A zinc-containing amalgam is contaminated by saliva during condensation. What is the characteristic consequence?

Correct answer: B. Delayed expansion beginning after several days

• A. The properties are substantially affected, not merely the appearance.
• B. CORRECT. This is the classic DELAYED (secondary) EXPANSION. Zinc reacts with water to liberate HYDROGEN GAS, which becomes trapped within the setting mass. Pressure builds gradually, so the expansion begins after about 3–5 days and can continue for months, reaching several hundred micrometres. The clinical results are protrusion of the restoration from the cavity, postoperative pain from pressure on the pulp, and marginal breakdown or fracture.
• C. The expansion is delayed and progressive rather than an immediate contraction.
• D. There is a marked dimensional change — that is the defining feature.

Note: Moisture contamination of ANY amalgam reduces strength. Only a ZINC-CONTAINING amalgam shows delayed expansion. This is why rubber dam isolation is not optional for amalgam placement.