WASHINGTON (AP) - Thomas Jefferson wrote "all men are created equal" to declare U.S. independence from Britain, yet he was also a lifelong slave owner who freed only nine of his more than 600 slaves during his lifetime.

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That contradiction between ideals and reality is at the center of a new exhibit opening Friday as the Smithsonian Institution continues developing a national black history museum.

It offers a look at Jefferson's Monticello plantation in Virginia through the lives of six slave families and artifacts unearthed from where they lived. The exhibit, "Slavery at Jefferson's Monticello: Paradox of Liberty," was developed with Monticello and will be on view at the National Museum of American History through mid-October.

It includes a look at the family of Sally Hemings, a slave. Most historians now believe she had an intimate relationship with the third president and that he fathered her children.

Museum Director Lonnie Bunch said his staff can test ideas by building exhibits before the National Museum of African American History and Culture is finished.

It will be the first museum added to the National Mall since 2004. A groundbreaking is planned for Feb. 22, and it's scheduled to open in 2015 near the Washington Monument.

Bunch said museum officials want to see how the public responds to subjects, such as slavery, as they try to present history for the widest possible audience.

Slavery, he said, is still the "last great unmentionable" in public discourse but central in shaping American history.

"This is a story we know we have to tell, and this is a story we know is going to be difficult and going to be challenging, but this new museum has to tell the story," he told The Associated Press. "In many ways, the Smithsonian is the great legitimizer, so if we can wrestle with slavery and Jefferson, other people can."