Despite the powerful continuity condition, the notion of closure operator is very general. It is therefore important to provide tools for improving a given operator. Fortunately, there is a natural lattice structure for closure operators that allows us to distinguish between properties stable under meet (idempotency, hereditariness, productivity), and those stable under join (weak hereditariness, minimality, additivity) . Hence it is clear that each closure operator has an idempotent hull and a weakly hereditary core, and analogously for the other properties. The passage to the hull w.r.t. to a meet-stable property will normally not destroy already existing join-stable properties, and the passage to the core w.r.t. to a join-stable property will normally preserve meet-stable properties.