Techniques such as the inlay of filaments in the mass of glass and the vetro de trina, or iced glass, made Venice famous in the 16th century. Later other regions and cities - such as Bohemia, Silesia and Sèvres - also had their name incorporated into the history of the glass arts.
Glass is a transparent or translucent material, smooth and shiny, hard and fragile, obtained by mixing silica with a lower proportion of alkali, to which lime or another hardening agent and reducing the melting temperature is added. In addition to transparency, it is characterized by isotropy, that is, it has the same physical properties in all directions.
The mineral substances that make up glass melt at high temperatures and do not crystallize when solidifying. When cooled, they maintain the amorphous state of liquids, but as their viscosity increases, they acquire the properties common to solids. Because of these properties, glass is usually defined as a "superfrozen liquid" or as a substance representing an intermediate phase between the liquid and solid, or crystalline, state.
There are many types of glass, with different chemical compositions and physical qualities, but they all have certain characteristics in common, such as the fact that they can be easily broken by blows or shocks and show conchoidal fracture. When cold, they are poor conductors of electricity and heat. They are almost unaffected by solvents, but are immediately attacked by hydrofluoric acid.
Composition. The basic ingredients in the composition of glass are silica or silicon oxide (SiO2), obtained mainly from pure white sand, and alkalis (the main ones are sodium carbonate, sodium sulfate and also slaked lime). Thousands of types of glass are industrially produced with these materials and different techniques. However, the high melting point and high viscosity make it very expensive to manufacture glasses with a high silica content. That is why the most common types of glass contain, in addition to sand, several other substances, which vary according to the application.
Sodium carbonate (sodium carbonate) and lime are used in the composition of glass for bottles, glass panes and lamp bulbs, which are low cost and large in consumption. The glass used in the manufacture of laboratory utensils and kitchenware, resistant to heat and fire, contains borosilicate. Glass with high resistance to shocks and high temperatures, as well as those intended for aircraft windows, contain aluminosilicate. High transparency glass, often called crystal, is made from lead and alkali.
The addition of other chemical elements to silica, soda, potash and lime greatly increases the variety of types of glass. Adding barium, in the form of precipitated carbonate, produces remarkable changes in the refractive indices and density of the glass. Colored glasses are obtained with the addition of manganese, cobalt, iron, nickel, antimony and other metallic components.
Historic. The oldest known objects of glass are Egyptian beads, from 2500 BC, and glazed clay vases with colorful designs. Around 1500 BC, Egyptian artisans colored a type of glass with metallic oxides, almost always opaque because of the bubbles present in the mixture, as they could not produce temperatures high enough to break them.
In the 1st century BC the Syrians were blowing glass to create objects of all shapes. Blow molding was also a technique perfected by the Egyptians. On the walls of the Beni Hassan hypogeum were paintings depicting artisans working glass: one of them blows a vase with the help of a bamboo. In Hellenistic times, Egyptian glass jars imitated the shapes of pottery, and Alexandria gained fame for richly decorated glassware in the millefiori technique and pressure-molded vessels.
Known in Rome since the time of Nero, glass quickly spread throughout the empire, manufactured in a pressed or blown mould, shaped on a lathe or chiseled. Almost all forms manufactured in various centers followed the Hellenistic style, but regional forms soon emerged. With the fall of the empire, the art of glass was swept away from Europe for some time. A resurgence, at first timid, took place from the 6th century onwards in the Rhineland and Gaul.
Glassmaking developed at this time mainly in Syria and Egypt, under the influence of Islam. The Egyptian-Arab artisans began to paint the glass with metallic varnish and the Syrians developed formulas of vitreous enamels with lead, until the production of lamps, cups and original flasks, of unsurpassed technical precision.
From the 13th century onwards, glassmaking developed in Venice, where it was concentrated on the island of Murano to preserve the secret of manufacturing and cultivate the high skill of the artisans. The first objects are of heavy shape, dark colored and enamel decoration. In the middle of the 15th century, crystallo was already made, similar to rock crystal, but full of bubbles; if blown too thin, it became transparent despite the use of dyes. The process has been brought to perfection in the manufacture of cups.
Venetian artisans based in Antwerp, Liège and Nuremberg created a skillful glassworks in Venetian technique, making it very difficult to distinguish their pieces from those produced in Murano. In Silesia and Bohemia the art of glass was perfected; a new crystal was produced, made of quartz, potash and table salt, whose strength and clarity were suitable for copper and abrasive engraving. In Barcelona, since the 16th century, a remarkable glass art has been developed, characterized by enamels in which green and yellow dominate. In the following century, it was mainly in Cadalso that the art of glass flourished.
In the 18th century, the high taxes levied in England on glass components determined the spread of a type of white, opaque glass, painted in a similar way to porcelain. Important technical improvements appeared in the following century, with the transition to mechanical mass production. Manual methods were abandoned, preserved only in centers for the manufacture of luxury objects, such as Baccarat, in France. The Arts and Crafts Movement in the middle of the century, and later Art Nouveau, reacted against the dominance of the machine and the lack of originality of design and new products. René Lalique and Émile Gallé introduced a new artistic version of the shapes and colors of glass, with an oriental influence.
The 20th century brought a combination of form and function to the glass industry. With Denmark, Finland and Sweden at the forefront, designers from Europe and America have taken the glass industry to new heights. In parallel with series production, glass began to participate in architecture, as a construction and decoration material.
Main types. Flat glass, used on a large scale in the construction industry - such as glazing and mirrors - represents a considerable part of sales in the glass industry. They are manufactured in large industrial units, through successive processes from the casting of raw materials to cooling, lamination and polishing. Hollow glasses -- in the form of tubes, flasks, bottles, lamp bulbs, etc. -- also occupy an important position in domestic and industrial consumption.
The variety of table glasses ranges from the common ones, used in the manufacture of glasses, to the commercially designated semi-crystals and crystals, whose more refined types are traditionally produced by manual processes and sometimes reach a high artistic level. The great resistance to heat characterizes thermal glasses, which find application in kitchen appliances, laboratory equipment and electronic components.
Optical glasses require special care in manufacturing and finishing; are crystals of high transparency, with very precise refractive and dispersion indices, used in the manufacture of lenses for ophthalmology and optical instruments. Rhinestone glass (from Strasser, its inventor) is even harder, denser and more refractive, and is used to imitate gemstones.
It is in the automobile industry that safety glass finds its greatest market. First built (1909) with the interposition of a sheet of celluloid between two glass plates, they started to use (since 1936) a sheet of polyvinyl plastic that does not change with exposure to the sun. The common feature of these glasses is their shock resistance and the ability to break apart without causing injury to people.
Among other types of glass, it is worth mentioning fiberglass, used mainly for thermal, electrical and acoustic insulation, drawn glass (also called glass wool) and optical fibers, through which information travels in the form of pulses of light. .
