''The Last Supper,'' according to art historian Kenneth Clark, is ''the central point of Leonardo's career . . . the most literary of all great pictures . . . the keystone of European art.''
And Leonardo's masterwork has endured a massive campaign of destructive forces that have fought against its survival for almost 500 years.
''The Last Supper'' -- or ''Il Cenacolo'' (''Supper Room''), as Leonardo called it - is now undergoing its eighth restoration since the artist completed it in 1498. In this instance the word restoration may be taken in its literal meaning: to bring back to a former position or condition. Earlier ''restorations'' consisted mainly of overpainting by inferior or even clumsy artists, and some of these ''improvements'' served only to plunge Leonardo's original into greater obscurity.
Vasari -- who saw it in 1556 (only 58 years after it was created) -- described it as ''so badly handled that there is nothing visible except a muddle of blots.'' In 1642, or 144 years after its creation, Scanelli wrote that only a few traces of the figures remained, so confused as to make comprehension almost impossible.
Carlo Bertelli, however -- the specialist in charge of the present restoration -- tells us that he and his team are discovering that far more of Leonardo's original work remains than was generally supposed. Professor Bertelli is superintendent of the Pinacoteca di Brera, Milan's major museum. He is also in charge of the artistic and cultural treasures of Milan and of five of the seven provinces in Lombardia. His work on ''The Last Supper'' is being sponsored and funded by the National Board of the Cultural Heritage of Italy, situated in Rome.
''As our work proceeds, we are discovering that the 'Cenacolo' will have a new identity,'' Professor Bertelli explained, ''its own true identity. Even though badly damaged by time, and reduced by portions that have totally disappeared, the painting still maintains an inexhaustible force that gives meaning and beauty to each remaining fragment. In the areas already cleaned we are finding a masterpiece that is quite new to us, considerably different from the image we have been used to.''
''The figures,'' he continued, ''are quite diverse from what we imagined them to be. Our excitement grows as we remove the mutilations made by earlier restorers, together with the dust of past centuries and the smog of recent years. We are now far more satisfied by the results of our work than we expected to be at the outset. The final effect should be extremely gratifying!''
The figure of Simon the Zealot at the far right of the painting has now found its original profile, noble and intense. His beard, which grew some two inches through the generosity of inept retouching, has now returned to its more becoming length. His left hand no longer makes a declamatory gesture, but is now restored to its original attitude of one who receives.
And speaking of gestures, it was Leonardo's conviction that no gesture was valid unless it was directly motivated by the immediate intellectual and emotional forces in thought. This conviction, masterfully expressed by Leonardo's genius, evoked the drama of this biblical crisis as no painter before or after him has ever equaled.
Earlier artists -- such as Ghirlandaio and Perugino -- presented ''The Last Supper'' at the moment of communion, each disciple serene, lost in his own contemplation of what Jesus was teaching. But not so Leonardo da Vinci (1452- 1519). He selected the most theatrical moment of the gospel narrative: ''Verily, verily, I say unto you, that one of you shall betray me'' (John 13:21). And at this moment each disciple was driven by the emotion of his own heart to strike the attitude we behold.
What are some of the destructive forces that have militated against the survival of Leonardo's masterpiece?
* It was commissioned by Ludovico il Moro, the reigning duke of Milan, to be painted on one of the refectory walls in the Convent of Dominican Friars, housed at the church of Santa Maria delle Grazie. This wall was treacherous, filled with humidity. Leonardo covered it with a calcium carbonate ground. He discarded the traditional method of al fresco painting, in which the colors are brushed onto wet plaster, a technique that prohibits later corrections. He chose rather to paint in tempera mixed with oil and varnish, adjusting and refining his conception as his work progressed.
* The monks in the monastery let the smoke from their kitchen drift over the painting and actually drove a door through the lower portion of it.
* Napoleon's soldiers threw stones and bricks at it when they were using the refectory as a stable. Thus parts of Leonardo's original were destroyed, and these open spaces will unfortunately be seen when the present restoration is complete. According to Professor Bertelli, this damage is to be found only in the lower portion of the picture.
* Leonardo's worst foes, as already noted, were the restorers, ineptly retouching, overpainting, and distorting his original.
* During World War II a bomb fell in the center of the refectory. Had it not been for the sandbagging placed against the wall, the painting would have been totally destroyed at that moment.
We asked Professor Bertelli how he and his team were able to determine the difference between Leonardo's original and the overpainting by later tamperers.
''We observe the painting by microanalytical methods and by infrared spectophotometry,'' he answered. ''Sometimes we make photographs using infrared color film. At other times we use microchemical techniques that identify the materials. Thus we can separate the pigments used for repainting from the original pigments used by Leonardo.''
''How long will it take for the job to be done?'' we asked.
''It took us one year to restore only the figure of Simon the Zealot,'' he said. ''Now we have cleaned the head of Matthew, and the results are extraordinarily beautiful. We are learning ways of moving our work forward at a faster pace. After all, the smoke, smog, dust, and overpainting must be removed a speck at a time. In another two years, more or less, we should have finished the task.''