Glass Composition

Glass containers are commonly made with a combination of various oxides or oxygen-based compounds and are commonly referred to as “soda-lime” glass. The combining of raw materials, including sand, soda ash, limestone and cullet, creates glass containers that are durable, strong, impermeable, easily shaped, and inexpensive. The proportion of raw materials is based on availability, chemical and physical consistency, sizing, purity and cost. The goal is to use the most economical and high-quality raw materials available. Some oxides will form glass without adding any other elements and are known as network formers. The most common of these is silica (SiO2).

Sand

  • Sand is the most refractory of the major raw materials, or the hardest to melt; it is critical that it conform to fairly rigid sizing specifications.
  • The particle size distribution is typically between 40 (0.0165 inch or 0.425 mm opening) and 140 mesh size (0.0041 inch or 0.106 mm).
  • Sizing specifications for the other raw materials are dependent on the sand specifications.
  • Since larger particles of different sizes tend to segregate during material flow, the other materials must be sized to minimize the effects of this segregation.

Cullet

Cullet, or recycled glass, improves furnace efficiencies, including energy consumption. All cullet, however, requires processing to remove non-glass contaminants and to create size uniformity:

Cullet is usually color separated, crushed to a maximum size of ¾ of an inch, and screened and vacuumed to remove contaminants.

Labels, aluminum caps, and non-magnetic metal are all considered contaminants.

Raw Materials and Their Uses

Principle Raw Materials Used in Glass

Minor ingredients such as fining agents, decolorizers, and colorizers are added to the typical container glass composition. The most common fining agents are sulfates in combination with carbon. Of the sulfates used, sodium sulfate, or salt cake, is the most common Sodium sulfate acts as a wetting agent to aid in melting the silica source and also as a fining agent.