What she meant was that the murals are frescos, created by painting pigment directly into wet plaster prepared on the walls.

You can't easily remove a fresco and sell it, and these days nothing that can be ripped out of Detroit and sold can truly be considered safe. (For example, the city has already harvested the poles from its fire stations and sold them for scrap.)

This week Detroit finally cleared the last legal hurdle preventing it from entering bankruptcy, with a court authorising an application filed in July when the city admitted it had no chance of repaying its 100,000 creditors the $US18 billion ($20 billion) it owes them.

Now the most powerful man in town is the unelected emergency manager, Kevyn Orr, whose task it is to decide whose pensions shall be cut, whose debts shall be paid and which treasures will be sold off in order to complete the process.

Detroit is the largest city in America ever to suffer such an indignity.

Mr Orr has now turned his eyes to the Detroit Institute of the Arts, one of America's great art museums, with a collection of 66,000 pieces largely gathered when Detroit was the largest industrial centre on earth.

Christie's Auction House has begun an initial appraisal of 496 works from the collection and come up with an estimate of $US2 billion, a smaller amount than was expected. The value of works purchased by the city from the initial appraisal was estimated as being between $450 million and $800 million.

The gallery has vowed to defend the collection in court if necessary, a process that could pit the DIA and its supporters against city workers who face losing pensions they have been contributing to for years.

The gallery declined on the recommendation of its lawyers, but pointed to previous statements it had made.

In August the gallery said it would co-operate with Christie's evaluation, but added that it held its collection in a charitable trust that could not be sold, and argued that Mr Orr would undercut the city's recovery by selling off its most important cultural institution.

Donors have already begun to offer funds to save the collection. On Friday, philanthropist A. Paul Schaap pledged $5 million.

Mr Orr has suggested that there may be other solutions than simply auctioning off the works. Shares could be sold in some of the gallery's masterpieces, with their various part owners allowed to borrow them from the museum.

The crisis in Detroit is so deep that it has become a kind of laboratory for inventive civic approaches from across the political spectrum.

In the city's centre, a small recovery has begun to be built around the massive investment by a handful of people who are buying and restoring some of the city's famed towers and moving their workforces into the new office and apartment buildings.

With the city government unable to provide even the most basic services the investors have rebuilt city parks, reinstalled streetlights, hired private security guards and paid for street cleaning and rubbish collection.

The early success of the downtown revitalisation and the abundance of cheap real estate has led to an influx of young creatives pushed out of such cities as San Francisco and New York by high prices.

In the near-empty and decaying suburbs that ring the city, urban farming groups have taken over vast swathes of land. Some seek simply to clear the abandoned buildings and halt the urban blight for those who remain, but other groups hope Detroit could become a new model city, able to feed itself from produce grown within its borders.