Let's say that an American couple were charmed by an old ceramic vase they found in a store in Mexico during their vacation, and they bought it. They were less than charmed when a customs official told them that they had taken the vase out of Mexico illegally - Mexico was claiming it as part of its national patrimony. Why were they not told that there was an export restriction when they bought it? Why were they able to buy part of a nation's ''patrimony'' in the first place?
A quirky sort of problem, but one which may affect more and more people who want to do a bit of collecting while abroad.
Leaders of countries around the world have become increasingly concerned about the loss of national treasures - historical and religious monuments - and works of art that form their homeland's cultural heritage, and they have enacted laws to protect these objects and keep them within their borders.
They are less concerned about the plundering of invaders than that which takes place in the marketplace, where foreigners with considerable amounts of money can deprive their people of the ability to discover their roots and understand the basis of their culture.
Many communist countries and others of the third world - including Brazil, Bulgaria, China, the Soviet Union, and Zaire - have established wholesale embargos of all art from their borders. Others, such as Greece, Iran, Mexico, Turkey, and the countries of Central and South America, have adopted wide-ranging restrictions covering art and artifacts that are from certain areas and date back a certain number of years. Many European countries, as well as Canada, are also restrictive, but they generally decide what can and cannot leave their borders on a case-by-case basis.
The rules change from one country to the next, but that is not the only thing that could confuse the buyer of art. The nations with embargoes and restrictions often have considerable black-market activity, and the sellers of this contraband may successfully persuade tourists that desired pieces are no problem to take out of the country.
An explorer may come upon an as-yet undiscovered ancient ruin (there are many in Turkey, for instance, and in Africa), but find to his dismay that the government lays claim to objects it never previously knew existed. The same is true for finds in coastal waters. Another complication is the fact that US Customs recognizes some claims by foreign governments and ignores others.
Talking with foreign consulates and cultural attaches stationed in the US before going abroad may not be all that helpful, at least when one is looking for specific information.
For instance, the Mexican cultural attache said recently that none of the works by the painter and muralist Diego Rivera are allowed to leave Mexico, but some works by his teacher, Geraldo Murillo (who went by the name Dr. Atl) may be exported. When asked which works could be exported and where one could find out, the attache responded: ''You can take the work and apply for an export license. If you get the license, you can take it with you.''
Stewart Seidel, US Customs assistant chief counsel for enforcement and operations, says the best thing to do is to ''contact the American Embassy in the country you are in. They can help interpret the country's laws, explain how to go about obtaining an export license if you need one, and let you know to what degree the United States will enforce the export laws of the country.''
He points out that the US will not enforce a general restriction on export unless there is a clear case of theft involved. Mexico, for instance, claims ownership of all pre-Columbian art and artifacts, and pre-Columbian works entering the US will be considered stolen from the Mexican government. There is no such claim, however, for works of the colonial period, although the Mexicans still consider them part of their cultural heritage. If a tourist legally purchases an object of the colonial era, the US will probably ignore any subsequent claims by the Mexican government for its return.
As far as our imaginary couple and their ceramic vase are concerned, the issue is how they got it out of the country and also how it came into their hands. As the question of ownership was in dispute (the Mexicans claiming it by law theirs and the would-be owners pointing to a bill of sale), the US would assume control of it, awaiting a decision in an American court. In all likelihood, someone in Mexico stole the vase and sold it to the American tourists either directly or through an intermediary. In a foreign country, where language may be a problem, finding out whom to trust when making purchases can be difficult.
According to Michael Downey, senior arts investigator of the Federal Bureau of Investigation, ''A lot of stolen artwork is brought into the country by people who innocently thought that they were just getting a good deal.'' A good way to ''keep out of trouble'' is not to buy any works of art or antiquity from someone you can't go back to, to be suspicious if a high-quality work is being offered way below value, and to check with major art galleries both in the US and abroad about what kinds of works can be purchased and taken out of the country, he says.
In some cases, the collector will simply be out of luck: A piece may have been bought quite legitimately, but just can't be taken out of the country. Some countries - like Egypt, Mexico, and Turkey - have more egregious cases of this than others, but often something can be worked out. Most nations have some ministry of culture from which special permission for an export license can be sought, and various considerations are taken into account.
England and Canada are no less concerned about their national patrimony than other countries, but they will only withhold an export license when purchase of the work at market value by a public collection can be arranged. In this case, the tourist-collector may lose the work, but not the money paid for it. Sometimes the state buys the work directly.
When a work with a ''clouded'' title does enter the US and a claim is made against it, all may not be lost for the collector. Many times a deal can be struck with the claimant (especially when it is an individual) which will cost some money but will prevent an expensive and protracted legal battle. Europeans are quite used to this sort of thing. With all the wars that have taken place there - and the plundering of valuables that so often accompanies wars - they are accustomed to dealing with the gray areas of the law.
The heightened concern in countries around the world with preserving their cultural heritages and the UNESCO treaty of 1972 (which the US signed last December) prohibiting the export of artwork whose ownership may be in dispute have very much changed the way in which art and antique collectors function. Never again will a Lord Elgin be able to take away half the Acropolis and store it in a museum a thousand miles away. The trick now is to understand what all the laws are about and how to protect yourself. Someone with money to spend still has a fair amount of flexibility, but maybe not quite as many choices as before.