Glass
By the end of this lesson, you should be able to:
2. Describe the properties of glass.
3. List types of materials used in a glass batch.
4. List natural and synthetic glasses.
The American Society for Testing and Materials defines glass as "an inorganic product of fusion which has cooled to a rigid condition without crystallizing."
Precisely Machined Glass
http://www.louwers.nl/welcome2.htm
Typical glass is non-crystalline (also known as "amorphous") if it is cooled quickly. Because it does not have crystals, it does not fracture along specific crystalline orientations. By scratching the surface of glass, it is possible to reduce the local surface tension and with applied pressure, fracture the glass along the scratch.
Making Straight Cuts in Glass
http://www.tipsondiy.co.uk/handyman/cutting_glass.html
Micrograph
of a Glass Fracture Surface
www.cs.alfred.edu/~maurojc/papers/microscopy/labreport2/index.html
Glass is not a solid at room temperature. Actually, it is a "super-cooled liquid" with very high viscosity. (Amorphous materials do not have clearly defined melting points.) But glass is always flowing. If you inspect a hundred year old window you might find it to be noticeably thinner at the top, and thicker at the bottom.
Glass is nearly perfectly elastic (i.e., no plastic range); at room temperature, you cannot bend it so that it stays bent.
Read an introduction to glass, and a very informative web site on Making Glass.
Raw Materials
Typical materials used in a glass batch include:
1. Glass Former: the primary material. (e.g., silicon dioxide {SiO2; silica}, melts at about 3110oF {1710oC})
2. Flux: lowers melting temperature / lowers viscosity. (e.g., sodium carbonate, calcium carbonate)
3. Decolorizer: complements or neutralizes the green tint from iron oxide impurities. (e.g., manganese dioxide)
4. Colorant: imparts color. (e.g., metal oxides or carbonates).
Cobalt carbonate ==> blue
Iron oxide ==> green
Manganese dioxide ==> purple.5. Firing agent: helps gather and expel gas bubbles from molten glass. (e.g., potassium nitrate and arsenic compounds)
6. Strengthener/Stiffener: increase strength or stiffness. (e.g., aluminum oxide).
7. Brightener: increase transparency. (e.g., barium carbonate and lead compounds).
8. Opacifier: reduces transparency. (e.g., zirconium, tin oxide, phosphorus and antimony compounds). No, this is not a pacifier for an opossum.
9. Cullet: "waste glass, rejects and recycled glass products." Cullet speeds melting.
10. Other.
Types of Glass
Some glass occurs naturally. Obsidian, fulgurites, teletites are examples of natural glass.
Obsidian
http://www.coolrox.com/rocks.htm
However, typical industrial and art glass is synthetic, or "human-made."
Types of Common Synthetic Glasses
Glasses can be categories as either "soft glasses" or "hard glasses," but this is a confusing use of terminology. "Soft glasses" are those that are more susceptible to thermal shock, and cannot withstand sudden large changes in temperature without breaking; "hard glasses," on the other hand, can withstand significant temperature changes without fracture. However, this use of the terms "hard" and "soft" does not correspond with a typical definition (i.e., hardness as resistance to surface penetration or local plastic deformation.) Two common "soft glasses" and four common "hard glasses" will be discussed.
Soft Glasses
1. Soda Lime
Soda lime glass is made from silica, soda ash and limestone. It is typically the least expensive glass, and thus the most common, accounting for 90% or more of all glass. It was first produced about 4000 years ago. Soda lime glass is relatively soft and easy to form. However it is susceptible to thermal shock and splintering. It is typically used for bottles and windows.2. Lead Alkali
Lead alkali glass is made from silica, soda ash, litharge, pearl ash, soda niter and feldspar. It tends to be very clear, brilliant and electrically resistant. It, too, is a soft glass, and is susceptible to thermal shock. Lead alkali glass is sometimes called "glass crystal," which is a misnomer since there are no crystals in typical glass. It has historically been used for glassware (wine glasses, etc.) and eye glasses. However, concerns about possible lead poisoning have decreased its use for glassware.
Hard Glasses
3. Borosilicate
Borosilicate glass is made from silica, boric acid, borax and alumina. It is hard and relatively resistant to temperature change, acid and electricity. It is typically used for oven ware and laboratory glassware. Kimex and Pyrex are registered trademarks used for borosilicate glass.
4. 96% Silica
Made from silica and borax, "96% Silica" glass is hard and highly resistant to temperature change, acid and electricity. It has high levels of ultraviolet light transmission, and thus is used in sun lamps. It is also used for certain chemical ware.
5. Fused Silica
Fused silica glass is made from pure silica. It is extremely costly but it is also very heat resistant, corrosion resistant, and hard. It is sometimes used for aircraft cameras and astronomical telescopes.
6. Aluminosilicate
Aluminosilicate glass is made from silica, alumina, borax, boric acid, limestone and magnesium oxide. It is extremely hard and extremely heat resistant. This makes it a prime choice for use as top-of-stove ware and high temperature thermometers.
Please note that in addition to these six, there are many specialty glasses.
Borosilicate
Glass: Organism Series
Sirius Blue, 1998
http://www.mickelsenstudios.com/catalog/sirius_blue.htm
Glass Processing
Processing glass typically begins with preparation and melting of the "glass batch." This is followed by machine forming, which replaces much of the historical hand forming. Glass can be processed (shaped) by blowing, drawing, rolling, floating, and grinding.
A
Glass Blower
www.planetlighting.com.au/noframes_site/glassworks.htm
Grinding then Polishing Glass
http://chestnutfarms.i2k.com/Travel/Prague/index.htm
Glass "Finishing" operations can be thermal, chemical, or mechanical, and can impart important properties. This use of the word, "finishing," is not meant to imply a protective coating, but also includes conditioning.
Thermal finishing can include annealing, which in the case of glass refers to a slow cooling of glass that tends to provide a uniform structure and reduces internal stresses.
Tempering is another finishing operation; this involves rapidly cooling the exterior surface of hot glass so that the surface is in compression. Tempered glass does not tend to fracture in long, dangerous shards, but in small rectangular prisms; therefore it is often required in car windows. (Windshields are typically made with a laminated layer of plastic that would keep fractured glass relatively contained.)
Specialty Glass Products
In addition to the plate glass that most of us may picture, many other materials are made out of glass, from optical fibers to thermal insulation.
Cellular Glass is an interesting insulating material.
Glass Ceramics, such as Pyroceram by Corning, are materials that have been transformed from noncrystalline back to a crystalline state by a process of controlled nucleation or devitrification; they can be drilled, tapped, and machined in ways not possible with amorphous glass. Glass ceramics are often opaque can be stronger than glass.
© Jim Flowers
Industry & Technology, Ball State University