Introduction / Context:
Quality glass requires removal of dissolved gases and bubbles from the molten batch, a step called “fining.” Classic exam items ask which additive acts as a fining agent. Correct identification shows familiarity with glass chemistry beyond the basic silica–soda–lime trio.
Given Data / Assumptions:
- Process: glass melting followed by fining (bubble removal).
- Goal: reduce seeds (bubbles) for optical clarity and strength.
- Candidate fining agents include compounds that release/absorb gases at high temperature.
Concept / Approach:
Arsenous oxide (As2O3) historically served as an effective fining (and decolourising) agent, helping small bubbles coalesce and escape. Although many modern plants have transitioned away from arsenic for environmental reasons, the textbook answer remains arsenous oxide. Potassium carbonate and soda ash act mainly as fluxes (alkali sources). Feldspar supplies alumina and alkali in some glasses. Calcium chloride is not a standard fining agent in this context.
Step-by-Step Solution:
Identify fining need: remove gas pockets for optical uniformity.Match traditional agent: arsenous oxide aligns with classic fining practice.Exclude fluxes (K2CO3, Na2CO3) and network modifiers not used for fining.Select “Arsenous oxide.”
Verification / Alternative check:
Older references list As2O3 and sometimes Sb2O3 as fining agents; both help bubble rise and escape during refining zones of the furnace.
Why Other Options Are Wrong:
Potassium carbonate / Soda ash: Primarily fluxing; not principal fining agents.Feldspar: Aluminosilicate raw, not used for fining.Calcium chloride: Not a standard fining additive.
Common Pitfalls:
Confusing fluxes and stabilizers with specialized fining additives; their roles in the batch are distinct.
Final Answer:
Arsenous oxide
Discussion & Comments