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Glassmaking And Decorating
Until the twentieth century there have been few major developments in the art of glassworking. After the introduction of the blowing iron about the first century B.C., there was no comparable invention until the pressing machine in the nineteenth century. In fact, almost every known technique of making or decorating glass up to this machine age was practiced during Roman times. The Venetians rediscovered Roman methods. improved some, and also developed better formulas.
The basic formula used from the beginning of the art is composed of silica (sand) fused with soda and potash. To this was added lime or lead, as well as oxides for coloring, and manganese or other chemicals for decolorizing. Cullet or broken glass is included in every formula. In spite of manganese, most glass has a greenish tinge which is due to iron in the sand. Another deterrent to good clear glass is the presence of "seeds" or "stones" and "cords" which are caused by unfused sand, foreign matter, or air pockets due to improper melting. In early glass we can often see bits of unfused sand or even dust from the atmosphere.
George Ravenscroft's lead formula, invented in England in the 1670s, produced a great change in glassworking. His formula had to be carefully prepared with fine sand, red lead or litharge, potash, and lime. The soda and potash glass continued to be used on the Continent and in early American glasshouses. However, both Stiegel and Amelung made some of the lead glasses. After 1800 and up to 1864 most American tableware was made with lead. In the latter year, William Leighton, Sr. improved the soda-lime formula so well that the making of expensive lead glass was discontinued in most factories. Pressed glass from that time on rarely contained lead which had given the earlier ware weight, brilliance, and resonance. Since 1900 some radically new formulas have been invented, but few have affected table or ornamental ware.
There are two major methods of decorating glass. First, the decoration is an integral part of the glass. Second, it is added after the article has been otherwise finished. The latter decoration is of two types. Painting, enameling, gilding, and transfer papers give an extrinsic colored decoration. Cutting, engraving, carving, etching, and sandblasting form patterns by removing part of the article.
Decorating glass as it is being formed into articles is accomplished in various ways. With the use of his regular hand tools, a skillful glassblower by blowing, spinning, and reheating can shape an article into different forms. By pinching, pulling out part of it, shaping the rims he can further change its contours. The addition of more glass, either of the same or of a different color, has given glass designers a variety of decorative types. This method includes the simple process of using a few threads around the neck or rim of the article and the complicated technique of imprisoning between two glasses, metals or other glass, either previously patterned or left in flakes or bits.