Reduces the coefficient of expansion in glass

• A. MnO
• B. Borate
• C. K
• D. Pb

Answer: (B)

Lowering how much glass expands with heat comes from making a more tightly connected glass network. Borate acts as a network former that bonds with the silica network to form borosilicate glass, which is highly interconnected and rigid. This tighter structure resists the bond-length changes that occur with heating, so the material expands less—hence a smaller coefficient of thermal expansion. That’s why borate-containing glasses (borosilicate glasses) are prized for thermal stability and resistance to thermal shock, unlike the other additives listed, which either don’t promote this tight network or act mainly as colorants or modifiers.