Porcelain. Dillon Edward
this latter type that has with few exceptions found favour in other districts where porcelain is made.
We have attempted in this chapter to give some idea of the nature of porcelain from a physical and chemical point of view, and in doing so have taken as our type the hard, refractory paste of Europe. When we come to describe the porcelain of the Chinese, we shall notice some important divergences from this type. We say nothing here of the soft-paste porcelains, seeing that so long as we confine ourselves to the question of chemical composition and physical properties, they lie entirely outside our definitions. It is only from the point of view of its history and of its artistic qualities that this group has any claim to the name of porcelain.
CHAPTER II
THE MATERIALS: MIXING, FASHIONING, AND FIRING
IT would be quite foreign to the scope and object of this book to attempt to describe in any detail the different processes that come into play in the manufacture of a piece of porcelain. There is the less cause for any such detailed treatment, inasmuch as the operations involved in the preparation of the paste and in the subsequent potting and firing do not essentially differ in the case of porcelain from those employed in the manufacture of other classes of pottery. The differences are rather those of degree—greater care is necessary in the selection of the materials, and these materials must be more finely ground and more intimately mixed. Again, the great heat required in the kilns necessitates, in the firing of porcelain, many precautions that are not called for in the case of earthenware or fayence. Without, however, some slight acquaintance with the processes of the manufacture, it would be impossible to avoid an amateurish and somewhat ‘anecdotal’ treatment of our subject. There are, indeed, many intimate features, many delicate shades of difference that distinguish the wares of various times and places, both in Europe and in the East, which can only be rationally explained by reference to the details of the manufacture.
At the present day there is only one district in Europe where true porcelain is manufactured on a large scale. This district lies on the western and south-western border of the central granitic plateau of France, especially in the Limousin and in Berry. Again at Sèvres, for the last hundred years and more, a succession of able chemists has carried on a series of experiments on the composition and preparation of porcelain. It is no wonder, then, if we find that the literature concerned with these practical departments is almost entirely French. One result of this is a greater richness in technical terms than with us. We find in France names for the various implements and processes of the potter’s art, that are something better than the workshop terms of the local potter. Again, the little that has been written in England upon the technology of pottery has been concerned chiefly with earthenware of Staffordshire.6
As for the English soft-paste porcelain of the eighteenth century, there is a remarkable dearth of information both as to its composition and as to its manufacture. We know in fact in much greater detail how the great potteries at King-te-chen were carried on at the same period, thanks to the letters of the Père D’Entrecolles, and to the information collected in Dr. Bushell’s great work, Oriental Ceramic Art (New York, 1899. I shall always quote from the text edition).
The following technical notes are based chiefly on the processes in use either at Sèvres or in the great factories of the Limoges district.7 To begin with the Kaolin, the ‘premier’ element in the composition of porcelain. The greatest care is taken to procure a pure white clay which should approach as near as possible to the more or less theoretical mineral kaolinite, i.e. to a hydrous silicate of alumina. With this object the rough china-clay brought from the pit is thrown into a large tank of water and broken up with wooden spades; the milky liquid is now decanted into a second tank, leaving behind most of the quartz and the other stony particles. On its way the soup-like liquid passes through the meshes of a sieve—these may be formed either of brass wire or sometimes of finely woven silk. On this sieve all but the finest particles are retained. The greater part of the kaolin is deposited in this second tank, but a certain portion still remains suspended in the liquid, which is again decanted; the remaining kaolin then settles down in the third tank, yielding the finest clay. To dry this slimy mass, it is first forced by hydraulic pumps into canvas bags, and these bags are then pressed between fluted wooden trays, strongly clamped together. We have now got a white chalky mass which may contain as much as 98 per cent. of the hydrated silicate of alumina.
The other materials, the china-stone8 and the quartz, have first to be reduced to the finest powder. To effect this they may, to begin with, be roasted to effect disintegration, then crushed in a stone-breaking machine, and finally passed through the grinding-pan in which they are ground fine between large blocks of chert which rotate upon a pavement of the same stone. The finely ground materials have now to be mixed in suitable proportions either by the old process of ‘slop-blending,’ where the different ‘slops,’ each of known specific gravity, are run in due proportion into the big ‘blending ark,’ or, as is now usual in the case of fine wares, by weighing out the materials in a dry state. On the relative amounts of the three elements, the china-clay, the china-stone, and the quartz, the nature of the porcelain after firing will depend. M. Vogt (La Porcelaine, Paris, 1893) gives a useful table showing the limits within which the materials may be varied. We may note that in the case of a normal china-stone or petuntse being used instead of felspar, very little additional quartz is required. These limits are: kaolin, 35 to 65 per cent.; felspar, 20 to 40 per cent.; and quartz, 15 to 25 per cent. The larger the percentage of the first material, the harder and more refractory will be the resultant porcelain.
This question of the composition of the paste has been the subject of many experiments lately at Sèvres. A somewhat animated discussion has raged around it. M. Vogt, who is the director of the technical department in the National Porcelain Works, is well qualified to speak on the subject. We shall not hesitate then to avail ourselves of the conclusions which he arrives at, the more so as they put tersely some important points of which we shall see the importance later on. I refer especially to the relations of the glazes and the coloured decorations to the subjacent paste.
These are, then, the results that M. Vogt arrives at:—
The two extreme types of porcelain, one with 65 per cent. of kaolin and the other with only 35 per cent., when taken from the kiln do not differ in appearance, though one has been subject to a temperature of 1500° C. to ensure vitrification and the other to only 1350° C. Their physical properties, however, are very different. The first, rich in alumina derived from the excess of kaolin, stands without injury variations of temperature, it suits well with a glaze made from felspar, a glaze hard enough to resist the point of a knife. These are excellent qualities for domestic use, but such porcelain does not lend itself well to artistic decoration. At the high temperature required in this case in the firing, the colours of the paste and of the glazes assume dull and tame hues, so as to offer little resource to the artist. In a word, in that part of the decoration that has to be subjected to the full heat of the kiln, the artist has command only of a restricted and relatively dull palette. Again, in the decoration of the muffle-stove the vitrifiable enamels do not become incorporated with the glaze on which they rest. If a decoration in opaque or translucent enamels is attempted, these enamels are apt to split off, carrying with them a part of the glaze. To sum up: the porcelain of which the hard paste of Sèvres, introduced by Brogniart, may be regarded as a type, though excellent for domestic use, is incapable of receiving a brilliant decoration.
Porcelain of the second type, more silicious and less aluminous, is fired at a lower temperature. In order to get a glaze sufficiently fusible to melt at such a temperature to a fine uniform surface, it is necessary to introduce a certain amount of lime into its composition; by this the glaze is rendered at the same time a little softer. But now the lower temperature of the fire will allow of a greater variety and greater brilliancy in the colours either combined with or used under the glaze. When we come to the muffle-fire we can employ enamels of the widest range of colour, yielding a brilliant decoration. On the other hand, this type of porcelain offers less resistance than the other to the action of hard bodies and to rapid changes of temperature—enough resistance, however, so M. Vogt thinks, for all ordinary usages. It is to this type that the porcelain of China, and Japan, as well
It is to the scattered notices and essays of Mr. William Burton that we must go for information in this country. In his new work on
The most complete work on the processes of manufacture is now Dubreuil’s
The