Image caption The cleaner had been told to keep at least 20cm away from the artwork

An art installation worth more than $1m (£624,000) has been damaged by an overzealous cleaner who tried to scrub away a deliberate patina.

The patina was meant to look like a puddle beneath a rubber trough placed under a stacked tower of wooden slats.

Artist Martin Kippenberger's When It Starts Dripping From the Ceiling remains on display in Dortmund's Ostwall Museum, where the damage occurred.

The work is on loan to the museum.

The private collector who owns the installation by Kippenberger - who died in 1997 - has agreed that it remain on display despite the incident.

Dortmund city spokesperson Dagmar Papajewski said it had not been decided whether the patina would be restored.

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

Ms Papajewski said the company that employed the cleaning staff had been working with the museum since October last year and had told their staff to stay at least 20cm away from the artwork.

Kippenberger was a prolific artist who produced work in a wide range of styles and media. His portfolio included sculpture, paintings, prints and installations.

His works have been displayed at New York's Museum of Modern Art, London's Tate Modern and London's Saatchi Gallery.