Jan Six bought the portrait of a well-dressed man, painted around 1634, with the help of an investor for a bargain £137,000 at a London auction in 2016.
Although it had not been previously spotted as a piece by Rembrandt, Mr Six’s personal experience and expertise of his work meant he knew there was something more to the painting.
“It’s that sort of blink of an eye feeling that I felt this is better than what they think,” he said.
The painting, Portrait Of A Young Gentleman, is undated and unsigned.
But Mr Six has had the backing of more than a dozen Rembrandt experts, including the former leader of the Rembrandt Research Project, who spent a year verifying its authenticity.
Given the value of other Rembrandt pieces, this is likely worth more than Mr Six and his investor paid.
Mr Six said the financial benefit will be “fantastic”, but added: “I love pictures. I love discovering them.
“We call them sleepers, when they are unknown and I wake them up. And that for me, that’s my joy and that’s my passion.
“I am completely obsessed with Rembrandt. And to have the ability in your lifetime to find one out of nowhere, it’s a dream.”
Mr Six, 39, said it was a particular style of collar on the man depicted in the painting that caught his eye when he received the auction catalogue from Christie’s. The collar was only in fashion for a short time in 1633, and was painted in a style Rembrandt used.
He said he “flipped through the catalogue and my eye got stuck on this picture because of how it looks but also because they called it circle of Rembrandt van Rijn and if you know a little bit about the 17th century you can sort of see that this type of fashion relates to the early ’30s.
“So at that time Rembrandt just came from Leiden and he didn’t have a circle, so I thought what they written down doesn’t make sense and the picture looks too good.”
He said: “There’s one fantastic flap of lace and there’s one curved edge and it’s so fantastic – it’s such a depiction of space – that you really want to put your finger on it.
“And the ability to get there to that end result, in my view and in many other experts’ views, only Rembrandt ever reached that level.”
He spent 18 months using X-ray techniques and analysis of paint samples to prove he had bought a real Rembrandt.
The existence of this painting was unknown because there was no literary reference to it, making it different to the discovery of previous “lost” pieces which were known about.
Mr Six believes Rembrandt was around 28-years-old when he painted the piece, and suspects it was cut out of a larger work, including the young man’s wife.
The work is now going to be displayed in Hermitage museum in Amsterdam for a month before Mr Six finds a buyer.
He added: “The real adventure started when the painting arrived in Amsterdam. Because it was not sold for a huge price.
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“It’s not signed, it’s not dated and it’s not described in the literature. You have to prove it’s a Rembrandt. And that was very exciting. It’s the most adventurous starting point.
“And gradually, through all the opinions and technical research, the conclusion was reached that there is no other possibility then that it’s a Rembrandt.”