The Science Stuff

Working with Glass

• Basic elements of sand, soda and lime are combined with coloring agents and melted together at approximately 2500°F. The glass paste will begin to solidify at around 900°F. It is usually worked when it is 2000°F.
  
  
• The glassblower collects a small amount of molten glass, called a gather, on the end of the blowpipe and rolls it against a paddle or metal plate to shape its exterior (marvering) and to cool it slightly. The glassblower then blows into the pipe, expanding the gather into a bubble, or parison. By constantly reheating at the furnace opening, by blowing and marvering, the glassblower controls the form and thickness. Simple hand tools such as shears, tongs (pucellas), and paddles are used to refine the form, often while the glassblower sits in the special “glassmaker’s chair,” one with extended arms to support the blowpipe.
  
  
• For finish work and fire polishing at the mouth of the furnace, the gather is transferred to a solid iron rod called a pontil, applied opposite the blowpipe, which is then removed. When the pontil is cracked off it leaves a “pontil mark” that may be later ground or polished away.
  
  
• When the piece is finished, it is placed in an oven (lehr) for annealing. Annealing is the process of slowly cooling the glass to room temperature to stabilize its delicate crystalline structure. If the piece is heated or cooled to quickly it will crack.
  
  
• To produce glass with surface texture, facets, grooves, and depressions are ground into the surface with rotating disks of various materials, sizes, and shapes. The steps are marking the pattern, rough cutting, smoothing, and polishing.
  
  
• Designs are engraved by means of a diamond point, a metal needle, or with rotating wheels, generally of copper.

The Nature of Glass

• Glass is not easily described. Its physical structure does not conform to liquid, solid or gas. Glass actually is more of a liquid than the solid it appears to be. Its complex nature has intrigued man from ancient times. Glass is neither a solid nor a liquid but exists in a vitreous, or glassy, state in which molecular units have disordered arrangement but sufficient cohesion to produce mechanical rigidity. Glass is cooled to a rigid state without the occurrence of crystallization; heat can reconvert glass to a liquid form. Color varies with the ingredients of the batch.
  
  
• The American Society for Testing and Materials defines glass as “an inorganic product of fusion, which has cooled to a rigid condition without crystallizing”. Glass can be considered, then, an unusual material which has the random atomic arrangement of liquid but which somehow has been “frozen” in place so that it is a solid and permanent substance. Glass can be transparent, translucent or opaque. It is non-porous, non-absorptive and impervious to the common elements and many harsh chemicals and liquids. It is exceptionally resistant to abrasion and surface scratches. It is one of the best electrical insulation materials, yet can be treated to conduct electricity. Glass has lower heat conductivity than most metals and can
  possess a very low, zero or even negative coefficient of expansion.
  
  Because it contains a large proportion of silica and is produced by the action of heat upon that silica, it is generally categorized as a ceramic.
  
  Glass, however, stands in a class by itself, quite distinct from other ceramics. Most ceramic materials are shaped cold and then fired to produce the desired result; glass is shaped at extremely high temperatures and then allowed to cool. It again may be made semi-plastic, plastic or even molten by the further application of heat. For this reason, glass is also considered a thermoplastic material, which softens when heated and hardens when cooled.

Ingredients of Glass

• This natural glass is composed of three elements of the earth-sand, soda and lime. These same elements in varying forms also make up the basic composition of manufactured glass products ranging from containers and glassware to windshields and windows for high-rise commercial buildings. About 50 other chemical elements are used in modern glassmaking, in major and minor ways, to affect color, viscosity or durability, or to impart some desired physical property. But nature’s original ingredients are still basic elements in the formulation of glass.
  
  
• Glass largely is an open chain of silicon atoms with atoms of various oxides occupying the spaces between. It is this loose structure that permits transparency. Silica, or sand, is the most important ingredient in glassmaking since it is the source of, and provides the structure for, transparency. But sand requires soda and lime for practical glassmaking.
  
  
• Today, an average batch mix used to manufacture flat glass products contains about 70 percent silica sand, 13 percent lime, 12 percent soda and small amounts of other materials. About one-quarter of the batch is in the form of cullet, or cleaned and crushed glass recovered from previous glassmaking operations.
  
  
• Silica or silicon dioxide, which is converted into glass by the action of heat, is very difficult to fuse, requiring extremely high temperatures. Ancient scientists discovered that other materials such as soda, when melted in close contact with sand, would permit the melting of silica at much lower temperatures. Such materials are known as fluxes, and soda was probably the first flux.
  
  
• The primary forms of soda used in glassmaking are soda ash (sodium carbonate) or caustic soda (sodium hydroxide). When a mixture of sand and soda dissolves in the molten soda it forms sodium silicate. Depending on the proportions of sand and soda, this sodium silicate is more or less soluble in water and is known as water glass. To overcome water solubility of glass, another element, lime, is required.
  
  
• Lime (calcium oxide) usually is introduced into the glass batch mix in the form of limestone. Its use in correct proportion causes formulation of a soda-lime-silicate composition that is virtually unaffected by moisture or acids. Lime also renders the glass more viscous at the working temperature, shortens the setting time and improves weathering properties.
  
  
• Because of its low melting range, the soda-lime-silicate composition undoubtedly was the type used by ancients to produce the earliest known vessels, vases, semi-precious glass stones and beads, and, much later, the earliest form of window glass. Today, soda-lime-silicate is the basis for float glass, and of course, products fabricated from it.

Producing Glass with Color

• Other materials are added to produce different properties in the basic flat glass product or to replace one of the basic elements to produce different types of commercial glass.
  
  
  • Lead, for example, in the form of lead oxide, may be used to replace lime, and is introduced to increase brilliance, density and index of refraction. Lead glasses include optical and ophthalmic glasses and the finest stemware and art objects. Lead glass is heavy and has an enhanced capacity to refract light, which makes it suitable for lenses and prisms.
    
    
  • Boron, substituted in whole or in part for the silica, increases the refractive index, deepens the color produced by various other coloring materials and greatly reduces the coefficient of thermal expansion.
    
    
  • Borosilicate glasses are used for such high heat resistant products as ovenware, laboratory glasses and range surfaces.
    
    
  • Metallic oxides are added to produce tinted or colored glasses.
    
    • Greens and aqua glasses usually have iron.
      
    • Amber and brown colors are produced by adding small amounts of iron and sulfur.
      
    • Lead antimony can produce yellow.
      
    • Light blues require copper.
      
    • Brilliant yellow-green glass has Uranium.
      
    • Dark blues require very small quantities of cobalt.
      
    • Amethyst glass contains manganese.
      
    • Opaque white can contain either tin or calcium.
      
    • Selenium is one metal oxide that is used to produce reddish colors. Some reds and pinks even have a bit of gold in them.