CHAPTER III.

COMPOSITION OF WARES AND GLAZES.

Hard and Soft Pottery and Porcelain.--COMPOSITION OF PORCELAIN: Kaolin--Its Derivation and Ingredients--Petuntse--How prepared in China.--The European Process.--Differences between Chinese and European Porcelains.--Chemical Analysis.--English Porcelain and its Peculiarities: Its Average Composition.--How English Clay is prepared.--French Artificial Porcelain.--Parian.--COMMON EARTHEN-WARE.: Table of Ingredients of different kinds.--General Table.--GLAZES: Classes.--Brongniart’s Classification.--Difference between Enamel and Glaze.--Silicious Glaze.--History.--Use of Oxides.--Egyptian Processes.--Metallic Lustre.--Stanniferous Enamel: Its History.

The division of pottery and porcelain into two great classes, hard and soft, is based upon the difference of their composition, their hardness of surface, and their power of resisting the action of fire. The simplest test is scratching with a knife or other instrument. Hard porcelain and pottery resist the metal, while the soft is marked. The former will also stand a temperature in the kiln at which the latter would crumble or fuse.

To understand the composition of porcelain, it is necessary to bear in mind that it is a compound of kaolin and petuntse, the former of which is infusible, and the latter fusible at a high temperature. The former constitutes the body of the piece, the latter gives it its translucency. The word “kaolin” is derived from _Kaoling_, the name of a mountain near King-teh-chin, one of the great centres of the manufacture in China. Kaolin is simply the result of the decomposition of granitic rock, and silica and alumina are its chief ingredients. Petuntse is pure felspar. The conditions in which these materials are found in China may be briefly stated. They are either in the form of stone or sand, from which the unsuitable parts are removed by the action of water. When they are thrown into the water, the fine particles which do not sink are collected and dried. The paste, before being used, is again put into water and strained through a sieve, so that only the finest is preserved, and used in making porcelain. The materials are obtained from different parts of the country, and blended according to their respective qualities, as ascertained by the most systematic investigation and experiment. The European process is similar, the kaolin being first washed clear of all argillaceous impurity, and then mixed with felspar and silicious sand. Of the further similarity between the two, MM. Ebelman and Salvetat say:

1st. The kaolin and petuntse used in making paste for Chinese porcelain are chemically identical with the materials used in Europe. The Chinese kaolin is evidently disintegrated granite. Chemically, petuntse resembles the pegmatite of Limoges; mineralogically, it is to be classed with petrosilicious felspar.

2d. The mechanical preparation of the pastes of China and Europe is based upon similar methods.

3d. The Chinese paste is the more fusible of the two.

4th. The Chinese glaze is also the more fusible, on account of the addition of lime to the petuntse, which the French use pure.

It may be added that the Dresden, Sèvres, and Limoges porcelains are baked at a higher temperature, and are harder than the Chinese.

The basis of the natural pastes of Germany and France is 46.66 parts of silex, 40 of aluminous earth, and 13.33 alkaline earth, although the proportions vary, and the following may be nearer an average: Silex, 66; alumina, 30; potash, magnesia, and lime, 4. In the glaze the proportions are different, the silica largely preponderating: Silex, 73.4; alumina, 15.7; potash, lime, and magnesia, 10.9.

The following table is given by M. A. Salvetat as the result of analyses made at different times by himself and others:

+--------------------------------------------------------------------------+ | Pastes. |Silica.|Alumina.| Oxide |Lime.|Magnesia.|Potash.|Soda.| | | | |of Iron.| | | | | |--------------------------------------------------------------------------| |China, 1st quality|69.00 |23.60 |1.20 |0.30 |0.02 |3.30 |2.90 | |China, 2d quality |70.00 |22.20 |1.30 |0.80 |traces |3.60 |2.70 | |China, 3d quality |73.80 |19.30 |2.00 |0.60 |traces |2.50 |2.30 | |China, 4th quality|68.94 |21.30 |3.48 |1.14 |traces |3.42 |1.78 | |Meissen |58.50 |35.10 |0.80 |0.30 |traces |5.00 |.... | |Vienna |59.60 |34.20 |0.80 |1.70 |1.40 |2.00 |.... | |Berlin |64.30 |29.00 |0.60 |0.30 |0.45 |3.65 |.... | |Limoges |70.20 |24.00 |0.70 |0.70 |0.10 |4.30 |.... | |Sèvres |58.00 |34.50 |.... |4.50 |.... |3.00 |.... | |Sèvres (sculpture)|64.10 |30.24 |.... |2.82 |traces |2.80 |.... | |Worcester |82.00 |9.10 |..... |1.30 |7.40 |.... |.... | |Paris |71.20 |22.00 |0.80 |0.80 |.... |4.50 |.... | +--------------------------------------------------------------------------+

The English artificial porcelain differs from the natural paste of China and the European continent chiefly in one particular. At first the compound used was white clay, white sand, and glass, the latter being employed to impart the necessary transparency. More recently bone came largely into use, and is now one of the distinctive ingredients of English paste. The phosphoric acid of that material was found to produce, in combination with the other materials, a clear, translucent body, of less strength than natural paste, but less liable to sink. The following may be taken as the mean composition: Bone, 47; kaolin, 34; felspar, 19. The kaolin is found in Cornwall, where a very large tract is formed chiefly of decomposed granite. The purest rock having been selected, it is placed on an inclined plane, upon which water can be turned. It is washed down into a trench, and thence into a catch-pit, and again into lower pits, in which successively the impure ingredients are retained, the water laden with the finer particles running into tanks, and there depositing its fine silt. The clay is partially dried, and cut into blocks, and in that shape reaches the potters. The manner in which the kaolin is prepared bears a very close resemblance to that adopted by the Chinese, as previously described. The glaze is composed of felspar, carbonate of lime, borax, and white-lead. Sometimes the kaolin is mixed with the bone and felspar in the proportions above specified, and sometimes the bone is made, in combination with silex and pearlash, into a frit.

The artificial or soft porcelain of France, exemplified in the old china of Sèvres, was produced by a very intricate and ingenious process. A frit was made of saltpetre, sea-salt, burnt alum, soda-ash, gypsum, and sand. This mixture, having been purified by partial vitrification, was ground, and mixed with chalk and marl. The glaze was as follows: Litharge, 38; sand, 27; calcined flint, 11; and the carbonates of soda and potash, 15 and 9 parts respectively.

The composition called parian, in which the potters of England and America have executed much beautiful work, varies considerably. Analysis of one specimen resulted thus: Silica, 58.57; alumina, 21; oxide of iron, 1; lime, 0.14; magnesia, 0.5; potash, 11.40; soda, 5.08.

The clay from which common earthen-ware is made is composed to a great extent of silica and alumina, with admixtures of iron, lime, and magnesia. An average combination is 60 parts silex, 30 alumina, 7 iron, and 2 lime. These proportions vary very widely, certain substances appearing in one place and not in another. In some, carbon is found; in others, quartz, sand, marl, or chalk, as the case may be. The work of classification, except in a very extended form, is thus rendered somewhat difficult. Possibly the following series of tables will serve our purpose most intelligibly.

Column headings:

S: Silica. A: Alumina. I: Oxide of Iron. L: Lime. M: Magnesia. W: Water. C: Carbon.

+--------------+-------+-------+-------+------+------+-------+------+ | Pottery. | S | A | I | L | M | W | C | +--------------+-------+-------+-------+------+------+-------+------+ | German | 63.90 | 12.76 | 10.24 | 1.04 | 0.52 | 9.98 | 1.02 | | Scandinavian | 64.02 | 10.77 | 11.23 | 2.48 | 0.05 | 9.97 | 1.00 | | Old Gallic | 62.22 | 18.36 | 5.71 | 1.17 | 0.47 | 10.56 | 0.78 | | Peruvian | 67.04 | 10.83 | 10.17 | 3.24 | 0.28 | 7.07 | 1.00 | | Etruscan | 64.02 | 12.49 | 8.53 | 3.00 | 1.83 | 8.13 | 2.00 | +--------------+-------+-------+-------+------+------+-------+------+

In the following carbon does not appear, and the proportion of silica increases:

+-------------+---------+----------+----------+-------+-----------+--------+ | | | | Oxide of | | | | | Pottery. | Silica. | Alumina. | Iron. | Lime. | Magnesia. | Water. | +-------------+---------+----------+----------+-------+-----------+--------+ | Roman | 64.00 | 17.77 | 10.23 | 4.86 | .... | 2.23 | | Middle Ages | 72.55 | 20.27 | 2.54 | 1.04 | .... | 3.00 | | Egypt | 81.00 | 13.50 | 1.00 | 3.00 | .... | 1.90 | | Egypt | 92.00 | 4.00 | .... | 2.00 | 0.60 | 0.40 | | Persian | 90.00 | 1.50 | 1.50 | 3.00 | 0.80 | 0.60 | | Jerusalem | 87.16 | 5.50 | .... | 3.00 | 0.78 | .... | | Arabian | 89.95 | 3.87 | .... | 2.00 | 0.51 | 3.00 | +-------------+---------+----------+----------+-------+-----------+--------+

The Egyptian, compounded as above, is that which has been commonly known as Egyptian porcelain. Many of the better known wares of Europe and the East have a common characteristic in the calcareous nature of their pastes. The silica decreases and the lime increases, while carbonic acid appears as a new ingredient.

Column headings:

S: Silica. A: Alumina. M: Magnesia. I: Oxide of Iron. C: Carbonic Acid.

+--------------------+-------+-------+------+------+-------+-------+ | Pottery. | S | A | M | I | C | Lime. | +--------------------+-------+-------+------+------+-------+-------+ | Lucca della Robbia | 49.65 | 15.50 | 0.17 | 3.70 | 8.58 | 22.40 | | Majorca | 48.00 | 17.50 | 1.17 | 3.75 | 9.46 | 20.12 | | Spain (old) | 46.04 | 18.45 | 0.87 | 3.04 | 13.96 | 17.64 | | Valencia (modern) | 51.55 | 20.52 | 1.24 | 2.63 | 10.42 | 13.64 | | Delft | 49.07 | 16.19 | 0.82 | 2.82 | 13.09 | 18.01 | | Persian | 48.54 | 12.05 | 0.30 | 3.14 | 16.72 | 19.25 | | Nevers | 56.49 | 19.22 | 0.71 | 2.12 | 6.50 | 14.96 | | Rouen | 47.96 | 15.02 | 0.44 | 4.07 | 12.27 | 20.24 | +--------------------+-------+-------+------+------+-------+-------+

Of the potteries which hold a place between the hard and soft wares are the Palissy and Henri Deux. The composition of the former is 67.50 silica, 28.51 alumina, 1.52 lime, 2.05 oxide of iron, with a very slight admixture of alkalies. That of the latter is 59.10 silica, and 40.24 alumina.

From what has been said, it will be seen that the difference between earthen-ware, stone-ware, and porcelain is to be attributed to a few minor ingredients, to the preparation, and to the degree of heat to which they are subjected. The following table may be studied for the sake of making comparisons:

Common earthen-ware Silica, 60; alumina, 30; iron, 7; lime, 2.

Blue clay Silica, 46; alumina, 38; iron, 1; lime, 1.

Staffordshire clay Pipe-clay, 40; kaolin, 25; quartz, 20; felspar, 15.

Stone-ware { Felspar, 25; quartz or silex, 25; soda, 25; plastic clay, 15; boracic acid, 10.

Porcelain Silica, 66; alumina, 30; potash, 3.4; magnesia and lime, 1.1.

Porcelain glaze Silica, 73.4; alumina, 15.7; potash, 7.4; magnesia and lime, 2.2.

English porcelain Kaolin, 34; bone, 47; felspar, 19; soda ash, 36.

Old Sèvres soft-paste { Saltpetre, 22; sea-salt, 7.2; burnt alum, 3.6; soda { ash, 3.6; gypsum, 3.6; sand, 60. { This was made into a frit and mixed--75 { parts frit, 17 chalk, and 8 of calcareous marl.

As to the glazes applied to clay or opaque ware, we have seen that they are broadly distinguished as translucent plumbiferous or alkaline, opaque stanniferous, and salt glaze. The distinction is also to be observed between glaze and enamel, although they are often confounded. Thus, according to M. Brongniart, there are three kinds of glaze--varnish, enamel, and couverte--all of which are vitrifiable. Varnish he describes as a transparent and plumbiferous material, melting at a lower temperature than that required for baking the paste; enamel, an opaque, generally stanniferous (containing tin) substance; couverte, a substance which melts at a temperature equal to that required for baking the paste. Birch, on the other hand, draws a distinction between glaze and enamel. In one place he speaks of “opaque glasses or enamels,” and again, “among the Egyptians and Assyrians, enamelling was used more frequently than glazing.” So, also, Fortnum, who, dividing pottery into soft and hard, subdivides the former into unglazed, lustrous, glazed, and enamelled. The glazed he again divides into silicious, or glass-glazed, and plumbeous, or lead-glazed, both of which are transparent. The word “glaze” is thus more correctly applied to the covering, which does not alter the color of the body upon which it is laid, and “enamel” to that which obscures the body.

Glass, or silicious glaze, is formed by fusing sand with an alkali--potash or soda. When to this is added the oxide of lead, transparent plumbiferous glaze is the result; and when to both of these oxide of tin is added, we have opaque stanniferous enamel. The glass and plumbeous glazes may be colored with a variety of other oxides, without losing their transparency.

When or where glaze was first applied to clay is not known. Like many other branches of knowledge and many nations, it has its roots in the East, but whether we are indebted for it to India, Egypt, or Assyria, cannot now be decided. Upon this question Dr. Birch says:

“The desire of rendering terra-cotta less porous, and of producing vases capable of retaining liquids, gave rise to the covering it with a vitreous enamel or glaze. The invention of glass has been hitherto generally attributed to the Phœnicians; but opaque glasses or enamels, as old as the eighteenth dynasty, and enamelled objects as early as the fourth (B.C. 3000-2000), have been found in Egypt. The employment of copper to produce a brilliant blue-colored enamel was very early both in Babylonia and Assyria, but the use of tin for a white enamel, as recently discovered in the enamelled bricks and vases of Babylonia and Assyria, anticipated by many centuries the rediscovery of that process in Europe in the fifteenth century, and shows the early application of metallic oxides. This invention apparently remained for many centuries a secret among the Eastern nations only, enamelled terra-cotta and glass forming articles of commercial export from Egypt and Phœnicia to every part of the Mediterranean. Among the Egyptians and Assyrians enamelling was used more frequently than glazing, and their works are consequently a kind of faience, consisting of a loose frit or body, to which an enamel adheres after only a slight fusion. After the fall of the Roman Empire the art of enamelling terra-cotta disappeared among the Arab and Moorish races, who had retained a traditional knowledge of the process. The application of a transparent vitreous coating, or glaze, over the entire surface, like the varnish of a picture, is also referable to a high antiquity, and was universally adopted either to enhance the beauty of single colors or to promote the combination of many. Innumerable fragments and remains of glazed vases, fabricated by the Greeks and Romans, not only prove the early use of glazing, but also exhibit, in the present day, many of the noblest efforts of the potter’s art.”

The use of oxides is also very ancient. The Egyptians employed that of copper for the production of their turquoise-blue, and possibly also for their green, manganese for violet, iron or silver for yellows, etc. The same processes were known in Babylon and Assyria. To the Persians and Arabians the application of metallic lustres was known at a very early period. Plumbiferous, or lead-glaze, was employed by the Babylonians, and the knowledge of its composition was in all probability imported thence among the Greeks, and by them may have been carried into Southern Italy.

The course of enamel is equally difficult of definition. Although used in Egypt, Babylon, and Assyria, it does not appear to have supplanted the lead-glaze; and for a long period all traces of it are lost, until it reappeared among the Arabs. We next meet with it as a distinctive characteristic of the potteries of Spain. It was also known to the Saracenic and Moorish potters of Sicily, and from either of these sources may have found its way into Italy.