“I am confident that you will never have seen an exhibition like this before. And given the level of complexity of loan negotiation and diplomatic intervention involved, you never will again.”

I delivered this statement last week at the unveiling of the Royal Academy of Arts’ autumn blockbuster exhibition, Bronze, which I curated. Filled with dozens of iconic objects, it tells the 6,000-year-old story of bronze as an artistic material. Never before has such a broad collection of national treasures – from the classical period to the present day, and from such disparate cultures around the world – been brought together beneath one roof.

It is hard to imagine any one person who might be familiar with all of the pieces in the show. It offers a single coherent overview of a technically complex material that has long enchanted artists from Cambodia to Nigeria. Bringing together an Etruscan sculpture that dates back to the 2nd century BC as well as bronzes by 20th-century sculptors – a wall-mounted spider by Louise Bourgeois, Alberto Giacometti’s sinewy The Cage – this truly is a once-in-a-lifetime exhibition.

But the challenges my team of curators faced in drawing together the 177 works of art from 98 lenders are almost as epic as the pieces themselves. Not only were we limited by the time we had to prepare – Bronze was hastily convened over just 14 months after an anticipated exhibition on the art, archaeology, history and influence of Syria became impossible to mount following last year’s Arab Spring – but also by what one might call local difficulties. Despite unrest across the Middle East, the earliest works featured in the exhibition, the Nahal Mishmar hoard (c. 3,700 BC), a selection of sacred ceremonial treasures excavated from the Judaean Desert, were lent from the Israel Museum in Jerusalem. It was one of the simplest to arrange.

Many of the loans had never been moved from their museums before because of their size and weight. Others had never been seen outside their countries, not least because of the great expense and dangers associated with shipping them by air. In some cases, getting permission for their export required changes in the law.

Last summer, we began collating lists of the world’s most desirable bronze works, and a plan of action was put in hand to ensure the best possible chances of securing them on loan. Then ensued months of negotiation with the institutions holding these landmark works. National treasures are not released by their guardian museums on the back of a polite letter of request. They require face-to-face discussions at which a persuasive argument must be made for lending the work.

As I and the show’s other curators, Professor David Ekserdjian and Cecilia Treves, travelled to museums to put our case (and, when politely turned down, our appeals), other colleagues met with private owners to reassure them we would take care of their pieces.

Curating is an exercise in diplomacy. Take the first work visitors encounter as they enter the exhibition – Dancing Satyr, from the Church of Sant’Egidio in Mazara del Vallo. Believed to be a precious object looted from a dignitary during the 455 BC sacking of Rome, it was rediscovered by chance in 1998, by fishermen as they dragged their nets along the bottom of the Straits of Sicily.

Just as much luck – and three trips to Italy – was involved in getting the 2,500-year-old piece to the UK. Months of discussions began with the Italian Embassy in London smoothing the way, establishing the support of the various arts and archaeology bodies responsible for Italian collections, as well as the museum directors and curators in charge of specific pieces. Without the compliance of these parties, and the quiet influence of others, Mario Monti, the Italian prime minister, would not have given the exhibition his official blessing. But this encouraged other countries to grant us permission to show their treasures, too.

Even then, negotiations went close to the wire. Christopher Le Brun, the president of the Royal Academy, went to Sicily wearing his medal of office to ensure the Dancing Satyr, as well as another crucial Italian piece, Ariete the Ram, made it into the exhibition.

It can be just as challenging getting things from across the UK. Laocoön and His Sons, a life-size bronze cast of a marble original held in the Vatican, had to be moved from Houghton Hall for the very first time since it was brought to Lord Cholmondeley’s Norfolk estate 400 years ago by Britain’s first Prime Minister, Sir Robert Walpole. Which is no mean feat for two tonnes of arms, legs and snakeheads.

It was by sitting down with Lord Cholmondeley and talking him through what the exhibition was trying to achieve and showing him where his work would sit in the context of other pieces – like a seating plan at a wedding – that he was persuaded. That, coupled with reassurances as to how delicately the work would be handled.

Transporting objects of such weight is always a complicating factor. As bronze is such a malleable material that can be melted and moulded or beaten into extraordinary shapes and sizes, the centre of an artefact’s gravity can be in a peculiar place, making loading and unloading even trickier.

And it’s not a question of bubble-wrapping these items for protection. They need to be transported by specialist couriers in made-to-specification museum crates padded with sophisticated materials such as plastazote, a thick but versatile foam, similar to the material used in yoga mats. Fine art transportation is not like calling a removals company – and the insurance requirements are unspeakable…

The statue of a giant Perseus standing on top of Medusa, whose head he has just cut off, came to us from Trenton Park in Staffordshire. But still it was no easy get, because of the sheer scale; he’s about 12ft tall. He went on an air-conditioned, climate-controlled truck and travelled with a huge entourage of conservators and curators. The pieces in this show are just like members of a touring rock band, only without the riders.

Juxtapositions across cultures, styles and time are at the heart of this exhibition. The Cult Chariot of Strettweg, an ancient burial gift featuring helmeted horsemen surrounding a central female figure, which dates back to 7 BC and is owned by the Universalmuseum Joanneum in Graz, Austria, demanded to be part of the show.

But we weren’t after just one bronze chariot – we were after two. The Chariot of the Sun, a religious artefact from the late Nordic bronze age, dates back to the 14th-century BC. Discovered in a peat bog in Trundholm in 1902, it is a centrepiece of Copenhagen’s National Museum of Denmark. The opportunity to see the two chariots together for the very first time was a tempting challenge.

Institutions are naturally disinclined to part with their most popular and well-known objects. But the museum world is a small one, built not only on the collection of marvellous objects that they hold and bring together, but the bonds of scholarship, friendship and mutual enthusiasm shared by curatorial professionals across the world.

It was with this in mind that I travelled to Graz last year to petition Peter Pakesch, my colleague in charge of the Universalmuseum Joanneum, to lend the Strettweg Chariot, an exceptional work that has never been allowed to leave Austria before. Even if the museum agreed, it would only be able to journey to the Royal Academy if we convinced every minister in the Styrian Government to change the law to permit its loan.

Once a curator at the Graz gallery, who appreciated the significance of the Royal Academy’s exhibition, began championing our request for the Chariot, the museum set about getting the law changed on our behalf. The process took about five months. When the exhibition ends in December, the work will return to Schloss Eggenberg where it will remain, bound once more by the law that prevents it from leaving Austria.

With the Strettweg Chariot as a considerable bargaining chip in my pocket, my next stop was Copenhagen. But museum curators there could not understand why I would not accept the very fine copy they had made of their Chariot of the Sun. Nor could they believe I would even imagine I could persuade them to part with such an important Danish icon.

But for a few short months, the two chariots will now sit proudly alongside each other at the Royal Academy – cultures and years apart, but joined through form and meaning.

When sourcing artefacts from further afield, we relied on the backing of the British Council and British Embassies. It is through the tenacious offices of Mark Gooding, the British Ambassador to Cambodia, for instance, that we have several loans from its national museum. When our negotiations hit an impasse, we began a conversation with Mr Gooding to see if there were diplomatic channels he could use to speed up the process. His support encouraged the various ministries to make sure it happened.

The sacred sculptures were finally blessed (literally) in Phnom Penh last Wednesday morning, and took their place in the exhibition yesterday. As the show opens to the public today, I hope you will now take yours, too.