The portrait of the Three Eldest Children of King Charles I had been hanging in the same spot under a staircase at Oxburgh Hall in Norfolk since at least the late 19th century.

The work was believed to be a copy of a well-known oil painting by Sir Anthony van Dyck, until experts at the National Trust discovered that it was actually an early printed mezzotint by 18th century printmaker Jacob Christoff Le Blon, lauded as “the inventor of three and four colour printing”.

It had been sent to the Trust’s Royal Oak Foundation Conservation Studio at Knole in Kent for examination and assessment for conservation treatment when experts there realised it was the work of Le Blon.

National Trust curator Jane Eade said: “Van Dyck’s touchingly affectionate portrait brilliantly synthesises the grandeur of the court with the innocence of childhood and was much copied.

“Only three Le Blon prints of it were known to survive so to have a discovered a fourth is really exciting, especially as it is the only version that remains hanging in its historic setting.” 

Le Blon was a painter and print maker, and was the first to create a three-colour printing process.

He used the mezzotint engraving process to create separate plates that were inked in blue, yellow and red.

“Until then, artists had inked colours one beside the other on a single printing plate,” the Trust noted.

The mezzotint needed conservation treatment, which was carried out by a specialist paper conservator.

Technical analysis was also undertaken to learn more about Le Blon’s techniques and the history of this particular object.

The Trust’s analysis has helped to identify the colours Le Blon is known to have used such as indigo and carmine.

All of the versions were hand-coloured after printing, with the Oxburgh version described as being “particularly fine”.

Shona Johnston, property curator at Oxburgh, told Printweek: “It’s really fascinating when you see it magnified. He’s such an interesting artist because he was first and foremost a painter.

“In revolutionising that print process he brought his understanding of paint to print. He created these amazing mezzotints by layering the colours, then he did really fine hand painting over the top. When you magnify the picture you not only see the print technique you also see this really, really fine hand colouring that takes the print to another level.

“It’s so connected to the way light and colour works, and this was in 1721!”

Le Blon spent years experimenting with ink recipes and methods of engraving to mimic the effects of an oil painting.

In 1718 he moved to London where, using the name James Christopher, he was granted a royal privilege by George I to practice his trichromatic printing. 

Royal patronage gave him access to Kensington Palace where he was able to copy paintings in its collection, including the Van Dyck.

Johnston said the print will be back on show at Oxburgh from tomorrow, on a special easel in one of the historic rooms “so people can get close and see it in all its glory”.

Also on display will be some “astonishing” 16th century textile fragments, which were found beneath the floorboards at the hall during recent building work.

She added: “We’re hoping that after this initial display where we celebrate its conservation and the discovery of Le Blon, we can actually revisit it and start to explore some of the amazing story of Le Blon and really celebrate him as a print maker.

“It’s a story that really lends itself to the visitor experience, talking to people about how you superimpose those three colours to create a multitude of colours, and that’s essentially what your [home or office] printer does too.”

The conservation of the Le Blon mezzotint and the textiles feature in the new series of Hidden Treasures of the National Trust which starts on Friday 10 May at 9pm on BBC Two.

Oxburgh Hall appears in episode six, and this will also be available on iPlayer from 10 May.