18GS.] Porcelain, Glass, and Pottery. 843 of the Western Empire. It was revived, however, in Spain through the Moors by the production of beautiful tiles of enameled earthenware, called by the Spaniards "Azulejo;"* and spread thence into Italy, in " Umbrian Ware," or "Majolica;" thence into France, Germany, Holland; and, finally, into England, where, under Wedgewood, it made greater progress and attained its greatest importance, as a branch of the Useful Arts. In this immethodical and desultory sketch of the histories of the Ceramic and Vitreous Arts, we have been guided in our order of treatment by the rela- tive general commercial values of the wares, rather than by anj^ thing more philosophical. From a chemical stand- point, they have much in common ; and we may properly place Glass and Pottery at the ends of the series, Porce- lain holding an intermediate position, and possessing properties in common with, and allying it to both. There are two chemical elements — sometimes designated by chemists by the name of Glass-Producers or Hyalo- gens, whose union with oxygen produces acids, which, at a high temperature, have the power of uniting with various salifiable bases ; and to the products thus formed is given the name, of passes. Of these two — which are called Silicon and Boron, and their oxygen compounds Silicic and Boracic acids — the former, alone, is used in practice. It is abun- dant everywhere, and is an old acquaint- ance of us all, merely under other names. We stj'le it variously Quartz, Sand, Flint, Agate, Jasper, or otherwise, ac- cording to the conditions under which we observe it. The salifiable bases are potash, soda, lime, baryta, magnesia, alumina and the oxides of iron and man- ganese, variously used and proportioned, according to the quality of the glass desired, and with a view to the purposes to which it is to be applied. ♦Marryat's History of Pottery, London, 1857. These bases and silicic acid, in the high heat of the glass-pot. unite to form artificial compounds, which are called silicates, analogous in composition to those abundantly distributed natural compounds, which form many of the most beautiful objects in the mineral- ogist's collection. The artificial sub- stance differs, in one essential particular, from its native relatives, for, while these last are crystalline in structure, glass is what is scientifically termed amorphous, that is to say, is non-crystalline. " It bears," says Dr. Knapp, " the same rela- tion to a natural silicate of like compo- sition, that melted sugar does to ordi- nary crystallized sugar." This feature of structure is the cause of the brittle- ness, since the melted mass, being rapidly cooled, has no opportunity to allow its particles to carry out their natural intention and desire of crystal- lizing. On the contrar}-, they are forced to remain in the relative positions in which they were placed during working ; and, consequently, there is such a want of uniformity, in attraction of particle for particle, that the slightest impulse, from without, frequently will subvert the cohesive force and cause the mass to break. The familiar philosophical toys, known as " Prince Rupert's Drops" and "Bologna Flasks," beautifully illustrate this. To obviate this evil, the glass-maker has resort to the process, that we call annealing, which he conducts, by ex- posing his wares, or products, to heat just sufficient to soften the glass ; and allows it gradually and slowly to cool. If, however, the mass is too rapidly cooled, the vitreous, amorphous struc- ture of the glass is destroyed ; and the crystalline structure established, pro- ducing a remarkable change in j)roper- ties ; the mass losing its lustre ; and, in a great measure, its transparency ;. and resembling, in so> many respects, porcelain, that it is called Reamur's Porcelain. Artistic talent has found a wide field
