| Introductory Science Concepts | Hands-On Activities |
Overview
Following a brief introduction to the history of glass making, students learn about the composition of glass and some of its properties. Students observe a demonstration of hot glass techniques, and then have the opportunity to produce a blown glass object of their own design.
History of Blown GlassA crafts instructor, in a 10-15 minute slide presentation, shows students images of blown glass objects from prehistoric to modern times. The craftsperson shows and explains how glass was first discovered and how humans experimented and developed techniques to manipulate molten glass into decorative and useful objects. To begin this session, a craftsperson shows students a 10-15 minute slide presentation of images of blown glass objects from prehistoric to modern times. The craftsperson shows and explains how glass was first discovered and how humans experimented and developed techniques to manipulate molten glass into decorative and useful objects.
[Physical
Science: Science as a Human Endeavor]
[Chemistry:
Science as a Human Endeavor]
Glass is described by scientists as a super-cooled liquid or an amorphous solid because it has properties of both the liquid and solid states. When molten glass solidifies, its molecules are still in a random state of organization usually associated with the liquid state. This lack of order is reflected in the physical properties of glass. Glass is brittle and shatters in an irregular way. Glass can adopt any shape; it lacks the smooth planes and definite angles of crystals. Glass softens and liquefies over a wide range of temperatures rather than over the small melting range characteristic of crystalline solids. Ordinary glass is composed of quartz sand (chemically, silicon dioxide), sodium oxide and calcium oxide in a complex mixture that contains both ordered and disordered regions.
[Chemistry
1.04]
[Physical
Science 5.04]
Pure quartz is composed of alternating silicon and oxygen atoms held
together with strong, covalent bonds in a three dimensional web-like structure.
The strong bonding prevents quartz from melting until very high temperatures
( in the range of 1500-1700oC, 2700-3000oF ) are reached. Such temperatures
are difficult to achieve in furnaces which burn ordinary fossil fuels or
wood. Addition of sodium carbonate to quartz lowers and broadens the melting
range so that the quartz-soda ash mixture liquefies at temperatures easily
achieved in a fuel fired furnace. In a heated mixture of silica (sand)
and soda ash, the soda ash first decomposes to sodium oxide and carbon
dioxide gas. The carbon dioxide bubbles out of the mixture. Then sodium
oxide reacts with the quartz, breaking some silicon oxygen bonds in the
network. This allows greater movement of the atoms within the matrix of
the mixture. The movement is manifested as a visible softening and then
liquefying of the solid mixture.
[Chemistry
1.07]
[Physical
Science 6.04]
To help visualize this phenomenon, students are given molecular models and asked to construct a network of atoms using the ratio of two oxygens to one silicon, the actual atomic ratio in quartz. The models are the same as the ones used for the Paper and Book Making session except that the black pieces represent silicon atoms rather than carbon atoms. Students are told that this transfer of identity is legitimate since carbon and silicon are in the same group on the periodic chart and that they both tend to form four bonds in their compounds. The rule that students must obey in their construction is that all bonds must be between silicon and oxygen; there can be no bonds between silicons or oxygens. Students are encouraged to create a structure that is three dimensional and quite rigid as it would be in a quartz crystal. They accomplish this task quite readily.
[Chemistry
2.01]
[Chemistry
4.01]
[Physical
Science 5.06]
Now students are asked to introduce additional atoms of oxygen to the structure. This represents the addition of sodium oxide to quartz. The additional oxygens must break bonds between silicon and oxygen atoms in the network and produce dangling chains of atoms that move much more freely than the atoms in the network solid. The chains tangle and become quite disordered. On a molecular scale, this is the process that produces the disordered mixture called "glass". No models are used to represent the sodium ions in the glass mixture. Their function is to provide a positively charged ion to maintain electrical neutrality. The sodium ions are free to move around in the molten mixture.
If soda ash were the only substance added to quartz to lower its softening temperature, the resulting glass would be somewhat soluble in water. Water solubility is not a desirable property for glass. Therefore calcium oxide (lime) or calcium carbonate (limestone) is also added to the mixture. Calcium oxide also reacts with quartz, lowering the temperature at which it softens, but the calcium ions are more strongly held in place by the oxide ions and render the glass insoluble in water.
Glasses characteristically soften and liquefy over a broad range of temperature. Thus liquid glass, removed from a furnace, remains plastic enough to be molded and shaped by a glassblower for several minutes. When the glass cools to the point of hardness, it is placed in a "glory hole", a hot furnace that does not contain a pool of molten glass. The glass softens in the glory hole and can be worked again for a few minutes. Trips to the glory hole may be many for an intricate object or a slow, inexperienced glassblower.
Over centuries of experimentation with various mixtures, glass makers
have developed specific mixtures of sand, soda ash, and lime or limestone
that produce soda-lime glasses with properties that are ideal for windows,
containers, and decorative items.
Modern soda-lime glass contains, roughly, 72% silica, 15% sodium and
potassium oxides (soda ash), and 13% calcium and magnesium oxides(lime).
Students are now introduced to the techniques of working with molten glass in the hot glass studio. The craftsperson demonstrates how to gather a gob of molten glass from the glass furnace at the end of a "punty". Since molten glass glows brightly, the students must wear dark goggles to protect their eyes from the heat and light. The goggles also allow the students to see the material in the furnace or glory hole rather than just a bright blur. The craftsperson shows students how to shape the glass with one hand while continually rotating the punty with the other hand.
The craftsperson then gathers another gob of glass from the furnace onto a blowpipe. The craftsperson shows students that by getting a feel for the rate at which molten glass flows, the glass blower can allow gravity to help shape the glass and he/she can blow air into the gob of glass and allow the heat of the glass to expand the bubble of air to create a hollow space. Again, with constant rotation of the blowpipe, occasional heating of the glass in the "glory hole" and application of shaping and cutting tools, the gob of glass is transformed into a vase, or a tumbler or some other object of the craftsperson's choosing.
Each student is given an opportunity to work with glass, both on a "punty" and a blowpipe. There are sufficient, knowledgeable assistants present so that each student has a personal tutor during the entire process. Students are somewhat hesitant, at first, but, with expert coaching, and plenty of encouragement from classmates, quickly catch on to the required techniques. When the objects that students have produced are completed, they are removed from the end of the pipe and placed in an annealing oven to cool at a slow rate, thereby reducing internal stress in the body of the glass and preventing it from cracking or shattering.
Dr. Claire OIander, Appalachian State University
