Glassmaking over the centuries

Many different useful and decorative articles have been made from glass over the centuries. The history of glass as a creative art has been determined partly by technical advances in its manufacture and decoration and partly by the history of taste and fashion.

Glass was first made in the ancient world, but its earliest origins are obscure. Egyptian glass beads are the earliest glass objects known, dating from about 2500 BCE. Later in Egyptian civilization, a type of glass characterized by feathery or zigzag patterns of coloured threads on the surface of the glass vessel was made.

The real origins of modern glass were in Alexandria during the Ptolemaic period and, later, in ancient Rome. Alexandrian craftsmen perfected a technique known as mosaic glass in which slices of glass canes of different colours were cut crossways to make different decorative patterns. Millefiori glass, for which the canes are cut in such a way as to produce designs reminiscent of flower shapes, is a type of mosaic glass.

Molded glass was also developed early, glass being pressed into a mold to form a particular shape. Various types of decoration involving engraving and colour were also possible.

Glassblowing was probably developed during the 1st century BCE by glassmakers in Syria. With this technique the possibilities of shaping glass into desired forms were endless. Glass could be blown into a mold or shaped completely free-form. The Romans perfected cameo glass, in which the design has been produced by cutting away a layer of glass to leave the design in relief.

He next major developments in the history of glass came during the 15th century in Venice. As early as the 13th century the Venetian island of Murano had become the centre for glassmaking. At first, Venetian glassmakers made use of many of the ancient and medieval decorative techniques to produce richly coloured and ornamental pieces having motifs characteristic of the Italian Renaissance.

Later they developed a clear glass similar to crystal, called cristallo, which was to form the basis for a thriving export trade and spread throughout Europe. Simple blown glasses of this type were much in demand in the 16th century. Such glass lent itself to decoration by the engraving of delicate designs; used from the early 16th century, the technique remained popular well into the 18th century throughout Europe. Diamond-point engraving was practiced in particular in the Netherlands and in Germany.

Late in the 17th century Bohemia became an important glass-producing area, and it remained important until early in the 20th century. By the 17th century England was making glass in the Venetian tradition that was notable for its simplicity. The glassmaker George Ravenscroft discovered about 1675 that the addition of lead oxide to Venetian-type glass produced a solid, heavier glass. Lead crystal, as it was known, thereafter became a favourite type of glass for fine tableware.

Enameling came into fashion in the middle of the 18th century in England, leading to the development of the type of glass sometimes called Bristol glass. In the 18th century glass cutting came into fashion. As this technique was perfected, great richness of effect became possible. Eventually, by the end of the 18th century, when the technique was further developed in Ireland, the whole surface of glass was being deeply cut to reflect light. This English and Irish cut lead crystal was imitated in Europe and in the United States and has remained popular to the present day. Waterford crystal is an important example of this type.

The Art Nouveau period saw some important changes. The Favrile glass invented by Louis Comfort Tiffany, with its flowing shapes derived from naturalistic forms and its lustrous surface, was much admired and particularly influenced glassmakers in central Europe. The French glassmaker Émile Gallé and the firm of Daum Frères were also important designers in the Art Nouveau epoch.

René Lalique, one of the leaders of French glass art, made glass characterized by relief decoration. The Steuben Glass Company of New York produced clear glass objects, often with engraved or incised designs.

Water glass, also called sodium silicate or soluble glass, a compound containing sodium oxide (Na2O) and silica (silicon dioxide, SiO2) that forms a glassy solid with the very useful property of being soluble in water. Water glass is sold as solid lumps or powders or as a clear, syrupy liquid. It is used as a convenient source of sodium for many industrial products, as a builder in laundry detergents, as a binder and adhesive, as a flocculant in water-treatment plants, and in many other applications.

Water glass has been manufactured since the 19th century, and the basic principles of making “silicate of soda” have not changed since that time. It is commonly produced by roasting various quantities of soda ash (sodium carbonate, Na2CO3) and silica sand (a ubiquitous source of SiO2) in a furnace at temperatures between about 1,000 and 1,400 °C (approximately 1,800 and 2,500 °F), a process that gives off carbon dioxide (CO2) and produces sodium silicate (Na2SiO3; usually represented by its two constituents,

Na2O and SiO2):Na2CO3 + SiO2→ Na2O∙SiO2 + CO2

This roasting produces fused glassy lumps called cullet, which can be cooled and sold in that form or ground up and sold as powders. Lump or ground water glass in turn can be fed into pressurized reactors for dissolving in hot water. The solution is cooled to a viscous liquid and sold in containers ranging in size from small jars to large drums or tanks.

Sodium silicate liquid can also be prepared directly by dissolving silica sand under pressure in a heated aqueous solution of caustic soda (sodium hydroxide, NaOH):

2NaOH + SiO2 → Na2O∙SiO2 + H2O

In either production route, the higher the ratio of SiO2 to Na2O and the higher the concentration of both ingredients, the more viscous the solution. Viscosity is a product of the formation of silicate polymers, the silicon (Si) and oxygen (O) atoms being linked by covalent bonds into large negatively charged chain or ring structures that incorporate the positively charged sodium ions as well as water molecules. Highly viscous solutions can be spray-dried to form glasslike beads of hydrated sodium silicate. The beads can be packaged for sale to commercial users much like ground cullet, but they dissolve more readily than the anhydrous form of water glass.

These properties make hydrated sodium silicates ideal for use in one of their most common consumer products: powdered laundry and dishwasher detergents. Dissolved water glass is moderately to highly alkaline, and in detergents this property aids in the removal of fats and oils, the neutralization of acids, and the breakdown of starches and proteins. The same property makes the compound useful in the de-inking of wastepaper and in the bleaching of paper pulp.

Small quantities of dissolved water glass are used in the treatment of municipal water supplies as well as wastewater, where it adsorbs metallic ions and aids in the formation of loose agglomerations of particles called flocs, which filter the water of undesirable suspended materials.

Liquid sodium silicate reacts under acidic conditions to form a hard glassy gel. This property makes it useful as a bonding agent in cemented products such as concrete and abrasive wheels. It is also an excellent adhesive for glass or porcelain.

A traditional use for dissolved water glass is as a preservative for eggs. Fresh eggs stored under cool conditions in a viscous silicate solution will keep for months.

There are many formulations of sodium silicate, depending on the amounts of Na2O and SiO2. Also, there are other silicate glasses in which the sodium is replaced by another alkali metal, such as potassium or lithium. Some glasses are better suited than others for particular applications, but they all share the same property of being a glassy solid that dissolves in water to form an alkaline solution.

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