As the musicians of the Minnesota Orchestra prepare to leave for Cuba on Wednesday, some are worried about their ability to return home with their instruments.

Top-flight classical musicians seek out the best-available instruments, which can be centuries old. Many were built at a time when there was little concern about the materials used to make them.

Those days are gone. About a year ago the Obama administration announced that it would more tightly enforce the Convention on International Trade in Endangered Species of Wild Fauna and Flora, an international endangered species treaty also known as CITES.

Mele Willis, the Minnesota Orchestra's artistic operations manager, said the treaty includes three endangered materials most commonly found on classical instruments:

"Brazilian rosewood, mostly in stringed instruments, though not exclusively," she said. "Elephant ivory, again mostly in stringed instruments, but some reed instruments as well. And tortoiseshell."

The complex international accord effectively banned the international transport of the materials even if they are part of an instrument. However, a grandfather clause allows musicians to travel with the instruments if they can prove among other things that their instruments were made before 1974, when the treaty went into effect.

Willis has been charged with documenting the details of instruments used by Minnesota Orchestra musicians before they go to Cuba. The historic trip marks the first visit of a North American orchestra to Cuba since President Obama announced in December that the United States would change its policy toward the island nation.

Minnesota Orchestra artistic operations manager Mele Willis, photographed at Orchestra Hall in Minneapolis Wednesday, April 22, 2015. Willis is co-coordinating the tracking and documentation of instruments during the orchestra's trip to Cuba. Jeffrey Thompson | MPR News

"The oldest is a stringed instrument that was made in 1710," she said "I think the newest is [from] about 1930."

Now that Willis has collected the proper documents, they should allow the instruments to pass safely through customs.

"We need to have exactly what material is found on their instrument: where it is, when the instrument was made, when they acquired it," Willis said. "It's significant. A lot of detail."

That includes an accounting of tiny amounts of material, like the white covers on the tips of bows, or the inlay on an instrument body.

In June 2014, shortly after the step-up in enforcement, a horror story reverberated through the classical music world. Customs officials at John F. Kennedy International Airport in New York seized seven bows belonging to members of the Budapest Festival Orchestra as they flew in to perform at Carnegie Hall. The officials said the bows did not have the required documentation — a claim the musicians disputed.

Dispatches from Cuba: Follow the orchestra's trip

The musicians, who borrowed bows for the concert, had to pay fines to collect their bows before their flight home.

Minnesota bow maker Matt Wehling, who has customers in the United States and in Europe, said his clients express increasing concern over the international treaty.

"I just got one the other day from someone who is in Europe and said, 'Can you send me some sort of certification saying I don't have elephant ivory on this?'" he said.

In his workshop in Northfield, Minn., Wehling likes to make bows from scratch, using special custom tools.

Yet because of the stepped-up enforcement of the international accord, increasingly he is replacing ivory on bows, some of them hundreds of years old, with what might seem to be an extraordinary material.

"Now people in this industry don't use ivory," he said. "They use parts from tusks of mastodon and mammoth."

As a result of climate change, a supply of the ancient bones of the animals are emerging from melting permafrost on the Russian steppe, and since mastodons are not endangered but extinct, they're not covered by the treaty.

A violin bow tip made from mastodon, center, instead of ivory, Thursday, March 26, 2015, in bow maker Matthew Wehling's workshop in Northfield, Minn. Jennifer Simonson | MPR News

Mastodon and elephant ivory are very similar. But as Wehling pulls out two small white rectangles ready to become bow tips, he said that similarity is a double-edged sword.

"That one's ivory, and that one's mastodon," he said. "You can see how difficult to would be for someone to look at some of these pieces and decide that they are different."

The Hungarian bows seized in New York all had documentation listing their ivory elements as mastodon, but federal authorities believed they contained elephant ivory.

Willis said that after completing the paper work, she determined that musicians should leave certain bows behind just to be safe because they contain tortoiseshell.

Although orchestra officials are confident in the documentation, the true test will come when the musicians return from Havana and go through customs.

All of the precautions taken by Willis have not completely allayed musicians' fears.

Second violinist Aaron Janse said he has replaced the ivory tips on his bows, but worries customs officials may not be able to tell. The worst thing he said is the uncertainty.

"I want to be able to bring my best instruments so I can personally do my best in the concerts," he said. "And I am going to. So we'll see if they make it back."