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How does calcium oxide make soda-silica glass more durable?

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In the production of soda-lime glass, sodium oxide (soda) is added to silica as a flux to lower the high melting temperature of sand, making the manufacturing process more efficient. However, a glass composed only of silica and soda is highly soluble in water, making it chemically unstable.123

Calcium oxide (lime) acts as a stabilizer for the silica network. By incorporating lime into the mixture, the glass is rendered insoluble, significantly increasing its chemical durability and resistance to water. While lime is essential for stability, manufacturers must balance its concentration carefully; adding too much can cause the glass to become prone to devitrification, where crystalline phases precipitate within the glass during cooling or forming. Consequently, most commercial soda-lime-silica glass is formulated with specific proportions—typically around 70–75% silica, 15% soda, and 9–10% lime—to achieve an optimal balance of workability, chemical stability, and durability.1432