An American billionaire believes that no artists depict our common humanity better than those of the Dutch Golden Age. That’s why he is sending his collection on a world tour

Thomas Kaplan is the first foreign collector to have an exhibition at the Louvre, and he wants to tell me all about it. Striding into “Masterpieces of the Leiden Collection”, he plunges through the crowd, eager to point out Gerrit Dou’s old scholar interrupted at his writing; Frans van Mieris’s exhausted traveller who can’t even pull up his wrinkled socks; the creased, lived-in face of the old woman in her white bonnet, the first Rembrandt Kaplan ever bought. He now has 11.

It’s clear Kaplan loves them all, especially for their humane qualities. If his three-piece suit and the discreet red flash of the Légion d’Honneur ribbon at his lapel already set him apart from the daytrippers in the crowded French museum, what marks him out even more is that he never stops talking. He reminds me again and again that he is a historian (“by avocation rather than vocation”, but with a BA , MA and DP hil from Oxford). He tells me about Rembrandt’s influence on Goya, Picasso, Francis Bacon, Damien Hirst and Zang Fenzhi.

Kaplan did not set out wanting to be a collector. His mother took him regularly to the Metropolitan Museum when he was a boy, but it was only a chance encounter 15 years ago with Sir Norman Rosenthal, then secretary of the Royal Academy, that led him to consider the idea. At 39, he was on his way to making a fortune, first out of silver and later in natural gas, and was thus steeped in the business of arbitrage. Reassured by Rosenthal that Old Masters were unfashionable – and therefore affordable – Kaplan and his wife, Daphne Recanati, started to buy.

The Kaplans focused on the Dutch Golden Age, on Rembrandt and his pupils in Leiden, the fine 17th-century painters known as fijnschilders, who created works that told stories from history, the Bible and every­day life: of butchers and fishmongers, milkmaids and musicians, cardplayers and travellers, men praying, women at the keyboard. Over nearly a century these painters produced a historical record remarkable for its humanity. According to Kaplan, “They offer beauty, storytelling and a peephole into Western culture and the universal themes of civilisation.”

Golden hoard MAIN IMAGE Kaplan in front of the “Rembrandt wall” at the Leiden Collection, his gallery in New York. ABOVE Kaplan with his Vermeer, the only one still in private hands

The Leiden Collection, named after Rembrandt’s birthplace, began with a small oval portrait on metal by Gerrit Dou, Rembrandt’s first pupil. Kaplan took advice from a few expert dealers, Otto Nauman in New York and Johnny van Haeften in London, and bought directly from them or had them bid at auction on his behalf to keep his identity secret. By 2007, when he sold his business, he was acquiring a painting a week. For at least five years, Van Haeften reckons, Kap­lan bought nearly three-quarters of all the fijnschilder pictures that came on the market. Many he acquired inexpensively, but he pushed the boat out if necessary. “When he really wants a picture you can’t stop him,” says Van Haeften. Steve Wynn, a Las Vegas casino magnate, offered Kaplan a Rembrandt self-portrait on condition that he also buy his lovely little Vermeer, “Young Woman Seated at the Virginals”. Kaplan went for it.

But the real holy grail for Dutch Old Master collectors is Rembrandt’s greatest pupil, Carel Fabritius, who painted “The Goldfinch” and was killed at the age of 32 in an explosion in 1654 that destroyed much of the city of Delft, including Fabritius’s studio. Just 13 of his works survive; “Hagar and the Angel”, the only one still privately owned, had been in the same collection for more than 200 years. When a dealer made a discreet phone call on Kaplan’s behalf to see if the picture might be for sale, the owner, a Viennese count, had just one question: “Will he pay a Rembrandt price?” He did.

Kaplan now owns more than 250 works. He won’t say how much he has spent, but the finest have recently been insured for over $500m. His office in New York has a special “Rembrandt wall” with a lighting scheme designed to glorify each picture – to make it “pop off the wall”. He doesn’t quite say so, but one of his ambitions is to make Old Masters sexy. There are good business reasons to do that, and humanitarian ones too: the acquisition of “Hagar and the Angel”, with its story of the saving of Hagar’s son Ishmael, whose story is central to both Jewish and Koranic traditions, persuaded the Kaplans that their collection had a message for the world.

They commissioned Arthur Wheelock, curator of northern European art at the National Gallery of Art in Washington, DC , to write an essay on every picture in the collection, and put his research online. The Kaplans also revealed their identity: the Louvre show is their “coming-out” party.

The Paris museum rarely borrows from private collectors, and never from foreigners. But even in France, museums are changing: they make a lot of money out of blockbuster exhibitions, which need to draw from collections all over the world. The director, Jean-Luc Martinez, is seeking global partnerships. By winning the Leiden show, the Louvre proves it is top dog, and the Kaplans’ pictures get sprinkled with the fairy dust the art world calls “validation”.

From Paris, the Leiden Collection will send 68 pictures to a show opening on June 16th at the National Museum in Beijing. Three months later, an even bigger show will open at the Long Museum in Shanghai. Further stops are planned for 2018, at the Pushkin Museum and the Hermi­tage in Russia, and finally at the Louvre Abu Dhabi.

Kaplan – “one of the great conveners”, as Jussi Pylkkanen, Christie’s worldwide president calls him – is following in the footsteps of Armand Hammer, another great American trader. The founder of Occidental Petroleum, he often lent his art collection to countries where he did business, especially Russia.

It is no accident that the show’s next stop is China, for the world’s new economic titan is becoming the focus of cultural diplomacy. Christie’s French owner, François Pinault, has made important gifts to China, and the company remains the only foreign auction house licensed to operate on the mainland. It was Christie’s that introduced Kaplan to Wang Wei, the founder of the Long Museum in Shanghai.

The Kaplans are lending the show to China free of charge and, unusually, are covering all the transport and insurance costs. This could prove a sound investment: if they ever wish to sell their paintings, three of the Rembrandts that have come up for auction in recent years have been acquired by a collector in Hong Kong, and the Kaplans’ generosity will not go unnoticed by the government. They no doubt hope that their relationship with China will prove as fruitful as that with the UAE , with which they have long been involved. Three years ago the Crown Prince of Abu Dhabi gave their big-cat conservation charity, Panthera, $20m.

Kaplan understands the commercial value of connections, but his diplomacy is not just transactional. His speech at the opening night of the Louvre exhibition ranged across the world’s political troubles. “Rather than silently acquiescing to the building of walls or the burning of bridges,” he said, “we are using the most powerful tools we have, Rembrandt and our passion, to build the connections that bind people together rather than tear us apart.” The fijnschilders would probably have gone along with that.