Porcelain. Dillon Edward
led to the introduction of other colours, derived both from new metallic bases and from fresh combinations of those already known. By far the most important of these new colours are those derived from the salts of chromium, but uranium and other rare metals have also been called into use. As with the sister art of painting, the beauty and harmony of the effects produced have not kept pace with the enlargement of the palette—the result was rather to accentuate the decline that had already set in from other causes.
There are two metals, iron and copper, that have always been of pre-eminent importance as sources of colour. Each of them forms two series of combinations differing entirely in hue, so that were we confined to the use of these two metals, our palette would still be a fairly complete one.
The protoxide of copper, especially when a certain amount of lime and of soda is present, forms a series of beautiful blue and green silicates. When the proportion of oxygen is decreased, as happens when the surface of the ware is exposed in the kiln to a reducing flame, a suboxide of copper is formed, which gives a deep and more or less opaque red hue to the glaze. So in the case of iron, the so-called sesqui-oxide is perhaps the most abundant source of colouring matter in the mineral kingdom: the colours produced by it range from pale yellow to orange, brown, and full red. When, however, the iron is present as a protoxide, the colour given to the glaze is entirely altered; it ranges from a pale sea-green to a deep olive.
The remaining two elements that have long played an important part in the decoration of pottery are cobalt and manganese. These metals, in the form of silicates, yield the well-known series of blues and purples. One important source of the famous underglaze blue of China and Japan is a black mineral known to us as wad, which occurs in earthy to stony concretions. This wad contains oxides of both cobalt and manganese, and the quality of the blue obtained from it depends in great measure upon the proportion in which the two metals occur.
The employment of antimony is comparatively rare, but, generally in combination with iron, it is an important source of yellow. In spite of the volatile nature of most of its salts, in the presence of silica this metal is able to withstand a high temperature.
But before considering the application of colour to the glaze, we must mention briefly a method of decoration which was in great favour at Sèvres some years ago—I mean the application of colour to the paste itself. This was done long ago by Wedgwood, sometimes to the whole mass of the paste, as was the case with his jasper ware, which some authorities class as a true porcelain. At Sèvres these coloured pastes have been generally applied to the surface only, in thin layers, or even as mere coats of paint. When laid on in successive coats, as in the so-called pâte-sur-pâte, the amount of colouring matter need not be large, from 2 to 5 per cent. When larger proportions of coloured oxides are mixed with the pâte, and this is painted on with a brush, the process differs little from the ordinary decoration under the glaze, into which it indeed may be said to pass. Coloured pastes of this description have never been employed by the Chinese, and it is not possible to obtain much brilliancy or decorative effect by their use. They are, indeed, foreign to the nature of porcelain, sacrificing the brilliant white ground which should be the basis of all decorative schemes.
When the colouring matter is subjacent to the glaze it must be of a nature to withstand the full heat of the subsequent firing; we are restricted therefore to colours ‘à grand feu.’ This practically confines us to cobalt and to certain combinations of iron and copper, as far as the ‘old palette’ is concerned. At Sèvres and elsewhere other metals have been made use of whose silicates withstand the extreme temperature of the kiln. By the use of chromium we have command of many shades of green. If to an oxide of tin we add a minute quantity of the sesqui-oxide of chromium, we can obtain, in the presence of lime, many shades from rose to purple; and a mixture of cobalt and chromium produces a fine black. There is, however, as yet no satisfactory yellow pigment known that will withstand the grand feu. At the best we can get a straw colour from certain ores of tungsten and titanium, and from uranium a yellow deeper in tint but uncertain in application.
The majority of the colours we have mentioned require a more or less oxidising flame for their full development. There are, however, two most important groups of coloured glazes, long the monopoly of the Chinese, but now successfully imitated in France and elsewhere, which require, for a term at least, to be subjected to a reducing flame.
The first of these glazes is the well-known Celadon, using that term in its proper and restricted sense, for certain shades of greyish green. The celadon of the Chinese is produced by the presence of a small quantity, about two per cent., of protoxide of iron in the glaze. An oxidising flame would change this protoxide to the yellow sesqui-oxide. We may note that a celadon of good tint can only be produced when a considerable quantity of lime is present in the glaze.
The other group, depending also upon a reducing flame, is constituted by the famous Sang de bœuf and Flambé glazes.
The colour of the first is given by the red sub-oxide of copper, chiefly suspended in the glaze. In the case of the flambé or ‘transmutation’ glazes, the strange caprices of colour have their origin, in part at least, in the contrast of the red sub-oxide and the green silicate of copper. In the case of both these glazes everything depends on the regulation of the draught of the furnace in which they are fired. The French have lately been at great pains to master the difficulties attendant upon the development of the effects sought after, and some success has been attained not only on a porcelain ground as at Sèvres, but these glazes have also been applied to fayence at the Golfe St. Juan and elsewhere. It has been proved by some experiments made at Sèvres, that in the firing, the critical period, during which so much depends upon the regulation of the draught, is just before the melting of the glaze. Once melted the glaze not only forms an impervious cover which prevents the smoky flame from discolouring the paste below, but the glaze itself is no longer sensitive to the action of the gases which surround it. It is therefore only during a short period preceding the moment when the glaze begins to melt, that it is necessary to promote a smoky and reducing flame. This is a point of considerable practical importance.20
The application of the Decoration under the Glaze is essentially a Chinese method. To it we owe the important family of ‘blue and white’ ware. The superiority of the Chinese in the management of the blue colour has been attributed to various causes. The result is no doubt influenced not only by the constitution of both paste and glaze, but also by the fact that the colour is painted upon the raw paste.
An important factor also is the care exercised by the Chinese in the selection and preparation of the blue pigment, by which not only the desired intensity but the richness of hue is secured. The quality of the blue depends in great measure upon the presence of a small quantity of manganese in the cobalt ore employed.
The only other colour that the Chinese have succeeded in using under the glaze is the red derived from the sub-oxide of copper. The full development of this colour has for long been a lost art, but a less brilliant red from this source, often little better than a buff colour, is sometimes found in later examples combined with the blue.
In the application of colours under the glaze there is one difficulty that the Chinese have surmounted even in their commonest ware, and this is the tendency of the cobalt blue to dissolve and ‘run’ in the glaze, giving to the design a blurred and indistinct appearance. It would seem that the sharpness of outline depends upon the consistency of the glaze at the moment when it first melts. At that point the glaze should be viscous and not inclined to flow, and this is what occurs in the case of the highly calcareous glazes of the Chinese.
Before passing to the enamel colours, we must say something of a class of glazes which may be looked upon as to some extent of an intermediate character. These are the glazes associated with the ‘San tsai,’ the ‘three colours’ first used in combination by the Chinese.
These coloured glazes were applied, not, as is usually the case in China, to the raw paste, but they were, it would seem, painted on the surface after a preliminary firing. Being applied with a brush, the whole surface of the biscuit was not necessarily covered, and glazes of all these colours could be used upon the same piece of porcelain. Glazes of this class were rendered more fusible by the addition of a certain quantity of lead, and on this ground, and still more in their
The colour of the ruby glass in our thirteenth century windows has a very similar origin. In this case the art was lost and only in a measure recovered at a later period. As in the case of the Chinese glaze, the point was to seize the moment when the copper was first reduced and, in a minute state of division, was suspended in floccular masses in the glass.