Porcelain. Dillon Edward
a certain reserve in favour of the earlier wares.
Our subject may seem a simple one compared with some kindred branches of the industrial arts, such, for example, as the history of glass-making, or that of cloisonné and other enamels. We come indeed at more than one time into contact with both these arts, and it is just at these points that some of our chief difficulties arise. It is in view of such questions as these, and indeed of many others equally important in the history of porcelain, that the necessity of a thorough understanding of the technical and even chemical side of our subject becomes evident. Of course, if in discussing the different kinds of porcelain we are concerned only with their merits or demerits as artistic products, we can put aside these practical questions as ‘beneath the dignity of our argument.’ But such a treatment of the subject would land us only too surely in vague generalities and in an arrangement based upon personal caprice. We require, above all at the start, a firm basis, and this can only be found in a thorough comprehension not only of the technical processes that are involved in the manufacture of porcelain, but of the physical and chemical nature of the substance itself.
But first we need some kind of preliminary definition of what is meant by the word. Porcelain, then, is distinguished from other fictile wares by possessing in a pre-eminent degree the following qualities: hardness, difficult fusibility, translucency, and whiteness of body or paste. Any specimen of ceramic ware that possesses all these qualities may be classed as porcelain, and from a practical point of view, the more it excels under these heads, the better specimen of porcelain it is.
These were the qualities by which the porcelain brought from the East in the seventeenth century was distinguished from any ware made at that time in Europe. Our ancestors dwelt especially on the practical advantages of the hard glaze and the elastic compact paste of the new ware, which compared favourably with the easily scratched surface and the crumbly body of the earthenware then in general use.
The greater infusibility that accompanies this hardness was not a point of much importance to them, but they marvelled at the translucency of the edges, as of some natural stone, and we find absurdly exaggerated accounts of the transparency both of the original ware and of the imitation that they claimed to have made. Finally, they noticed that the whiteness of the surface was not given by an artificial layer more or less closely adhering to an earthy base, but was the natural colour of the paste to which the thin layer of transparent glaze merely gave the effect of the polish on ivory or on marble. What then was this hard, white, translucent substance? What wonder if from one end of Europe to the other, scheming minds—chemists, alchemists, physicians, potters, and charlatans—were at work trying to make something that should resemble it? The history of this long search is a very interesting one, but it would be impossible to explain its failures, its partial failures (these last resulting in a compromise—soft-paste porcelain), and the final success of Böttger, without, as it were, going behind the scenes, and giving some account of porcelain from a modern, scientific point of view.
And first let us say that, although when treating of porcelain from the historical and especially from the æsthetic standpoint (and this after all is our principal business in this book), it is well to take a wide grasp and include a whole class of china—I mean the soft-paste ware—which does not come up to our standard of hardness and infusibility, this is not the case when we are considering the physical, and especially the chemical, nature of porcelain. By confining ourselves, for the present, to true hard porcelain, we have the advantage of dealing with a substance which chemically and physically may be compared to a definite mineral species. Nay more, we propose here to confine ourselves to the consideration of the hard pastes used at the present day in the wares of France and Germany, neglecting for the present the softer and more irregular porcelain of the Chinese.
First as regards hardness, the surface of the paste of a true porcelain, when free from glaze, can be scratched by a crystal of quartz, but it is untouched by the hardest steel. That is to say, it would be classed by the mineralogist with felspar, and given a hardness of 6 to 6·5 on his scale.1
The freshly broken edge shows a white, perfectly uniform substance, a glassy or vitreous lustre, a finely granular texture, and a fracture conchoidal to splintery. When struck, a vessel of porcelain gives a clear, bell-like note, and in this differs from other kinds of pottery. When held against the light it allows, where the piece is sufficiently thin, a certain amount to pass through, but even in the thinnest splinters porcelain is never transparent.
If a thin section be made of a piece of porcelain, and this be examined under the microscope by transmitted light, we see, scattered in a clear, or nearly clear, paste, a vast number of minute, slender rods, and between them many minute granules (Church’s English Porcelain, p. 6). These belonites and spherulites, as they have been called, doubtless reflect the light which would otherwise pass through the glassy base in which they float, and the partial reflection and partial transmission of the light may not be unconnected with the lustrous fracture so characteristic of porcelain. Their presence points to the fact that we are dealing with a more or less definite substance, one which may be compared to a natural mineral species, and not merely with a semi-fused clay, something between stoneware and glass. Now when we come to treat of the chemical constitution of porcelain, we shall find that this view is confirmed. This structure is developed in the paste by the exposure, for a considerable period of time, to a temperature of from 1300° to 1500° centigrade, a temperature which is sufficient to reduce all other kinds of pottery, with the exception of some kinds of stoneware, to a glassy mass. In the case of porcelain, this great and prolonged heat allows of a complete rearrangement of the molecules in the softened mass. The process may be compared to that by which certain minerals and rocks are formed in the depths of the earth.
We see, then, that not only from the standpoint of history, but on the basis of the physical properties and intimate constitution of the material, we are able to draw a sharp line between porcelain and other fictile wares. This distinction is even more definitely shown by a chemical analysis.2
We are dealing, as in the case of so large a part of the rocks and minerals of the earth’s surface, with certain silicates of the alkalis and alkaline earths, with silicates of alumina above all. All natural clays used for fictile purposes consist essentially of silicates of various bases, such as alumina, lime, iron, potash, and soda, more or less intimately combined with water, and with the addition, generally, of some free silica. If the clay be good in working quality and colour, the next point the potter has to look to is the question of its fusibility. It may be said generally that the simpler the constitution of a silicate, that is the smaller the number of bases that it contains, the greater will be its resistance to fire. Silicate of alumina is unaltered at 1500° C., a temperature which may be taken as the maximum at the command of the potter. The fusing-point is reduced by the addition of silica, especially if some other bases such as oxide of iron or lime, or again an alkali, are present even in small quantity. But beyond a certain point the addition of silica raises the fusing-point, and it is important to note that it is this excess of silica that renders certain stonewares and fire-clays so infusible. In the case of porcelain, on the other hand, the resistance to high temperatures depends more upon the percentage of alumina present, and the absence or small amount of other bases. Thus in comparing the composition of different porcelains, we find that it is those that contain the most silica that are the most fusible, or rather, to speak more accurately, that become ‘porcelainised’ at a lower temperature.3
The relation of porcelain to stoneware on the one hand, and to ordinary pottery on the other, will be made clear by the following figures, which give the composition of stoneware, Meissen porcelain, and of a red Samian ware:—
The refractory stoneware contains a large excess of silica over the amount required to combine with the alumina and the ‘other bases.’ In the easily fusible Roman pottery, the ‘other bases’ nearly equal in amount the alumina, while the Meissen porcelain not only contains less silica than the pottery, but the ‘other bases’ only amount to a sixth part of the alumina present.
But it is not enough for the manufacturer to discover a clay of which the chemical composition corresponds to that
Some English porcelain is stated by Professor Church to have a hardness equal to that of quartz. See below, ‘Bristol Porcelain.’
We have thought it well, once for all, to treat briefly of the scientific aspect of our subject, but those who are not interested in this point of view may pass over the next few pages.
I shall return to this point in a later chapter. I lay the more stress on this fact, as it is often stated that the hard and slightly translucent stonewares, such as the Fulham ware of Dwight, which contains as much as eighty per cent. of silica, form one degree of a series of which true porcelain is the next term. The fact is, those who sought to make porcelain by a refinement in the manufacture of stoneware were as much astray as those who started from a fusible glass frit.