action of fire on a certain kind of clay. No one is likely to confound it with earthenware or delf. While those wares are soft, opaque, and of impure colors, porcelain is always white, is perfectly clear, and is harder than steel. The fundamental distinction between the three wares is that earthenware is obtained by the simple action of fire on common clays; delfs are earthenwares more or less colored and glazed with a leaden enamel, which is rendered opaque by tin; while hard porcelain is obtained from a white clay, kaolin, and is enameled with feldspar.
Kaolin, a natural hydrated silicate of alumina, is absolutely refractory and opaque; it constitutes the resistant part of porcelain. Feldspars are silicates of alumina and potassa, fusible at a very high temperature into a beautiful transparent glass. If, now, we mix a quantity of feldspar with kaolin, cover the mixture with a layer of feldspar, and heat the whole at a very high temperature, the feldspar will melt and communicate to the opaque clay a clearness greater or less according to the quantity of it present, and to the superficial part of it that beautiful glaze with which all are familiar. A part of the action in this process is chemical, and consists in the production of a new crystalline silicate formed by a combination of all the substances present. The discovery of porcelain in China is traced back to a high antiquity. The Chinese have certainly made it regularly for at least a thousand years; many authors fix the discovery at fifteen hundred or eighteen hundred years ago, but no evidence exists to justify our going further back than a thousand years. The first pieces that came to Europe were probably brought by the Venetians at the end of the thirteenth century. Charles VII, King of France, received a present of Chinese porcelains, about the middle of the fifteenth century, from the Sultan of Babylon; but it was not till the sixteenth century that the importation of these Oriental products by Portuguese and Dutch merchants assumed a real importance. The discovery of tender porcelain was made in France toward the end of the seventeenth century, but whether by Louis Poterat or by Reverend, at Paris or Rouen, is disputed. This ware has no relation with real porcelain; it contains neither kaolin nor feldspar, but is an artificial product, a kind of glass made from a mixture composed essentially of sand, lime, potash, soda, and a small quantity of marine marl. This mixture, made plastic by the addition of manganese or other fluxes, is baked without glazing, and is covered after baking with a glazing composed of silica, lead, potash, and soda. The beauty of the material, its perfect glaze, and the facility with which verifiable colors are fixed in it, make of tender porcelain a ware exceptionally adapted to decoration.
The discovery of tender porcelain did not arrest the investigations of men of science and potters, who saw very well that it had none of the characters of the porcelain of China. A bed of kaolin was found in Saxony in 1709, and Böttger set up at Meissen the first European factory of hard porcelain. Fifty years later, in 1758, Guettard at Alençon, and afterward, in 1769, Madame Darnet at Saint-Yeux, near Limoges, discovered the French beds, and the industrial fabrication of hard porcelain was begun in France in 1774. Tender porcelain gave way quickly in the rivalry with hard porcelain. This was unfortunate, in an artistic point of view, for the latter material is very unaccommodating to the decorator. A more important object was, however, to create for domestic economy an absolutely healthful industry, and much is due to the illustrious Brongniart for having by his investigations put the manufacture of kaolinic porcelain on a firm scientific foundation.
Natural kaolin is never a pure clay, but contains also sand, undecomposed feldspar, etc., in variable quantities, and must first be purified. For this purpose the mass is pounded, and the products are separated one after another by successive levigations with water. The clay, which is extremely slow in settling, is drawn off first, and may be obtained almost pure; the other deposits, composed of more or less feldspathic sands, are brayed in mills, and are destined to enter in their turn into the preparation of the pastes. The nature of porcelain, its physical and chemical properties, vary infinitely according to the proportion of its two more plastic as it grows older. The qualities required of a good paste may be communicated by diluting it and stirring it with water and decanting, or by prolonged beating and manipulation. By treading it out or beating it we not only give it complete plasticity and homogeneity, but we also clear it of air-bubbles which would otherwise swell out in baking and cause much damage.elements (kaolin and feldspar), and as other substances, lime, silicious sands, potsherds, etc., are added, as is often done. Every country and every factory has its composition, which is adapted to the use to which the porcelain is destined, to the degree of resistance that is to be demanded of it, and to the kind of decoration it is lo receive. Generally, porcelain is more solid as the proportion of clay increases, and requires a higher temperature in baking; if, on the other hand, the proportion of feldspar is increased, it becomes more fusible, may be baked at a lower temperature, and submits more readily to decoration, but its plasticity and the possibility of working it easily diminish rapidly. The mixture of the materials should be perfect; when this is the case, the mass will keep for a long time, and become
The next thing is to give our ware the form which has been determined for it in a design previously made. This requires a knowledge of the whole process of fabrication. It would be a mistake to suppose that porcelain can be baked in any desired form. It becomes soft in baking, and has to be supported; and, as it is to be covered with a glass that melts at the same moment, places which need not be enameled must be found for fixing the supports in every piece, unless we are willing to risk having it spoiled. We may thus comprehend one of the difficulties in the manufacture of porcelain, and one of the points in which it differs most from delf.
Articles of porcelain may be shaped without molds or with them. By the former method all the shapes are obtained that may be produced by turning. The clay is first shaped on the wheel by the hand into a rough block of the general shape which the object is destined to assume, and is then left to dry, slowly and with care, to keep it from cracking. When it has been dried to a suitable degree of consistency, it is put upon the wheel again and carefully worked into the exact shape desired, with the moldings and ornaments called for by the design, and by the aid of the most simple instruments.
Articles whose shape does not adapt them to manipulation on a revolving wheel, such as objects of statuary and many lighter objects, may be shaped by molding them. The mold is made of plaster of Paris; to it, when dry, is applied a layer of the porcelain paste, which is pressed into it carefully and as evenly as possible; the earth espouses all the details of the sculpture, and, after a few moments, the plaster having absorbed the water from the paste in contact with it, a shrinkage takes place, thanks to which the proof detaches itself almost spontaneously. The operation is sure to be successful in the case of simple forms whose outlines offer no impediment to taking them from the molds. If, however, we purpose to obtain objects in relief, statuettes, groups of figures, or sumptuous vases, the sculptural decorations of which constitute their chief ornament, the process becomes more complicated. In this case the molder has to divide his pattern into a number of parts, the superficies of which must be determined by the possibilities of taking off the molds; then he must make as many molds in plaster as he has parts of his model; these molds will in their turn serve for the reproduction of each of the parts, which have afterward to be joined and cemented by the aid of the paste diluted in water. After this the seams at the junction of the parts must be rubbed away, and the whole work finished up by a restorer who must necessarily be an artist. I can not leave this part of my subject without mentioning the property which porcelain has of shrinking when baked. The shrinkage amounts to about ten or fifteen per cent.
A third method of shaping porcelains is by casting, which was discovered at Tournay, 1784, and in which Brongniart has made numerous improvements. Nothing is more simple than the manufacture of a small object by this process. Thus, if we take a plaster mold of a cup, and pour into it a quantity of barbotine, or porcelain-clay mixed with water, the mold will absorb the water from the clay in contact with it, forming a shell less liquid than the rest of the barbotine, and which sticks to the plaster. When this shell has attained a suitable thickness, the rest of the barbotine may be poured out: what remains in the mold constitutes the cup. We leave it to dry, and in a little while it will have gained consistency enough to be taken out of the mold without being deformed. Ware thus made is extremely delicate; the slightest pressure with the lingers may destroy it. This process is used at Sèvres for large pieces, but special manipulations are required for such work; for the weight of the shell which should adhere to the mold when the liquid is poured out, and which should be thicker and heavier in proportion to the greater size of the vessel, is very apt to cause it to separate from the plaster and fall. The least awkwardness might destroy the piece, and this should be avoided at any cost. MM. Milet and Delacour have devised a method, which has been used at Sèvres since 1857, for avoiding such accidents by turning compressed air against the interior of the mold at the moment the barbotine is poured out, to take the place of the liquid and hold the porcelain shell against the plaster. M. Regnault has simplified this operation by exhausting the air on the outside of the mold, which effects the same purpose and is more convenient of execution. The absence of seams, the purity of the outlines, and the clearness of the surfaces obtained by this process, make it one of inestimable value when we wish to get an object of art, and lift it far above the process of molding. The details of the operation are very exacting, but none of them should be neglected. Their importance may be realized by reflecting that a hidden fault in the interior of a large piece, a bubble of inclosed air, a lack of homogeneity in the paste, or other flaw, is not perceptible till after the baking, when the vessel has been decorated, and may perhaps have become of very great value. The slightest defect in the casting may destroy this value.
The objects, having been properly shaped and fitted, have next to be transformed into porcelain by the action of fire, the function of which is to combine the different elements of the paste and determine the fusion of the glazing. The baking is done twice. In the first operation, when the temperature is relatively only moderately high (1,800 to 2,160), the earth is converted into what is called biscuit baked porcelain; it becomes very tough and sonorous, and extremely flexible; in this condition it is submitted to the enameling process. That operation is of the simplest character, and consists in a quick immersion in water holding in suspension a feldspathic rock, which has previously been reduced to an impalpable powder. Yet it requires great care, for the thickness of the enamel must be adapted to the piece; it may be neither too great nor too little, under penalty of accidents; it must be as even as possible, with neither bulges nor thin places. These qualities can not be obtained by dipping alone, so the object has to be retouched with a brush. The next step is baking in the sharp-fire, where the temperature of from 2,880° to 3,240°, at which feldspar fuses, must be reached. A few facts will enable us to comprehend the steps and the difficulties of this operation. Porcelain-clay can not be baked in direct contact with the flames, ashes, and smoke, without being greatly altered. It must, then, for the biscuit-baking as well as for the sharp-fire, be inclosed in protecting envelopes called gazettes, or casettes—cases of as refractory clay as can be got, in which the pieces are adjusted with great care on suitably arranged supports. It must be remembered that porcelain is baked at the temperature at which it becomes soft; the softening must then be anticipated at the time of fitting the vessel in the casette, and all the parts of the object must be supported so as to prevent any possible deformation. At the same time the supports must be prevented from sticking to the piece; and it is only by the aid of many kinds of artifices that the object can be effected without the supports leaving visible marks of their having been applied.
The furnace is divided into two stories, the upper one of which is the dome, or biscuit-baking compartment, and is warmed by the surplus heat that escapes from the lower story and passes through the vent-holes in its roof. The lower story, where the baking with the sharp-fire is done, is called the laboratory, and is heated by a number of fires placed along the circumference of the furnace, called alandiers. The casettes filled with articles to be baked are arranged vertically and as symmetrically as possible, and properly supported in the interior of the laboratory; when the furnace is filled, the entrance is closed by a double door of refractory materials, and the fires are kindled at the different alandiers. The temperature should be raised very slowly and very regularly, in order to avoid unequal dilatations, which would develop breaks in the objects. The heating is watched through little openings left in the walls of the furnace for that purpose, through which the color of the fire is observed. It may be seen to pass in succession from a dark red to orange, bright orange, and white. At the white heat, which is reached in from twenty-four to sixty hours, according to the kind of furnace and fire, the porcelain is near its baking point. Since no apparatus has yet been invented for ascertaining the precise temperature within the furnace, the condition of affairs inside has to be determined experimentally by means of trial-pieces, which are put in for that purpose. These pieces become glassy a few hours before the baking is completed, but are apt in that condition to fly to pieces. If the baking is arrested at that point, the pieces would all be excessively brittle, and the batch would be spoiled; so the heat is kept up till the trial-pieces come out glazed and clear, without being brittle; but, if the cooking is prolonged much beyond that point, it will be at the risk of changing the character of the porcelain, and of other serious accidents. It is a very delicate matter to determine the right point for stopping the heat without running upon one danger in the effort to avoid the other. When the cooking has been judged complete, the fire is covered up, the vents are stopped, and the furnace is left to cool of itself—a process requiring from four to eight days.
An equally important consideration with that of the temperature is that of the nature of the gases existing within the furnace. If only white porcelain is baked, it is generally best to have an atmosphere of reducing properties, because the small quantities of iron, titanium, etc., included in the clays, will then be least oxidized, and will not color the mass as would be done with an oxidizing flame; if, however, the porcelain is decorated, it is generally an advantage to have an oxidizing atmosphere; and, as both kinds are generally baked at once, it is only by the best management—by, for example, artificially introducing into the piles gaseous substances adapted to one or the other object—that a satisfactory result can be reached. The nature of the fuel employed is variable. Different kinds of wood and coal are used. Efforts are made to adopt gaseous fuels, with which alone we can expect to be able to obtain a complete mastery over the baking.
Having reviewed as succinctly as possible the principal points in the fabrication of porcelain, we now come to the description of the processes employed to enrich that beautiful material. The art of fixing colors on pottery differs essentially from that of fixing them on cloths, wood, and paper; besides, special qualities, distinguishing it from all other kinds, are exacted of ceramic decoration. A perfect adherence, an absolute resistance to atmospheric agents, a glaze that shall permit the colors to be confounded with that of the object itself, are the characteristic qualities of a handsome ceramic decoration. Since the glazing of porcelain is a rock, one of the hardest substances of the mineral kingdom, it is easily understood that absolutely special processes must be adopted to make a color adhere. The result can be reached only through the intervention of an elevated temperature; and 'this fact eliminates at once from the palette of the ceramist all organic coloring matters and all minerals of slight stability. We must have recourse to oxides, metallic silicates, and metals. The fixation of these colors is always the result of a chemical action, of a combination that takes place at a high temperature, between the body or the glazing of the porcelain and the materials employed to decorate it. Several different methods are applied for this purpose, but they may be divided into two classes: decoration by the sharp-fire, and decoration by the enameling furnace. The former method consists in the application upon the porcelain of coloring substances which are fixed and developed upon it at the same temperature as that at which the porcelain is baked. It is the method that gives the most highly prized results, for with it the color is covered by the enamel, and takes a high luster and deep setting, becoming, as it were, of one body with the object. The magnificent blue of Sèvres, certain browns, the blacks, and a few other shades are obtained by this process.
The color may be mixed in the paste, or put on the fashioned object, before enameling, or mixed with the glazing; or it can be put upon porcelain that has been baked and will then be baked a second time in the sharp-fire. This is the process we employ at Sèvres for our blues.
One of the most brilliant varieties of decoration by the sharp-fire consists in what is called the process of applied pastes, or in painting with barbotine upon the raw or biscuit-baked porcelain; by successive, rightly tempered applications we can lay on a considerable thickness of material, on which the artist can with the chisel give a fine finish and thus add great value to his decoration. When the barbotine is applied upon a tinted bottom, charming effects, making the porcelain look like real cameos, can be obtained in the clear material. If coloring oxides are added to the barbotine, a real picture can be obtained. Unfortunately, the number of colors that can be used in the sharp-fire is, on account of the excessive temperature, very limited.
In decoration in the enameling-furnace, the painting is always done on baked porcelain, consequently on enamel, and the heating takes place at a relatively low temperature. Since the glazing is not to be melted, as in the sharp-fire, an intermediate agent or flux is required to make the colors adhere. This is generally a silicate or a silico-borate of lead, or for metals a sub-nitrate of bismuth. When the temperature is raised, these substances melt, attack the glaze, and combine with it, determining by their reaction the adherence of the color. The temperature in these operations is determined by the nature of the fluxes and of the colors; and, as some colors are more sensitive than others, it is frequently necessary to bake in two successive and different fires.
The palette of Sèvres is complete, and is competent to reproduce with a marvelous excellence the chief works of the greatest painters. Notwithstanding the beauty of the sharp-fire colors, and the richness of the palette, much is still left to be desired in the decoration of French hard porcelain. The sharp-fire colors are too limited in number and too delicate to admit of any great variety of effects being drawn from them; and the colors of the enameling furnace, in spite of their richness, have a capital fault. They are opaque, and they cover the porcelain and hide all of its highly prized qualities.
Improved processes of decoration have been devised by M. Salvetat and M. Ebelmen, of which the manufactory of Sèvres has possession, and which, I believe, will permit the substitution for painting of a lively and brilliant decoration, as distinct as that of delf, but which will be as superior to that as the material to which it is applied is more precious and delicate. Using these processes, the artists of Sèvres will be able to go beyond making commonplace copies of what has been made in the East, and to create a genuine French school of porcelain, restoring that material to the high rank which the artistic delfs have nearly taken away from it.
I consider that the manufactory at Sèvres, being a national establishment, supported by the country, ought to lay aside the character of a factory, and become a school of ceramics, devoting itself to the search for new processes, for original forms and decorations, to the creation of workmen and artists who shall be masters in their business; and that it is its absolute duty to give to French industry the results of its investigations. Thus might it become a most useful element in the national industries, and a glory to France and the republic.