Materials science – ordinary (soda–lime) glass formulation Which of the following is not a standard raw material intentionally used in the manufacture of ordinary soda–lime glass (window and container glass)?

Introduction / Context:
Ordinary glass used for windows and bottles is typically the soda–lime–silica composition. Understanding which ingredients are deliberately added versus which appear only as impurities helps in quality control, color control, and furnace operation.

Given Data / Assumptions:

• Soda–lime glass is the reference composition.
• Common intentional batch ingredients include silica (from sand), soda ash, and limestone/dolomite.
• Minor coloring/decolorizing agents may also be used, but the question asks about standard raw materials for ordinary clear glass.

Concept / Approach:
Ordinary soda–lime glass is produced from three principal raw materials: silica (SiO2) for the network former, soda ash (Na2CO3) as the alkali flux to lower the melting temperature, and limestone (CaCO3) to improve durability (providing CaO). Iron oxides, if present in sand, are typically treated as impurities and cause green/brown coloration; they are not intentionally used for making clear, ordinary glass.

Step-by-Step Solution:

Verification / Alternative check:
Typical batch recipes list sand (SiO2), soda ash (Na2CO3), limestone/dolomite (CaCO3/MgCO3) and sometimes minor Na2SO4 as a fining agent; Fe2O3 is kept low for high clarity, confirming it is not a standard deliberate addition for ordinary clear glass.

Why Other Options Are Wrong:

• Soda ash: Essential flux that reduces melting temperature.
• Limestone: Provides CaO to increase chemical durability.
• Silica: The primary network former; the main component of glass.

Common Pitfalls:
Confusing fining agents or colorants with principal raw materials. Small iron content can be used for tinting, but that is not the standard for ordinary clear glass production.

Final Answer:
Iron oxide