What is Glass?
Glass is a rigid material formed by heating a mixture of dry materials to a viscous state, then cooling the ingredients fast enough to prevent a regular crystalline structure. As the glass cools, the atoms become locked in a disordered state like a liquid before they can form into the perfect crystal arrangement of a solid. Being neither a liquid nor a solid, but sharing the qualities of both, glass is its own state of matter.
Three Classical States of MatterTOP
Gaseous state: individual molecules separated from one another by relatively great distances and moving in a chaotic fashion. No interaction between molecules except for collisions with one another.
Liquid state: molecules are held close by attractive forces, but are not held rigidly in position. They move about, changing from one disordered state to another.
Crystalline state: strong attractive forces hold molecules rigidly in position. Each molecule occupies a definite position, in a perfectly ordered three-dimensional lattice.
Quartz (a solid) vs. Glass Structure (in the glassy state)TOP
Glasses have the mechanical rigidity of crystals, but the random disordered arrangement of molecules that characterizes liquids.
From bottles to spacecraft windows, glass products include three materials (watch: Raw Materials of Glass):
1) Former - This is the main component of glass, which has to be heated to a very high temperature to become viscous. Silicon dioxide (contained in sand) is the most common former.
2) Flux - Helps formers melt at lower temperatures. This is usually soda ash or potash, which was traditionally made from marine plant ashes, or by burning bracken or trees, respectively.
3) Stabilizer - Keeps the finished glass from dissolving, crumbling, or forming unwanted crystals. Calcium oxide in the form of limestone, a mineral, is a common stabilizer.
The mixture of dry materials used to form glass is called the batch. Batch is heated in a furnace to about 2400˚F. Broken glass, called cullet, is added to the batch to facilitate the melting process. An imbalance in the batch due to an excess of alkaline flux or too little stabilizer will cause crizzling, a chemical instability resulting in a fine network of cracks and deterioration of the glass.
The color of glass may be changed by adding metallic oxides to the batch (watch: Coloring Glass). Examples of common colorants include:
Iron - Colors glass green.
Copper - Colors glass light blue.
Manganese dioxide - Can decolorize colored glasses. However, in higher amounts, this element can create purple and, in even higher amounts, glass that appears black.
Cobalt - Colors glass dark blue.
Special Types of GlassTOP
Chemical composition determines what a glass can do. There are already tens of thousands of workable glass compositions and new ones are being developed every day. The following elements help to create special types of glass.
When added to glass, lead makes glass brilliant, resonant, and heavy. Glasses containing a large percentage of lead are known interchangeably as crystal, lead crystal, and lead glass (watch: Rummer with Raven Seal - Technique).
Boron aids in the production of borosilicate glass, a glass known for its resistance to thermal shock. Glass cookware and labware are the most well-known applications of this glass (see: 96.4.167).
AnnealingTOP
Immediately after glasses are formed, they are most often annealed, or slowly and evenly cooled, in order to reduce internal stresses (watch: Annealing and Tension in Glass). If one area of a piece of glass is thick (staying hotter longer) and another area is thin (cooling down quickly), and that piece of glass is notproperly annealed, the steep temperature gradient between those areas causes stress and the piece will most likely crack apart.
Glass PropertiesTOP
Mechanically Strong
Glass has great inherent strength. Weakened only by surface imperfections, which give everyday glass its fragile reputation. Special tempering can minimize surface flaws.
Glass has great inherent strength. Weakened only by surface imperfections, which give everyday glass its fragile reputation. Special tempering can minimize surface flaws.
Hard
Surface resists scratches and abrasions.
Surface resists scratches and abrasions.
Elastic
Gives under stress---up to a breaking point---but rebounds exactly to its original shape.
Gives under stress---up to a breaking point---but rebounds exactly to its original shape.
Chemical Corrosion-Resistant
Affected by few chemicals. Resists most industrial and food acids.
Affected by few chemicals. Resists most industrial and food acids.
Thermal Shock-Resistant
Withstands intense heat or cold as well as sudden temperature changes.
Withstands intense heat or cold as well as sudden temperature changes.
Heat-Absorbent
Retains heat, rather than conducts it. Absorbs heat better than metal.
Retains heat, rather than conducts it. Absorbs heat better than metal.
Optical Properties
Reflects, bends, transmits, and absorbs light with great accuracy.
Reflects, bends, transmits, and absorbs light with great accuracy.
Electrical Insulating
Strongly resists electric current. Stores electricity very efficiently.
Strongly resists electric current. Stores electricity very efficiently.
