Table: Typical Chemical Composition

of Container Glass:

 Weight %

 Oxide Range + / -

 SiO2 66-88 1.00

 Al2O3 0-7 0.50

 CaO 0-15 0.50

 MgO 0-5 0.50

 Na2O 8-18 0.50

 K2O 0-4 0.50

 Fe2O3 < 0.5 0.05

 Cr2O3 < 0.1 0.02

 SO3 < 0.2 0.02

All other oxides < 0.1 0.02

 C < 0.1 0.02

Source: CBOT Recyclables Exchange

Chemical Composition of Container Glass

Material: Recycled Glass

Issue: Understanding the chemical composition of container glass is critical to recycling for a number of

reasons. First, the chemistry has significant implications for managing the behavior of molten glass in any

remelt application, from large-scale container manufacturing down to cottage-industry pressed glass. Second,

the composition has important implications for evaluating health concerns related to glass handling, such as

free silica content (see Analysis of Glass Dusts Best Practice). In addition, product manufacturers are required

to produce Material Safety Data Sheets (MSDS) which describe chemical composition and special handling

precautions. Finally, the chemistry is important in determining the compatibility of glass with new and novel

secondary uses.

Best Practice: More than 95% of all manufactured glass is made

from sodium oxide, calcium oxide, and silicon dioxide, commonly referred

to as a soda-lime-silica composition. Due to universal similarities in

manufacturing techniques for glass containers, particularly the viscosity

requirements of high-speed “press and blow” production equipment, the

chemical formulations of glass food and beverage containers have

relative uniformity and compatibility. Container glass is formulated to set

up quickly and to hold its shape when mechanically blown into molds. A

random mix of container glasses is anticipated to be relatively

homogenous, provided there are no significant contamination levels. 

The principle element of container glass, silica, refers to a naturally

occurring mineral consisting of silicon dioxide (SiO2). The majority of

silica occurs as quartz, a crystalline form of SiO2. Crystalline forms of

silica, also known as “free” silica can contribute to causing certain lung

diseases under conditions of prolonged exposure to airborne particles. 

While SiO2 is a primary ingredient in the manufacturing of bottle glass,

when glass is formed, the crystalline structure is changed to an

amorphous structure and the SiO2 is no longer considered crystalline.

The function of soda ash content in container glass formulations is to lower the fusion point of the silica sand and

improve workability of the glass at moderate furnace temperatures. Lime is included in an amount sufficient to adjust

the viscosity and increase durability of the finished glass. With the development of increasingly rapid

manufacturing machinery, soda content has been lowered and lime increased, as shorter working times are required. 

There has also been economic motivation to reduce soda contents, as it is the most expensive major ingredient in glass.

The addition of post-consumer cullet to raw materials promotes rapid melting of the batch, lowering energy

requirements. However, after one or more reheatings, the dissolved gases and the volatile alkalis in the cullet are driven

off, making it necessary to add chemicals to the mix in order to reinstate lower viscosity and improve working

characteristics.

Coloring agents are a minor constituent in container glass, but can effect chemical compatibility and furnace

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performance during remelting. Amber bottle glass is produced with a sulfur-iron colloidal solution, under reducing

conditions. Green and clear glass are produced in oxidizing reactions with small amounts of Cr2O3 dissolved in the

glass. These are general guidelines. Of domestic container production, 63% is clear, 23% is amber and 13% is green,

but varies significantly geographically and seasonally.

Implementation: While major variations in the chemical formulations of container glass are not anticipated, the

composition of specific post-consumer sources of container glass should be periodically tested to confirm the actual

chemical composition. It is especially important to understand the chemical nature of soda-lime container glass during

the development of alternative uses.

Benefits: For many uses of recycled glass, including remelt applications and abrasive blasting grit, knowledge of the

chemical composition of the post-consumer glass cullet is critical to accessing end-use markets. In remelt applications,

this information provides the basis for evaluating color modification potential (see Color Modification of PostConsumer Recycled Glass Best Practice). In industrial mineral applications, this information is important to assess

glass’s compatibility with the chemistry of the industrial process.

Application Sites: Glass beneficiation facilities, glass container and fiberglass insulation manufacturing

facilities, studio glass shops.

Contact: For more information about this Best Practice, contact CWC, mailto:info@cwc.org.

References:

Methods for Sampling and Testing Glass Cullet, Clean Washington Center, 1996.

Cullet Remelting Process Assessment, Clean Washington Center, 1996.

Issue Date / Update: November 1996

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