The vast papal art exhibition that toured New York, Chicago, and San Francisco over the last two years has finally returned home to the Vatican museums here. And when the last of the 237 works - a huge Caravaggio oil painting depicting the burial of Christ Jesus - arrived at the end of last month , the feeling of relief was almost palpable.
''We will never again send such a large collection out,'' says Walter Persegati, treasurer of the Vatican museums. ''Our next consignment abroad is now under discussion. It will probably be an exhibit in Canada in 1986 of no more than 20 or 30 works on the theme of papal Rome in the 16th century.''
Keeping hundreds of priceless works safe from the elements, shipping damage, vandalism, or theft proved to be an overwhelming task, he explains.
Preparations for transporting the show - ''The Papacy in Rome,'' which included paintings on canvas and wood, sculptures, frescoes, mosaics, tapestries , bronzes, ceramics ranging from the early Greek and Roman periods to contemporary works - were carried out by teams of experts working with scientific precision. Apart from the normal job of packing priceless and often large, unwieldy, and delicate works into specially padded crates, Mr. Persegati explains, ''we also had to deal with the risk of forced rerouting of our trucks by art thieves or terrorists, or demonstrations by well-intentioned, if naive, people who did not want to see such unique treasures leave the country.''
In the seclusion of the tiny Vatican railway station, the treasures were hefted piecemeal into trucks at 5:30 a.m. over a two-week period in November 1982. Italian customs officials came to the Vatican to seal the specially prepared pallets, and the trucks carrying them (never more than two at a time) were escorted by police to the airport.
Aldo Aroldi, director of Bruno Tartaglia, the company responsible for the initial shipment to New York of the Vatican works, says: ''We have been shipping art since 1907, but still every shipment can present new problems in this business.'' Packers worked in close collaboration with art restorers to determine how best to secure a piece of art in its packing case. Several points were considered: the state of repair, the change in climate and humidity level that might assail a centuries-old painting, and the shape and protuberances of a piece of sculpture, for example.
''The Greek statue of the Apollo of Belvedere in the Vatican collection is made of 13 pieces of marble,'' Persegati points out. ''Restorers had to work out the main pressure points of the sulpture, which had to be packed in one piece, and the packers constructed a wooden box with contoured wooden bars to fit the sculpture so that it could not move.''
The Apollo traveled in a series of three boxes, one inside the other, and the space between them was filled with a shock-absorbing plastic material called Ethofoam.
''The most difficult things to pack, however, are oil paintings on wood panels,'' Mr. Aroldi says. Something like a Leonardo da Vinci painting requires careful nursing during transport and on arrival. The wood is subject to humidity and temperature changes, and any expansion or shrinking could cause the paint to crack and chip. All oil paintings were packed with silica gel, a plastic substance that absorbs excess humidity or gives off vapor in a dry atmosphere to help keep the painting in its accustomed atmosphere. Once the painting arrives at its destination, it is left inside its crate to ''acclimatize'' for 12 to 20 hours before it is unpacked.
''A host museum must always guarantee stable temperature conditions, ensured by temperature-measuring instruments inside showcases,'' Persegati notes. ''Or, as in the case of many oil paintings, silica gel was sealed inside the back of the picture on display.''
One spot where the danger of breakage was very high, however, was inside the Vatican itself. ''The Vatican museums are an example of how a museum should not be planned,'' explains Persegati. Heavy sculptures like the Egyptian lions of Nectanebo - perhaps the heaviest works ever transported for an exhibition - had to be carried for hundreds of yards inside cramped spaces that made the use of sophisticated machinery impossible.
The dozen or so staff members of the Bruno Tartaglia company are practiced at getting their priceless cargoes packed. What they still balk at, however, is the red tape involved in moving any piece of art from Italian soil.
''Whatever the physical difficulties of packing and carrying a piece of art, they are nothing compared with the bureaucracy you have to go through every time a piece of art travels either in or out of the country,'' says one staff member. ''The rules constantly change. You never seem to go through the same procedure twice.''
Regulations include authorizations from three ministries, from the Italian State Exchange Office, customs and, in many cases, embassies.
Insurance is another problem. Most art carriers transport irreplaceable goods , and so work hard to reduce breakage risks to zero. Premium figures are not often mentioned, but recently news surfaced of a Caravaggio painting whose insurance costs for shipping to New York for the Caravaggio show in February amount to 800 million lire (about $437,000).
Fortunately damage to the works of world-class artists is rare. Persegati recalls one minor but still memorable incident that occurred while the Vatican works were being unloaded at the Metropolitan Museum of Art in New York. The truck containing the delicate sculpture of the Augustus of Prima Porta passed under a low concrete beam, which gave the packing case a hard knock, damaging the outer case. ''Immediate X-ray controls showed no damage whatsoever,'' he remembers with obvious relief. He adds that incidents of this kind show the careful planning and ability of all those concerned with the packing and shipping of art.