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Page:Popular Science Monthly Volume 21.djvu/323

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PORCELAIN AND THE ART OF ITS PRODUCTION.

whether by Louis Poterat or by Reverend, at Paris or Rouen, is disputed. This ware has no relation with real porcelain; it contains neither kaolin nor feldspar, but is an artificial product, a kind of glass made from a mixture composed essentially of sand, lime, potash, soda, and a small quantity of marine marl. This mixture, made plastic by the addition of manganese or other fluxes, is baked without glazing, and is covered after baking with a glazing composed of silica, lead, potash, and soda. The beauty of the material, its perfect glaze, and the facility with which verifiable colors are fixed in it, make of tender porcelain a ware exceptionally adapted to decoration.

The discovery of tender porcelain did not arrest the investigations of men of science and potters, who saw very well that it had none of the characters of the porcelain of China. A bed of kaolin was found in Saxony in 1709, and Böttger set up at Meissen the first European factory of hard porcelain. Fifty years later, in 1758, Guettard at Alençon, and afterward, in 1769, Madame Darnet at Saint-Yeux, near Limoges, discovered the French beds, and the industrial fabrication of hard porcelain was begun in France in 1774. Tender porcelain gave way quickly in the rivalry with hard porcelain. This was unfortunate, in an artistic point of view, for the latter material is very unaccommodating to the decorator. A more important object was, however, to create for domestic economy an absolutely healthful industry, and much is due to the illustrious Brongniart for having by his investigations put the manufacture of kaolinic porcelain on a firm scientific foundation.

Natural kaolin is never a pure clay, but contains also sand, undecomposed feldspar, etc., in variable quantities, and must first be purified. For this purpose the mass is pounded, and the products are separated one after another by successive levigations with water. The clay, which is extremely slow in settling, is drawn off first, and may be obtained almost pure; the other deposits, composed of more or less feldspathic sands, are brayed in mills, and are destined to enter in their turn into the preparation of the pastes. The nature of porcelain, its physical and chemical properties, vary infinitely according to the proportion of its two consituent elements (kaolin and feldspar), and as other substances, lime, silicious sands, potsherds, etc., are added, as is often done. Every country and every factory has its composition, which is adapted to the use to which the porcelain is destined, to the degree of resistance that is to be demanded of it, and to the kind of decoration it is lo receive. Generally, porcelain is more solid as the proportion of clay increases, and requires a higher temperature in baking; if, on the other hand, the proportion of feldspar is increased, it becomes more fusible, may be baked at a lower temperature, and submits more readily to decoration, but its plasticity and the possibility of working it easily diminish rapidly. The mixture of the materials should be perfect; when this is the case, the mass will keep for a long time, and become