Leonardo would have flipped.

If only the Great Experimenter, whose constant tinkering with new media left his "Last Supper" tragically faded and his "Battle of Anghiari" a hopeless omelet of dribbled paint, had seen the International Institute for Conservation of Historic and Artistic Works in action, he could have breathed easier.

About 650 art conservationists and others from 25 countries have been meeting here all week discussing the latest ways to rescue everything from corroded ancient bronzes to moldy photographs.

They are a good group. They talk Finnish and German, Japanese and Australian, and they applaud fiercely at each other's presentations and fiercely challenge them on excruciating details. Even one Swedish expert whose talk had to be read in translation didn't escape. A colleague interpreted the questions for him and relayed his answers.

Actually, the query addressed to Frantisek Makes about his "enzymatic consolidation" method of restoring a spectacularly obscured 17th-century Swedish landscape pointed up a problem that has become increasingly serious in the art world. Makes used yeasts and bacteria literally to eat away the dried oils and the lining paste of flour, glue, sugar and oil that had seeped through the paint layer.

"Are these organisms a danger to health?" someone asked.

Makes indicated that one could catch pneumonia from them, that the situation is being studied, that no one knows exactly how bad it is. It is understood that in recent years artists have created hazards for themselves in experimenting with volatile chemicals. Now it appears the conservators are venturing onto new ground, too.

Yesterday, Gustav Berger and William Russell mesmerized the audience with their study of canvas stretching. This is what makes large paintings (and some small ones too) ripple and sag with age. It is called "canvas creep." Curiously, the huge 19th-century cycloramas, the documentary films of their day, rolled up and hauled from town to town, exposed to all sorts of weather changes and casually stitched together, have come through in remarkably good condition.

Working with the 100-yard-long "Battle of Atlanta," the scholars studied its unique method of suspension: It was hung from a giant circular beam, its bottom edge attached to a similar ring of iron pipe, and unreeled as from a great spool. Thus it was stretched vertically while it contracted laterally.

A model frame was built to measure stress and strain on warp and woof, and from this Berger and Russell got the information that led to an ingenious stretcher frame on springs.

Localized restraint is what hurts a painting, not rolling or the climate, they concluded, and another speaker with another subject seemed to bear them out when he showed slides of a large fragment of a Monet lily painting that had been rolled up for 40 years. Careful wetting and three weeks of gradual drying on a cold table brought it right back, it appeared from the before-and-after pictures.

One also learned from the printed papers:

* An epidemic of black spot disease in bronze antiquities is sweeping through European museums.

* Forgeries of amber chunks enclosing spiders and flies and even frogs have exasperated collectors since the 16th century. Today infrared spectroscopy spots most imitation amber, but certain museums have introduced a new vexation: They routinely polish the oxygen-sensitive stuff and even paint it with nail polish. The Botfeldt Method of Non-Conservation is recommended: "Keep hands in pockets."

* To this day, people who ought to know better think they are rescuing gold-leaf picture frames by touching them up with radiator paint, which isn't gold at all and which soon tarnishes.

* Showcases themselves can be part of the problem. Lead bullets at the Air and Space Museum built up crystals of lead formate in three years, and at another Smithsonian building, "after three years of exposure to the Washington weather," the lead roof is covered on the outside by lead sulfate and on the inside by lead carbonate. And a silver spoon at the Museum of American History has "developed a magnificent dendritic growth of silver sulphide crystals covering the surface with a glistening black fur quite unlike normal silver tarnish."

* Whatever medium you work in, you can be sure someone somewhere is figuring out how to save it for the ages, whether watercolors, pastels, copper pigments on paper -- or newsprint. Yes, they've got out their ultraviolet lamps and bleaches and vapors and ovens and Macbeth Reflectance Densitometers, and if nobody stops them, they'll make ordinary groundwood newsprint paper immortal. Yesterday's news is forever.