Which additive is used to reduce glass expansion?

• A. MnO
• B. Borate
• C. K
• D. Pb

Answer: (B)

The ability of glass to expand with heat depends on how strongly its internal network holds together. A more interconnected and rigid network resists change in size as temperature rises, so the glass ends up with a lower coefficient of thermal expansion.

Borate does exactly this in glass chemistry. When boron oxide is added to silica, it forms borosilicate networks with BO3 and BO4 units that bridge and reinforce the silicate framework. This creates a tighter, more connected structure, so the material expands much less when heated. That’s why borate-containing glasses (like borosilicate glass) are known for their low thermal expansion and high thermal shock resistance.

The other additives don’t produce that same effect. Alkali oxides such as potassium tend to disrupt the network and increase expansion, while manganese oxide and lead oxide influence color or other properties more than the fundamental thermal expansion behavior.