Saving the stones of Rome. Plastic gets a bad rap as a coating for ancient arches

`CONSERVATION work is 99 percent tedium and 1 percent terror - terror that you might be doing something wrong,'' says Tom Roby, of Philadelphia, the sole American conservator among the otherwise Italian group at the Arch of Septimius Severus. As the debate continues over whether Michelangelo's Sistine Chapel is being returned to its original beauty or destroyed by conservators, a quieter debate persists over the use of a consolidating product that has long been used in the restoration of paintings but has shown mixed results on stone: Paraloid.

``When we began, we tried Paraloid,'' said Roberto Nardi, who works for Rome's private Center for Archaeological Conservation. ``But after six months, the stone became too dark and we reversed the process.''

Half a mile away, visible across the Roman Forum, the Arch of Constantine is being coated with Paraloid.

Critics of the product say it is highly sensitive to outdoor ultraviolet light and discolors and damages the stone. Even worse, they charge, the chemically organic material settles densely in the stone's outer layers. There, it becomes harder than the stone itself, forms a crust, and forces the stone to flake.

``Paraloid has become a kind of dirty word because it's plastic,'' said John Horne, also from Philadelphia, one of three non-Italians working with the conservation team at the Arch of Constantine. ``But it has good binding qualities, and if it's not used, the stones will continue to crumble.''

Key terms in the debate are ``organic'' and ``inorganic.''

Both organic and inorganic materials are used to ``consolidate'' crumbling stone. Both have their supporters and critics.

Inorganic material is chemically closer to the stone it adheres to, but deteriorates quickly (and requires frequent redoing). ``Inorganic'' supporters say this is fine, because the stone is not threatened by alien chemical compounds.

Organic material is chemically different and adheres better. But critics claim its demerits outweigh its merits.

Paraloid, a plastic, is organic.

``Theoretically, Paraloid is reversible, since it's soluble, but in reality, when you apply it and leave it on a monument this size, it isn't,'' said Alessandra de Vita, a conservator at Trajan's Column.

In Rome's laboratories, a search is under way for chemical solutions that will provide greater penetration than Paraloid and therefore avoid crust formation. A solution may be ``monomers'' and ``oligomers,'' according to Ulderico Santamaria, a chemist at the Laboratory for Testing Materials at Rome's government-run Central Institute for Restoration. These chemical compounds are made up of molecular units smaller than Paraloid and can make their way through smaller spaces that lie deeper within the stone, Mr. Santamaria said.