If you plan to donate your valuable art or mineral collection to a museum, be sure to give a sizable endowment, too.

That way, the museum can afford to study, preserve, catalog and store your collection.

In the 19th and 20th centuries, some benefactors who donated collections to museums "never provided sufficient endowment to maintain those collections," said Bill DeWalt, director of Carnegie Museum of Natural History in Oakland.

Endowments are essential, Dr. DeWalt added, "because the storage, maintenance and active research on those collections is very expensive."

In the natural history museum's budget, "We probably spend more than $2 million a year in salaries for the people who care for the collections," Dr. DeWalt said.

That figure does not cover costs for storage in climate-controlled rooms.

"We had a collection of model lighthouses that we sent to the lighthouse museum," Dr. DeWalt said.

"Should we ever sell anything, we reinvest it in the collection for the preservation of the core collections," Dr. DeWalt said.

At the Philadelphia Academy of Natural Sciences, which sold 18,000 minerals last month to a private buyer, the annual income from the collection's endowment was a mere $15,000, according to Ian Davison, interim president and chief executive officer.

"That comes nowhere close to paying for the cost of storage. The financial cost of maintaining a large collection of minerals is considerable even if you're not doing anything with it. It occupies space and requires security," Dr. Davison said.