Kitabı oku: «Porcelain», sayfa 3

Firing and Furnaces.—So far in our treatment of the operations involved in the manufacture of porcelain, the same general description has been applicable, with trifling exceptions, to the processes in use both in Europe and in the far East, and to soft as well as to hard paste. But now that we have to describe the firing of the ware, a division into three classes is necessary:—

1st. The Chinese system. This is the simplest plan. The glaze is applied at once to the air-dried ware, which is then subjected to but one firing—that of the ‘grand feu.’

2nd. The French system for hard paste. The unglazed vessel is exposed to a heat varying from dull to full red, generally in the dome over the main body of the furnace. It is then glazed, and again fired to the full point required by the paste. This is essentially a French process, and the preliminary fire is known as the feu dégourdi.

3rd. The English system used for bone pastes. In this case it is the first firing that is the most severe. The ‘biscuit oven,’ therefore, in which this is effected, must not be confused with the feu dégourdi just mentioned. After dipping, the ware is heated again in the ‘glozing’ or glazing oven, but only to a temperature sufficient to melt the glaze.

In the case of ware decorated with enamel colours over the glaze, there will be required in all these cases one or more additional firings at comparatively low temperatures in the muffle-stove.

The furnaces, ovens, or kilns in which porcelain is fired are always of the reverberatory type; that is to say, the fuel is burned in a separate chamber or fireplace, and the products of combustion pass over or among the ware that is being fired. Such furnaces differ on the one hand from the arrangement in a blast furnace, or that often used in the burning of bricks, where the fuel is mixed with the material to be heated, and on the other hand from the muffle-stove, where the object exposed to the heat is protected from the direct flame by the box of fireclay or iron in which it is placed.

Kilns of many shapes and sizes have been used for firing porcelain, but they may most of them be included in one or the other of the following broad classes.

1st. The old bee-hive ovens of China, the use of which appears to have been abandoned in that country by the end of the seventeenth century. These ovens were generally small, in some cases only holding one vase. A row of them may be heated from one fireplace, and they are then built on a rising slope. This type has survived to the present day in Japan.

2nd. The oblong horizontal furnaces, often of considerable dimensions, used during the present dynasty in China. They resemble in section the ordinary type of reverberatory furnace found in metallurgical works. A very similar form was long employed at Meissen.

3rd. The large conical furnaces, now in general use in the porcelain factories of Europe. They may be heated by either direct or by reversed flame.¹³

In China the fuel is generally pinewood, in billets of uniform size. In many European kilns wood is still used: birchwood, cut in lengths of fifteen to twenty inches, is the only fuel used at the present day at Sèvres. In England, however, the difficulties attendant on the use of coal appear to have been overcome.

The reader will find in the third volume of Brongniart’s great work (Traité des Arts Céramiques, Paris, 1877) several plates giving plans and sections of all these types of furnaces. From a careful examination of these engravings more is to be learned than from any amount of verbal description. A thorough grasp of the process of firing is of the greatest assistance in understanding the problems and difficulties that arise in the manufacture of porcelain, and we shall have to return to the subject when we come to treat of the several wares.

Whatever differences there may be in the shape of the furnaces, when it comes to filling the interior with the ware to be baked, there is one precaution which has been adopted in nearly every country.¹⁴ The ware must be protected from the direct heat of the flame by means of a case of fireclay in which it is placed. These are the seggars (French cassettes; the process of filling and arranging them is called encastage), to the preparation of which so important a department has to be set apart in all porcelain works, and whose manufacture adds so much to the working expenses.

The seggar proper is a cylindrical pan of fireclay, in shape and size like a hatbox. They are piled, in the furnace, one over the other, and these piles or ‘bungs’ are arranged in the furnace so as to allow a free circulation of the hot gases between them, but otherwise they are packed as closely together as possible. These seggars may be used several times over. When broken, the fragments are ground up and mixed with fresh fireclay or argile-plastique to form new cases—without this addition the clay would be too plastic or ‘fat’ for the purpose. The greatest precautions are taken in the packing of the seggars in the furnace. The giving way of one pile from any inaccuracy in the arrangement may destroy the contents of the whole oven. So again infinite care must be taken in the arrangement and support of the objects in each seggar. The bottom is covered with ground flint or other infusible material, and the vessel is supported, when necessary, by various forms of struts, props, or crow-claws, which sometimes leave their mark on the base or side of the finished object. In spite of these precautions, a large quantity of defective pieces or ‘wasters’ are produced in all works, and these are usually cast aside. The finding of such fragments in after days is sometimes the only proof we have that porcelain or pottery has formerly been made at the spot. But the proof is final, for defective pieces and ‘crow-claws’ are not objects likely to have been imported from a distance. Again, the indelible marks left on the porcelain, either on the edge which rested directly on the seggar or at the points where the object was supported by the crow-claws, often give valuable hints as to the provenance of the piece in question.¹⁵ In the case of valuable wares these rough edges and marks are removed as far as possible by grinding on a small wheel, and then polishing the surface with pumice or with putty.

CHAPTER   III
GLAZES

BEFORE attacking the somewhat complicated subject of the nature and composition of glazes, it will be well to take up again the thread of the mechanical processes that are involved in the making of a piece of porcelain.

The materials that enter into the glaze are reduced to the finest powder in mills similar to those in which the china-stone and flint are ground for the preparation of the paste. If any substance soluble in water, such as borax or salts of the alkalis, enter into the composition of the glaze, these must be first partially fused in combination with the other materials to form a frit, a kind of imperfect glass. These frits, which enter so largely into the composition of soft-paste porcelain, are formed with the object of bringing the soluble constituents into an insoluble form before mixing with water to form the slip. There are indeed other practical reasons that render a preliminary partial fusion desirable.

The finely ground elements of the glaze, mixed in due proportion, are worked up with water to form a creamlike slip into which the vessel to be glazed is now dipped. In China, in many cases, the glaze-slip is blown upon the surface in the form of a spray. This is done by means of a bamboo tube, covered at one end by a piece of silk gauze, through which the liquid is projected by the breath of the operator (French, insufflation); in other cases the glaze may be painted on with a brush. In China, as we have mentioned, the glaze-slip is generally applied to the raw surface of the thoroughly dried but unbaked ware, but in other countries there is, almost without exception, a preliminary firing of greater or less degree to produce a biscuit.

We shall restrict the use of the word glaze to the vitreous coating applied directly to the surface of the raw paste or of the biscuit to enhance the decorative effect of the ware, and with the more prosaic object of allowing the surface to be easily kept clean. In the case of porcelain this coating is always more or less transparent.¹⁶ There is here no necessity for concealing the natural white colour of the paste. In the case of many kinds of pottery, however, as in the ‘enamelled fayence’ of Delft and Italy, the glaze is rendered opaque by the addition of oxide of tin, so that the ill-favoured ground is concealed by a white shiny surface which may be made to resemble closely the natural surface of porcelain. A glaze of this kind is often called an enamel, but as we are not concerned with such an expedient we shall confine the use of that word to the various forms in which a vitreous decoration, whether translucent or opaque, is superimposed upon the glaze and fused into it, more or less thoroughly, by a subsequent firing in a muffle furnace.

The English word ‘glaze’ is only another form of the word ‘glass,’ and we may say at once that, in composition at least, there is often little difference between the two substances. The French word for ‘glaze’ is couverte or vernis; the last term applies well to the thin skin of glaze found on Greek pottery. The Chinese have several expressions, but it is a curious fact that the characters with which most of these terms are written contain the radical for ‘oil,’ and indeed the word ‘oil’ itself is often used in the sense of ‘glaze.’

Mr. Rix puts it well when he says that the glaze is to the enameller of porcelain what his canvas is to the painter; while in the case of a decoration ‘sous couverte,’ the glaze corresponds to the varnish which, while protecting his work, gives brilliancy to the colouring (Journal of Society of Arts, vol. xli.). It is, moreover, the vehicle by which the design is harmonised and rendered mellow. The effect is produced at once and endures practically for all time.

The hardness and fusibility of glazes differ widely, and they are conditioned by the nature of the wares that they cover. It is evident that there must be a close relation between the fusing-points of paste and glaze, and that the latter should be the more fusible of the two. The difference of melting-point should, however, not be too great. The melted glaze should rather, by penetrating into the already softened paste or by a chemical action upon its surface, form a more or less uniform mass with it. In cooling, the contraction of the glaze should follow that of the subjacent paste. This is a most important point; any discordance may lead to splitting, cracking, and ‘crazing.’

The beauty of the surface of porcelain depends on the fact that the glaze has become intimately united with the paste during the long exposure of both to a high temperature. We should not be conscious, in regarding a fine specimen of porcelain, of a greater or less thickness of glass covering an opaque substance; we should rather see in it the polished surface of ivory or of some precious marble.

It would seem that it was the beauty of the glassy surface, enhancing the brilliancy of the colouring, rather than any practical advantage connected with its use, that first led to the application of glaze to pottery. The turquoise and green glazes of the Egyptians (the colour is derived from a silicate of copper along with soda and sometimes lime) were known to the men of the Early Empire. They were applied to a fritlike mass of sand held together by silicate of soda, to which the name of porcelain has sometimes been very wrongly given. Objects of steatite, of slate, and even of rock crystal were sometimes covered with a coloured glaze of this kind, but it was never applied to the clay vessels in daily use. These were made, then as now, from the unctuous clay of the Nile bank. For this restriction there was a very good reason, namely that a glaze of this nature, composed chiefly of alkaline silicates, will not adhere to a base of ordinary clay. It was not until Ptolemaic and Roman times that, by the discovery or adoption of a glaze containing lead, the ancients were enabled to glaze their pottery. So in Assyria, the employment of glazes was almost confined to the decoration of the surface of brickwork, the bricks being of a loose and somewhat sandy texture.¹⁷

In these glazes, and indeed in much earlier examples from Babylonia, both tin and lead have been found. The respective virtues of the silicates of these metals were doubtless appreciated, that of tin to form a white opaque enamel hiding the material below, and that of lead to enable the glaze into which it enters to adhere to a paste formed of a plastic clay.

With the Chinese the aim was rather æsthetic than practical. They sought by means of the marvellous glazes that cover their ancient porcelain to imitate the surface of natural stones; their early celadons were in a measure intended to take the place of the precious green jade, so highly esteemed by them.

At the time when the manufacture of porcelain was first introduced from China there were (apart from the salt-glazed stoneware, which lies quite outside our inquiry) three classes of glaze in general use either in Europe or in the nearer East:—

1. Glazes consisting essentially of alkaline silicates without either lead or tin. Such glazes could only be applied to a fritty silicious base, and in India and Persia their employment seems to have been a survival from Egyptian and Assyrian times.¹⁸

2. Opaque enamel glazes, the opacity being due to the presence of tin; a considerable amount of lead also is generally found in these glazes. We are not concerned here with the obscure origin of this group, but in the sixteenth century this enamelled fayence was in general use for the better class of table-ware. It includes the Italian majolica, the French fayence of Nevers and Rouen, and above all the earthenware of Delft.

3. The oily-looking lead glazes with which the common earthenwares were covered. These were essentially the glazes of the Middle Ages in Europe, and their employment could probably be traced back to the lead-glazed ware sparingly used by the Romans. We have already noticed the use of a similar glaze in Egypt as far back probably as Ptolemaic times.

There were practical objections to all these glazes. It is true that at Delft, by the use of the tin enamel, a ware could be turned out closely resembling, in external aspect, the blue and white porcelain of China, but the enamel was soft and would in time chip off at the edges, showing the dark earthy clay beneath. On the other hand, the alkaline glazes of the East were not much known in Europe; they can only be used upon a very tender and treacherous base. In India and Persia, however, a ware thus glazed still competes with the hard porcelain of the Far East. In spite of the great objections to the glazes of our third class, those containing lead—objections arising from their softness and from the danger of poisoning to those employed in their manufacture—their use has tended rather to increase. Not only is lead the principal constituent of the glazes still universally used for common pottery, but it forms an important element in the glaze of our finer earthenwares as well as in that of those bone pastes which rank with us as porcelain.

The glaze which had been brought to perfection by the Chinese at an early period differs from all those yet mentioned by its hardness, its high fusing-point, and in its chemical composition. Speaking generally, the glaze of porcelain differs in composition from the paste which it covers only sufficiently to allow of its becoming completely liquid at the extreme heat of the furnace; and just as the paste of Chinese porcelain has a wider limit of variability than that made in Europe, but is on the whole of a ‘milder’ type than the latter, so we find that while the glazes of the Chinese are as a whole less refractory and not quite so hard, there is still a wide range of variation in these qualities.

If, then, we theoretically regard porcelain as a compound of a silicate of alumina with an alkaline silicate of the same base, we may say that the glaze of porcelain is formed by the latter body alone, that it is, in fact, merely a fused felspar. But as in the case of the paste, so in the glaze there is generally present an excess of silica, derived from the quartz contained in the petuntse or pegmatite, and this silica enters into combination with some other bases which are present in the constituents of the glaze, thereby increasing its fusibility and modifying the contraction in cooling. The most important of these additional bases is lime, so that the more fusible type may be called a calcareous, as opposed to a more refractory or purely felspathic glaze. As much as 21 per cent. of lime has been found in some Chinese glazes, the amount of alumina being proportionately reduced.

There is more or less lime in the glaze of most kinds of European hard porcelain, but the exceptionally hard and refractory paste made at Sèvres since the time of Brongniart is covered by a glaze of corresponding hardness from which that earth is absent. This hard paste has, however, of late been replaced in part by one of a milder type, and with this latter a calcareous glaze has been adopted even at Sèvres, the object of the change being, as we have said, to allow of a more brilliant decoration.

There is a perceptible difference in the aspect of these two types of glazes after firing. The hard, non-calcareous glaze has a slightly milky look. The softer calcareous type is more brilliant, and approaches in transparence and limpidity to the lead glazes of soft porcelain. A glaze of this last kind was used at Sèvres for a few years after the first introduction of the hard paste, and perhaps also at Dresden in quite early days.

The principal objection to a hard refractory glaze, such as that so long in use at Sèvres, arises from the difficulty of properly incorporating the enamel colours with its body. The restriction of the number of pigments that can be employed, both under and on the surface of the glaze, in consequence of the high temperature at which the latter melts, is another drawback. The dulness, the ‘painted on’ look of so much of the decoration on European hard paste porcelain, is in great measure a consequence of the employment of a glaze that is only softened at a high temperature. As an example of a medium type of glaze we give the composition of that used at Berlin in 1836. This consisted of kaolin, 31 per cent.; quartz, 43 per cent.; gypsum, 14 per cent.; and ground porcelain, 12 per cent. A glaze long in use at Dresden is of a very similar character. Felspar, it will be seen, does not enter into its composition, and such a glaze can contain but little potash or soda. With this we may contrast the hard glaze of Sèvres, composed simply of ground pegmatite, a rock consisting mainly of felspar. This glaze yields on analysis 74 per cent. of silica, 17 per cent. of alumina, and as much as 8 per cent. of potash.

The glaze on Chinese porcelain is prepared by mixing certain special varieties of petuntse with an impure lime, prepared by burning limestone with dry fern as fuel. It contains, as we have seen, from 15 to 21 per cent. of lime, 5 to 6 per cent. of alkalis, 11 per cent. of alumina, and 66 per cent. of silica.

We give these examples to illustrate the principal types of glazes used for hard paste porcelain. It will be noticed that the constituents are drawn from widely different sources.

The glazes of soft paste porcelain always contain a large amount both of lead and of potash or soda, so that they approximate in composition to a flint glass. The alkalis, generally introduced as carbonates, necessitate a previous fritting of part at least of the materials. Boracic acid plays an important part in the glaze of most modern English wares: it is generally introduced in the form of borate of soda or borax. This acid replaces in part the silica, just as in the paste the glassy materials are replaced by bone-earth.