William the Franciscan Friar who has come to investigate a murder in the monastery, is busy getting his glass lenses ready to help him with his failing eyesight. Humberto Eco, in his classic “The Name of the Rose” explains with great detail about life in the 14th century where he imagines the origins of using convex glass circles to create early eye glasses. (Ref.1)
Of course glass was discovered long before the 14th century. During 1500 BC, the first creations with glass is credited in ancient Mesopotamia, modern-day Syria. When lightning struck sand, a brittle shiny object called obsidian was formed. People in and around Mesopotamia shaped it as knives and sharp objects for hunting. Glass making was tedious in those days using clay pots and long hours working with sand. The invention of a glass blower in 30 BC changed glass production considerably allowing different shapes and forms to be created now.(Ref.2). However, the first recorded uses of glass-like material like obsidian from volcanic areas and fulgurites (lightning striking sand) is from 7500 BC.(Ref.2)
What is glass? Since it is created from sand which is plentiful in beaches, the main element is Silicon, Si in the form of compounds called silicate SiO44- (Ref.3). Both Si and O are the two most abundant elements on earth. Sand contains quartz, made up of an orderly arrangement of SiO44- anions in a tetrahedral shape combining with other Si and O ions to have the general formula SiO2, silica and is crystalline. Glass has the same formula, but it is amorphous and the arrangement of the anions is a short range. The process of creating glass sometimes involves adding sodium carbonate Na2CO3 to sand and heating it to a high enough temperature and cooling it rapidly.(Ref.4)
Glass manufacture needs three components: the former, flux and stabilizer. The former is silica or SiO2, and alone it needs very high temperatures to melt and therefore a flux e.g., oxides of alkali metals like Na2O (soda/sodium oxide) or K2O (potash/ potassium oxide) or Li2O (Lithia/ lithium oxide) is added. Even though now the melting point temperature is lowered, the resulting glass may be water-soluble and of low durability. A stabilizer like CaO or BaO or MgO or PbO,( oxides of calcium, barium magnesium or lead; also called lime, baria, magnesia, litharge respectively) is also added. Common glass (for windows, jars, bottles, bulbs) contain SiO2 as former, Na2O as flux and CaO as stabilizer.(Ref.3) As shown in the above paragraph, sometimes Na2CO3 is used as a flux.
Glass is neither a solid nor a liquid, since the arrangement of the anions and cations are almost like a liquid but it has an appearance of a solid! Molecular physics and thermodynamics (behavior under different temperatures) are still not clear but here are some observations:
Many solids have a crystalline structure on a microscopic scale just like Na+Cl–. The ions/molecules/atoms are arranged in a regular lattice and when the solid is heated, they vibrate until the melting point and the crystal structure breaks down. The ions/molecules/atoms flow and the solid to liquid state is a sharp transition, which explains the fixed melting point for so many solids. This is called a first order phase transition. The density also changes distinctly.
Let us look at the reverse process; from liquid back to the solid state. As it cools, the viscosity (also called resistance to flow) increases, which is understandable since the ions/molecules/atoms are moving closer together. However, this increase in the viscosity could prevent crystallization. And it could keep cooling below the freezing point called supercooling, and may never crystallize. The viscosity keeps rising and forms a thick syrup and finally an amorphous solid. The molecules will have a disorderly arrangement, almost glass-like?
Therefore, glass could be considered a super-cooled liquid with no first order phase transition, but a second order phase transition; between super-cooled liquid state and the glass state, not as dramatic as the liquid to crystalline solid state. The temperature at which this glass transition occurs depends on how slowly it is cooled.
So to summarize we have three types of molecular arrangements:
- Crystalline solids- ions/molecules/atoms are ordered in a regular lattice.
- Fluids- ions/molecules/atoms disordered and not rigidly bound.
- Glasses- ions/molecules/atoms disordered but rigidly bound. (Ref.5)
The jury is still out on the exact definition of glass!
Activities for Middle School Teachers:
Make glassy material using some sugar syrup; also make crystalline candy with sugar syrup and study the difference in the procedure.
Silicon is present in many semi-precious stones including quartz; list the elements that cause different colors/ semi precious stones.(Ref.6)
Students can study architectural wonders in the world constructed with glass: Louvre in Paris France and La Estancia Glass Chapel in Cuernavaca, Mexico are examples.(Ref.7)
Study the different civilizations like the Mayas and in Mesopotamia and look at the role of obsidian, glass during those periods. (Refs.8,9)
Nuggets of information:
In 1664, an Englishman, Mr.Ravenscroft added small amounts of lead oxide (PbO) during the glass making process to create brilliant glass. This led to the creation of crystal bowls and goblets. Different grades of glassware can be created by adding different oxides:
- When boron oxide is added, laboratory glassware is produced that withstands high temperature.
- When aluminium oxide is added the glass withstands even higher temperature to keep on stove tops.
- High content (almost 100%) silica glass, resistant to thermal shock and chemical attack is used to make spacecraft windows, fiber optics. (Ref.3)
Fiber optics or optical wave grids are today’s communication technology that has replaced the humble copper wire. A single strand of glass fiber, coated with a protective plastic resembles a human hair. The glass fiber has an inner core of ultra-pure fused SiO2 coated with another SiO2 glass to act as a light reflective barrier. Lasers are used to convert electrical impulses and sound waves to pulses of light that travel through the inner core. 1lb of glass fiber transmits the information that would formerly require 200 tons of copper wire! These fiber optic cables criss-cross our entire earth. (Ref.3)
Cooks who create desserts and work with sugar probably know how to create glassy shards of brittle concoctions using the super-cooled process with sugar syrup, a liquid. This same sugar syrup can be used to make crystalline candy.
In old churches, the glass is found to be thicker at the bottom of the window and people thought glass must be a liquid and flowed from top to bottom over the years. Actually this is not true. In Medieval times, panes of glass were often made by the blown glass method. A lump of molten glass was rolled, blown, expanded and flattened. Finally it was spun into a disk, which made it thicker at the edges. When installed, the thicker side was placed at the bottom side of a window-pane. (Ref.4)
Si, like C, (being in the same Group in the Periodic Table) forms a lot of different covalent bonds with O. Apart from quartz, talc, there are different kinds of naturally occurring asbestos that contain silicates. (Ref.4)
One of the most interesting properties of quartz is piezoelectricity, which means that when you apply pressure to a crystal of quartz, the orderly arrangement of atoms/molecules/ions is disturbed and the crystals conduct electricity. The opposite is also true; when you apply electricity to quartz crystal, there is a shift in the position of the atoms/molecules/ions. These principles are used in a quartz watch that keeps perfect time, as well as in voice-recognition software that we use frequently. (Ref.10)
Glass containers are considered the best for storing spices and other foods. Glass, especially clear glass in a dark cupboard or dark-colored glass, stores herbs well and does not impart any odor to the food, like plastic.
Today, most supermarkets sell liquids in plastic or paper coated with plastic containers that were previously sold in glass containers. The one place where glass use continues is in the making of all kinds of mirrors. (Ref.11) and in the manufacture of LED (light emitting diodes) lighting.(Ref. 12)
References:
1. Eco, Humberto, The Name of the Rose, (Bompiani, 1980, English,1983)
2. http://www.texasglass.com/glass_facts/history_of_Glass.html
3. http://www.chemistryexplained.com/Ge-Hy/Glass.html
4. http://www.science.uwaterloo.ca/~cchieh/cact/applychem/silicate.html
5. http://mineral.eng.usm.my/web%20halaman%20mineral/silica%20sand.pdf
6. http://math.ucr.edu/home/baez/physics/General/Glass/glass.html
7. http://omnigp.com/20-beautiful-glass-buildings-world/
8. http://authenticmaya.com/Obsidian.htm
9. http://www.historyofglass.com/
10. http://www.explainthatstuff.com/piezoelectricity.html
11. http://www.ehow.com/how-does_4569519_how-mirrors-made.html
12. http://www.bizjournals.com/prnewswire/press_releases/2012/10/04/CG87004
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