Tate Modern, London
Searing artistic responses to the agony of America's racial struggle sit alongside powerful abstracts by forgotten artists. This compelling show puts the battle for civil rights in a brutal, brilliant new light
Sam Gilliam's 1969 painting April 4 is an epic cascade of purple tears, a huge curtain of sorrow. Agony stains it. Melancholy seeps through its delicate clouds of colour. You don't need to know what its title means to be moved by it.
When you know it was painted to mark the first anniversary of the murder of Martin Luther King on 4 April 1968, this abstract painting becomes a funeral elegy for assassinated hopes. It is one of the most powerful things in an exhibition that unconvers an entire lost history of American art.