Porcelain. Dillon Edward
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.
CHAPTER IV
DECORATION BY MEANS OF COLOUR
IF we were treating the subject purely from a practical point of view, with the glazing and firing of a piece of porcelain the manufacture might be held to be terminated. This would be strictly true, for instance, of the white porcelain of Berlin, so largely used in the chemical laboratory; a great deal, too, of the china in domestic use receives no decoration of any kind. But for us there remains still to examine the element of colour and the way in which it is applied to the decoration of porcelain.
This is effected in three different ways: by the employment of coloured glazes; by painting on the surface of the paste before the glaze is applied (this is the decoration sous couverte); and finally by coloured enamels applied to the surface of the glaze. These methods may be combined, but as this is rarely the case, such a division forms the basis of a convenient classification, more especially for the wares of China and Japan.
In the case of both the paste and of the glaze, we have been dealing with a restricted group of elements, with alumina, lime, potash and soda; and apart from impurities unintentionally introduced, all the combinations of these bodies are colourless. We have now to consider the effect of introducing certain of the heavy metallic bases which combine with the excess of silica to form coloured silicates.
The metals that give to Oriental porcelain its brilliant hues are few in number. Indeed, in all lands and at all times, iron, copper, cobalt, and manganese have been the principal sources of colour in the decoration not only of porcelain, but of most other kinds of pottery. As equal to these four metals in importance, but not strictly to be classed as colouring materials, we may place tin, the source of most opaque whites, and lead, which is the main fluxing element for our enamels. Next in importance to these metals come antimony, long known to the Chinese as a source of yellow, and finally, but this last only since the beginning of the eighteenth century, gold, as the source of a red pigment.19 This exhausts the list, not only for the Far East, but for all the pottery of Europe up to the end of the eighteenth century.
It was in a period of artistic
A glaze of this nature was in the Saracenic East applied to a layer of fine white slip, which itself formed a coating on the coarse paste. Such a combination, often very difficult to distinguish from a tin enamel, we find on the wall-tiles of Persia and Damascus.
Metallic gold has, of course, been applied to the decoration of porcelain in all countries.