Art glass production technologies
Glass is one of the most common art materials, featuring an amazing wealth of technical and decorative aspects. Its history spans about five thousand years. Pliny the Elder associated the origin of glass with ancient Phoenicia. According to legend, “Phoenician merchants carried a load of natural soda extracted in Africa across the Mediterranean Sea. For the night stay, they landed on the sandy shore and began to cook their own food. For lack of stones at hand, they rounded a bonfire with large pieces of soda. In the morning, raking the ashes, the merchants discovered a wonderful ingot that was solid like stone, sparkled in the sun and was clean and transparent, like water. It was glass*.
Glass is an artificial amorphous material obtained at a temperature of 1500-1700o С by cooking a special mixture called a “batch” that consists of 75% sand (silica), 10% limestone or chalk (calcium oxide) and 15% soda (sodium carbonate). Silica is the main component of the batch. Its melting point is 2000oС, which is significantly higher than the temperature in traditional glass melting furnaces. Therefore, the melting point of silica is lowered by addition of soda (sodium carbonate). The third required element - calcium in the form of limestone or chalk - serves as a stabilizer, giving the glass strength and making it more resistant to environmental and chemical influences. Glass, obtained only from sand and soda, is destroyed by atmospheric moisture and can dissolve in water.
In practice, the glass usually contains five to six or more components. Moreover, the addition or replacement in the glass formulation of one component with another has a significant impact on not only on the properties of the material, but on the method of processing as well. Thus, the soda-limestone (calcium-sodium) glass produced in Venice, which flourished in the 15th-16th centuries, was soft and easy to process. It was decorated with filigree patterns, plaques and painting with polychrome enamels. Hardness, high degree of transparency and good optical properties are distinguished by potassium-calcium glass (the so-called "Bohemian crystal"), which appeared in the 1670-1680s in Bohemia. It allowed applying engraving, carving and grinding for glass decoration. Such glass is called "potash" or forest glass, since the role of soda is played by potash (potassium carbonate) - the ashes of tree species. Lead-potassium glass, called “crystal,” was invented by the end of the 17th century. This kind of transparent colorless glass is characterized by high refractive index obtained due to high content of lead oxide in the batch (from 24 up to 30%), which partially or completely replaces limestone. Strong luster and high refractive index of this kind of glass is often emphasized by faceting.
Glass can be colorless and stained, transparent or opaque. To obtain opaque glass, various substances are introduced into the batch - phosphorus, tin, arsenic, antimony, bone meal and so on. To obtain color (stained) glass, various metal oxides are added (copper - for blue shades, chromium - for green, manganese - for violet, cobalt - for blue, etc.).
Various techniques s were used to create (form) glass products; some of them are still used. Thus, in Mesopotamia and then in Egypt, core technique was used to create hollow vessels. It flourished in the 4th-2nd centuries BC. A clay core was put on a metal rod, which was shaped into a future vessel. Then a portion of hot glass was collected on a metal rod and, bringing it to a slowly rotating core, stretched glass tows “wound” circle by circle, closely, in a spiral. Connecting, they created a vessel body. The uneven surface of the formed object was smoothed out by re-heating and rolling it on a flat surface of stone, marble or iron. A crown was formed and a stem and handles were attached additionally warming the vessel.
Molding technique emerged in the 9th century – glass objects were created by pouring hot glass into a mold (clay or metal, a single-use or split-type reusable). Until the 1st century, in antique glass making bowls with thick walls — two-handed goblets, skyphoses and Kantharoses were cast in the similar way. After cooling, the surface of the objects was polished; the details were finalized by carving.
In Egypt, Syria and Rome from the II century BC to the 1st century AD glassmakers created bowls imitating colored stone products. To do this, they combined casting with mosaic technique. A beam was collected from glass trunks of different colors, which in cross section gave the intended pattern. The stems were welded, turning into a monolithic billet. It was warmed up and stretched. The obtained long rod was dissected into plates with the desired pattern, which were laid in the mold. The gaps were filled with crushed glass. When heated in a furnace, the plates fused with the background, creating bizarre patterns.
The method of glass products creation by filling the form with crushed low-melting glass, which, when slowly heated, melted and filled all the voids, was called "sagging". In this way, as a rule, glass sculptures or items with reliefs were made.
Since the time of the Roman Empire, glass-blowing (tube) technology was known that makes it possible to make small objects from fusible glass billets-tubes (i.e. rods) by heating them by oil (and later gas) lamp burner. Subsequent molding was realized manually using various tweezers and forceps.
The most ancient technology in glassmaking was compression moulding - a method of molding products from molten glass by extruding it into molds. Manual compression moulding of glass of pasty-like consistency into mold was known back in the 16th century BC in ancient Egypt, where small-sized objects were made in this way: beads, amulets, mosaic tiles. In the 16th-18th centuries in Europe, manual pressing was used to make small items, such as pendants for chandeliers. Mechanized compression moulding method was developed in the United States in 1820-1825. In the second half of the 19th century, glass compression moulding developed industrially in all countries "Shaped and pressed" products were produced by many factories in the United States, Europe and Russia.
Molding of molten glass products manually with the use of a special tube pontil tube is called blowing. Pontil was invented around 64 BC and allowed a master blower to create an object directly at the furnace, in the so-called “hot form”. A lump of molten glass called a “bullet” was taken from a pot with the help of a tube and then a glass item was blown out.
Glass blowing is an ancient traditional way of manual molding of products, which remains practically unchangeable to our days. Glass products, completely made (blown and decorated) from glass melt directly at glass melting furnace, are called “hot-glass (furnace) glassblowing”. To achieve the desired configuration of a glass object, a glass bubble is placed in the mold.
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