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.
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 | +-------------+---------+----------+----------+-------+-----------+--------+