Glasses have three characteristics that make them more closely resemble "frozen liquids" than crystalline solids. First, and foremost, there is no long-range order. Second, there are numerous empty sites or vacancies. Finally, glasses don't contain planes of atoms.
The simplest way to understand the difference between a glass and a crystalline solid is to look at the structure of glassy metals at the atomic scale. By rapidly condensing metal atoms from the gas phase, or by rapidly quenching a molten metal, it is possible to produce glassy metals that have the structure shown in the figure below
The amorphous structure of glass makes it brittle. Because glass doesn't contain planes of atoms that can slip past each other, there is no way to relieve stress. Excessive stress therefore forms a crack that starts at a point where there is a surface flaw. Particles on the surface of the crack become separated. The stress that formed the crack is now borne by particles that have fewer neighbors over which the stress can be distributed. As the crack grows, the intensity of the stress at its tip increases. This allows more bonds to break, and the crack widens until the glass breaks. Thus, if you want to cut a piece of glass, start by scoring the glass with a file to produce a scratch along which it will break when stressed.
Glass has been made for at least 6000 years, since the Egyptians coated figurines made from sand (SiO2) with sediment from the Nile river, heated these objects until the coating was molten, and then let them cool. Calcium oxide or "lime" (CaO) and sodium oxide or "soda" (Na2O) from the sediment flowed into the sand to form a glass on the surface of the figurines. Trace amounts of copper oxide (CuO) in the sediment gave rise to a random distribution of Cu2+ ions in the glass that produced a characteristic blue color.
Sand is still the most common ingredient from which glass is made. (More than 90% of the sand consumed each year is used by the glass industry.) Sand consists of an irregular network of silicon atoms held together by Si--O--Si bonds. If the network was perfectly regular, each silicon atom would be surrounded by four oxygen atoms arranged toward the corner of a tetrahedron. Because each oxygen atom in this network is shared by two silicon atoms, the empirical formula of this solid would be SiO2 and the material would have the structure of quartz. In sand, however, some of the Si--O--Si bridges are broken, in a random fashion.
Modifiers (or fluxes) such as Na2O and CaO are added to the sand to alter the network structure by replacing Si--O--Si bonds with Si--O- Na+ or Si--O- Ca2+ bonds. This separates the SiO2 tetrahedral from each other, which makes the mixture more fluid and therefore more likely to form a glass after it has been melted and then cooled. These so-called "soda-lime" glasses account for 90% of the glass produced.
Al2O3 is added to some glasses to increase their durability; MgO is added to slow down the rate at which the glass crystallizes. Replacing Na2O with B2O3 produces a borosilicate glass that expands less on heating. Adding PbO produces lead glasses that are ideally suited for high-quality optical glass.
The most common way of preparing a glass is to heat the mixture of sand and modifiers until it melts, and then cool it quickly so that it solidifies to produce a glass. If the cooling is rapid enough, the particles in the liquid state can't return to the original crystalline arrangement of the starting materials. Instead, they occupy randomly arranged lattice sites in which no planes of atoms can be identified. The result is an amorphous (literally: "without shape") material.