Crystal covers for Imperial columns?

Crystal towers? Space-age glass bubbles covering acres of land? Trapdoors opening and shutting out of the ground? These are some of the more dramatic proposals being made by Italian conservators who argue that permanent or semipermanent coverings ought to be placed over Rome's most threatened stone monuments.

One proposal is to cover the 10-story tall Column of Marcus Aurelius (erected AD 165-168) with a sheath of Plexiglas. The idea has drawn praise - and shrieks of horror.

The underlying question in the debate: Is an ancient monument under glass still an ancient monument or has the ancient reality disappeared beneath a modern cover?

``I favor the covering of the Marcus Aurelius Column, because of its uniquely threatened position,'' said Marisa Tabasso, a chemist who directs the Laboratory for Testing Materials at Rome's Central Institute for Restoration.

``I'm totally opposed,'' says Annamaria Mecchi, a consulting chemist working at the same laboratory. ``If one has to resort to such extremes to preserve a monument, I'd prefer to accept the fact that it will slowly disappear and ultimately be gone. We cannot turn all of Italy into a museum under glass.''

The transparent Plexiglas tower to cover the Column of Marcus Aurelius has been proposed by a University of Rome professor, Giangiacomo Martinez, the architect and conservator in charge of the current conservation project at the column.

Parts of the column cleaned three years ago have already turned black, while parts finished two years ago have turned gray, Mr. Martinez notes. ``If we don't cover the column, the carvings in the next generation will suffer terrible damage,'' he warns.

Martinez defends his project as a ``temporary'' solution until the pollution around the column decreases.

But critics say such a Plexiglas covering would obscure the object it protects and become a permanent fixture in the landscape.

``A Plexiglas dome was also proposed for the Acropolis in Athens, but Plexiglas becomes opaque under sunlight,'' says conservator Tom Roby. ``So the dome would obscure what it was covering.''

``You know what architects say - `Nothing is so permanent as the temporary,''' said Jukka Jokilehto, the Finnish director of archaeological conservation at the Rome-based International Center for the Study of the Preservation and Restoration of Cultural Property.

``The Eiffel Tower ... was built as a temporary structure for the World's Fair, and Helsinki's Glass Palace, now a protected building, was originally designed as temporary barracks,'' said Mr. Jokilehto. ``These monument coverings could become monuments themselves.''