Section 2.4: 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 creates glass containers that are durable, strong, impermeable, easily shaped, and inexpensive.
• Some oxides will form glass without adding any other elements and are known as network formers. The most common of these is silica (SiO₂).

SiO₂ Glass

While silica alone can be used to form glass, it has some undesirable features, which make it unsuitable for use in making glass containers.

Advantages

• Inexpensive Raw Materials
• Single Component
• Low Expansion
• Excellent Durability

Disadvantages

• Hard to Fine
• Hard to Melt
• Hard to Form

Fining refers to the removal of the gas bubbles (seeds) from molten glass. Expansion refers to the "Thermal Expansion Coefficient",which is the relative change in volume per unit change in temperature. Chemical durability is the resistance of glass to be altered or harmed by solvents or products.

Na₂O - SiO₂ Glass

In order to overcome some of the disadvantages of a pure silica glass, other oxides are added. Among these are the fluxing agents, the most common of which is soda, (Na₂O). As with a pure silica glass, a soda-silica glass has advantages and disadvantages:

Advantages

• Low Melting
• Low Viscosity

Disadvantages

• Expensive Raw Materials
• High Expansion
• Poor Durability

Chief among the disadvantages is its poor chemical durability. Sodium silicate glass, in the proper combination, is actually soluble in water and is aptly called water glass.

A network modifier, Na₂O in this case, opens up the silica network by breaking some of the Si-O bonds in the network, which leads to low viscosity and poor chemical durability.

Na₂O - CaO - SiO₂ Glass

In order to overcome the poor chemical durability, another class of oxide is added to strengthen and rebuild the network. These are known as network stabilizers. The most common network stabilizer is calcia (CaO), the basic element in limestone. The results of adding CaO are as follows:

Advantages

• Low Expansion
• Good Durability

Disadvantages

• High Melting
• High Viscosity
• Tendency to Devitrify (process in which a glass, noncrystalline or vitreous solid, transforms to a crystalline solid)

Na₂O - CaO - Al₂O₃ - SiO₂ Glass

Advantages

• Resists Devitrification
• Good Durability

Disadvantages

• High Viscosity

The network formers, modifiers, stabilizers and intermediates comprise the major oxides used in typical container glasses and make up the bulk of the composition. A typical container glass has the following composition:

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

When combined with carbon, sodium sulfate releases SO₂ and CO₂ gas by the following reaction:

2Na₂SO₄ + C → 2Na₂O + 2SO₂ + CO₂

These gases rise up through the melt and collect smaller bubbles of oxygen along the way. The oxygen reacts with the SO2 to form SO3, which is soluble and is absorbed in the glass. The CO2 rises to the top of the melt and is released into the atmosphere.

continue to Section 2.5: Composition Control