A modern art installation valued at euro800,000 ($1.1 million) was damaged after an overzealous cleaning woman scrubbed away a patina intended to look like a dried rain puddle, a Dortmund official said Friday.

Martin Kippenberger's "When it Starts Dripping from the Ceiling" remains in place at the city's Ostwall museum, despite the damage sustained earlier this month when a cleaner scrubbed away the painted puddle beneath a rubber trough placed under a stacked tower of wooden slats.

The work by Kippenberger, a German-born artist who died in 1997, was on loan to the museum from a private collector, who agreed that it should remain on display despite the incident, said Dortmund city spokeswoman Dagmar Papajewski. In the meantime, insurance adjusters are assessing the damage.

It has not yet been decided whether the patina would be restored, or if the artwork would be left in its newly "cleaned" condition, Papajewski said.

"It will be up to the collector to decide," Papajewski said. "We can't consult Kippenberger about it."

Papajewski said the company that employed the cleaning staff had instructed them to stay at least 20 centimeters (8 inches) away from the artwork and it was not clear why the patina had fallen victim to the woman's scrubbing brush.

"This company has been working with us since last October," Papajewski said.

Kippenberger's works have been displayed at New York's Museum of Modern Art and London's Tate Modern.