A Byzantine bowl once owned by a British Duke ambassador to Russia who believed it had carried the holy water of the founder of the Holy Roman Empire has become the most valuable single object to enter the National Museums Scotland collections.

The huge 16th century sardonyx bowl, bought by the 10th Duke of Hamilton in 1807 and mounted on a gold stand, was turned into its current tazza five years later, when the diplomat united it with an enamelled gold foot derived from a vast gold monstrance bequeathed by King Philip II of Spain.

Sir Angus Grossart, the Chair of National Museums Scotland, said the deal with the estate of Edmund de Rothschild strengthened the group’s “strategic vision”.

“The Hamilton-Rothschild tazza is the single most important acquisition that National Museums Scotland has made in many decades,” he declared.

“Acquiring this wonderful work of art demonstrates our enhanced international ambitions for our collections and underlines our aspirations.”

The Duke was compelled by the legend that the bowl had been the Bénetier de Charlemagne –  the holy water stoup of the Emperor Charlemagne, the founder of the Roman Empire.

He used it for the baptism of his children in 1811 and 1814, symbolising the premier peer status he believed the House of Hamilton held in Scotland, where he saw the family as the true successors to the Stuart kings.

Sensibly, the tazza was the most highly-insured item inside Hamilton Palace during the first half of the 19th century, although it was loaned to a prestigious exhibition at the former site of the Victoria and Albert Museum in 1862.

“It greatly enhances our already significant international collections of European art and design,” said National Museums Scotland Director Dr Gordon Rintoul, admiring the “remarkable” two-piece.

“We have recently welcomed more than two million visitors through the doors of the transformed National Museum of Scotland, and I know that this wonderful object will be a huge draw for thousands more. It will be a key focal point for further new galleries which we are planning.”

The tazza has gone on immediate temporary display at the National Museum of Scotland.