Raw Materials

Glass containers are made from abundant raw materials:

• Sand
• Soda Ash
• Limestone
• Cullet

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.

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

Colorizing/Decolorizing Glass

• Color can distinguish a glass container, shield its contents from unwanted ultraviolet rays or create variety within a brand category.
• Color can be obtained by simply adding small quantities of different oxides:
  o Chromium (for green)
  o Cobalt (for blue)
  o Nickel (for violet/brown)
  o Selenium metal (for red)
• The raw materials used in commercial glass making contain iron oxide as an impurity, which imparts a yellow/green color to the glass.
• To offset the yellow/green when making flint (or “colorless”) glass, other colors are introduced by adding selenium (for red) and cobalt (for blue) in proper proportions to yield a gray glass that appears colorless, hence the term “decolorization.”

Amber Glass

• Amber is the most common colored glass, and is produced by adding together iron, sulfur, and carbon.
• Amber is a “reduced” glass because of the relatively high level of carbon used.
• All commercial container glass formulations contain carbon, but most are “oxidized” glasses.
• Amber glass absorbs nearly all radiation consisting of wavelengths shorter than 450 nm, offering excellent protection from ultraviolet radiation (critical for products such as beer and certain drugs).

Green Glass

• Green glass is made by adding non-toxic Chrome Oxide (Cr+3); the higher the concentration, the darker the color.
• Green glass can be either oxidized, such as Emerald Green or Georgia green, or reduced, as with Dead Leaf green.
• Reduced green glass offers slight ultraviolet protection.

Blue Glass

• Blue glass is created by adding cobalt oxide, a colorant so powerful that only a few parts per million is needed to produce a light blue color such as the shade used for certain bottled waters.
• Blue glasses are nearly always oxidized glasses. However, a light blue-green glass can be produced using only iron and carbon and omitting the sulfur, making it a reduced blue.
• Creating a reduced blue is seldom done because of the degree of difficulty in fining the glass and controlling the color.

Most colored glasses are melted in glass tanks, the same method as flint glasses. Adding colorants to the forehearth, a brick lined canal that delivers glass to the forming machine of a flint glass furnace, produces oxidized colors.