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t was through his association with Der Sturm that he established contact with Dadaists, although he never officially joined the movement after feuding with Richard Huelsenbeck (1892–1974), who excluded him from the Berlin wing. After 1923 his work was influenced by De Stijl and Constructivism. That same year he began his magnum opus, the extraordinary Merzbau (destr.), an architectonic assemblage which gradually overwhelmed his Hanover home. Forced to flee Nazi persecution in 1937, he moved first to Norway and then in 1940 to Britain, where he later died in obscurity.
can be made opaque white by the deliberate growing of a large mass of crystals within the glass, and throughout the history of glassmaking many different chemical means of achieving this have been used. The most important technique in use at the end of the 20th century is the inclusion of a considerable quantity of fluorine into the batch, which on cooling forms a mass of crystals that are fluorides of every element in the glass. 23
Blues are very easily achieved: cobalt oxide is the basis of a strong, rich blue. Copper oxide dissolved in glass produces a huge variety of colours depending on the concentration and the level of oxygen allowed to remain in combination with the copper. Fully oxidized copper oxide (cupric oxide) in a pure soda–lime glass gives a pure, sky blue, which becomes increasingly turquoise if the glass contains either lead, boron or titanium. At concentrations of c. 0.2%, and if carefully stripped of its oxygen using a small quantity of tin as a reducing agent, colloidal particles of metallic copper will precipitate to produce a very rich, translucent red. The ancient Egyptians’ only method of producing an opaque, blood-red glass was by the precipitation of crystals of the intermediate cuprous oxide. The Venetians discovered that it was possible to precipitate crystals of pure metallic copper in glass, which grew as optically perfect, small flat ‘mirrors’, dispersed randomly throughout the glass, imparting a sparkling effect. This became
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In layman’s terms, glass is understood to refer to the manmade silica-based material used to make such items as window panes, bottles and drinking vessels. To the materials scientist, however, the term glass refers to a specific state of matter, often called the ‘glassy’ state; its defining property is that, regardless of its chemical composition, the material has solidified from the liquid state without forming any crystals, and thus at the atomic level lacks the regular ordered structure of normal crystalline solid materials. The ‘glassy’ state of matter is therefore the random,23 three-dimensional network of atomic bonds in the liquid state, which is preserved in the solid state; glass is therefore sometimes described as a ‘super-cooled’ liquid. It is a truism of science therefore that almost anything can be made into a glass if it is cooled down fast enough from the liquid state to prevent the formation of crystals as the material solidifies; the required rate may very well be measured in millions of degrees per second.
Occasionally, and under fairly dramatic circumstances, glass is formed in nature. Some volcanoes produce the mineral obsidian, a natural form of black glass, which when viewed in very thin section can be seen to be a translucent grey. It has been used by Native Americans to make carvings and tools, as large quantities were formed by the vast volcanoes that made up the Jemez Mountains of northern New Mexico. Natural glass is also formed when lightning strikes a sandy desert during a violent electrical storm. The immense energy discharged at the point of contact causes the sand to vaporize down the electrical path the lightning takes, leaving a fulgurik, a tube of melted sand, which solidifies without crystallizing t
Sand, the main constituent of manmade glass, is silicon dioxide, often referred to as silica, which has a melting-point of c. 1720°C. The melting-point (fusion point) of pure silica can be significantly lowered by over 1000°C by adding alkalis that serve as fluxes, notably the carbonates of sodium and potassium. A simple binary composition of silica and sodium carbonate (soda) will produce a non-durable glass, which will react easily with water. Some silica and potassium carbonate glass rapidly and completely dissolves in water. These binary compositions can be made durable by the simple and relatively low concentration of calcium carbonate (lime). Simple glass of the soda–lime–silica type is by far the most common type manufactured; the composition generally approximates to 75% silica, 15% soda and 10% lime. Glass of this type is suitable for windows and such industrially made wares as bottles; it is not, however, very suitable for handworking as it hardens very quickly on cooling (short working-range) owing to its relatively high lime content.
For
handworking, glassmakers require a glass with an extended working-range, which allows sufficient time for the various forming processes to be completed without the need for constant reheating (annealing). This has been achieved in different ways depending on the materials available. Glassmakers based around the Mediterranean seaboard during the Roman period worked glass with a soda content frequently over 20% and lime significantly less than 10%. Soda was easily available either being imported directly from Egypt or prepared by burning certain types of littoral plants.
Early northern European glassmakers burnt beechwood or oak in pits and used the potassium-rich ash (potash) as the flux. Characteristically potassium imparts a longer working-range to glass than sodium. Throughout England, samples from furnace sites as wide apart as Surrey and Yorkshire indicate that medieval glassmakers mainly used a flux with a very high concentration of lime, sometimes as high as 22%. The glass produced had to be worked while extremely hot as it tended to crystallize if left for any significant period below 1290°C.
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For several centuries glassmakers made use of lead in glass in a wide range of concentrations to achieve glass that was highly suitable for handworking. During the 1990s the use of such toxic, heavy metals in glass as lead and barium became a controversial issue because of environmental considerations, and the possibility of lead leaching out of the glass container into its contents. The benefits derived from the use of lead are that it imparts a very long working-range to the glass, especially when it completely replaces lime, and that lead, being a heavy metal, increases the density of the glass, which in turn raises the refractive index of the glass, thereby making the glass far more brilliant.
It was the introduction of lead into the manufacture of English glass by George Ravenscroft in 1676 that eventually gave rise to the cut-glass industry in England. Full lead-crystal contains a minimum of 30% lead and is fluxed with potassium; when cut and polished it is sparkling and brilliant. Lead is also used, albeit in much lower concentrations, by glass artists, simply as a means of achieving highly workable glass: for example many glass compositions contain concentrations of lead in the range of 4%–10%.
2.
Hot liquid glass is a universal solvent for oxides of every element. A group of elements known as transition metals share the characteristic of having incomplete inner electron shells. Metal-oxides from this group frequently have a colouring effect when dissolved in glass. Usually it becomes more difficult to achieve certain colours proceeding from blue in the spectrum. Not all colours, however, are produced by oxides of transition metals: particularly at the red end of the spectrum no transition-metal oxide dissolved in glass will create a red colour. Other colouratic techniques involve the precipitation of a metal in the glass as a mass of colloidal particles each containing between 50 and 200 atoms, another involves the formation of sulphides or sulpho-selenides of particular metals.
Glass