17.1 Glass Types
The term glass implies an amorphous structure. Crystallization is defeated by either rapid cooling or diversity of components. Quartz is a crystalline form of silicon dioxide that is formed under intense pressure and heat. Silica is an amorphous form of silicon dioxide that is formed during rapid solidification.
Glass is largely composed of the nine most common elements within the earth's crust (Table 17.1). Additional components of glass are listed in Table 17.2. Iron is not desirable for optical glass due to its green color. Arsenic (As) and antimony (Sb) are fining agents that remove bubbles and impurities such as iron. Figure 17.1 displays a periodic table of common elements in metal-oxide glass.
Silica provides the bulk material for most optical glass. The liquidus temperature of silica is 1715° C, and the melting point is 2000° C. The liquidus temperature defines the transition from crystalline to amorphous structure. The liquidus temperature is also called the glass temperature Tg, above which the structure is amorphous. The melting point defines the transition from solid to liquid.
Soda lime is an early form of glass. The liquidus temperature of silica is reduced to 1000° C by the addition of soda Na2O. The viscosity is reduced by addition of lime (CaO). Potassium oxide (K2O) may replace soda. A low liquidus temperature and low viscosity are beneficial for blowing, shaping, and floating. A trace amount of iron oxide content creates a bluish-green tint, which is prominent at the edges of soda lime glass. A large amount of iron-oxide content creates brown bottle glass.
Crown glass was originally formed by a rod with molten glass on the end. The linear momentum of the rotation stretched the molten glass into a disk, but not evenly. The process left a crown in the middle. A high transmittance for windows was the primary goal in crown glass.
The original flint glass was comprised of silica and flint. Flint occurs naturally as quartz with trace elements. The essential trace elements raise the refractive index of silica without the addition of color. Naturally occurring flint provides sparkle without color.