How should I remove calcium scale without damaging plaster?

Use a pumice stone wet and gently, rubbing in small circles — it’s softer than plaster so it won’t scratch if you keep it wet and don’t press hard. For bigger areas, use a scale remover with phosphoric or dilute muriatic acid, following the label exactly and neutralizing afterward. Avoid concentrated acid sitting too long or wire brushes, which cause pitting. Patience is cheaper than replastering.