Knees apart and hand placed confidently on her knee, this imperious lady looks down – but not at us. Her twisted pose is tense and defensive: she looks as though she is clasping an imaginary shield while showing off the luxurious fabric of her sleeve, woven with gold. Six dolphin-like creatures adorned with spikes and rubies for eyes guard her throne, baring sharp teeth. A scallop shell hung with coral and crystal beads crowns the maritime throne.

She once looked down from the walls of the studiolo (study) at Belfiore, the hunting retreat belonging to the dukes of Ferrara, part of a decorative scheme showing the nine Muses. The theme was chosen by Leonello d’Este, Duke of Ferrara, and was begun by a painter called Angelo da Siena. After Leonello’s death in 1450 and Angelo’s in 1456, Leonello’s brother Borso hired Tura to produce the work.

The Muses were mythological figures, the daughters of Jupiter and Mnemosyne (Memory). They embodied divine inspiration for the arts, each representing a specific art form. Leonello’s choice showed off his fashionable education in the culture of ancient Greece and Rome, which he received from the renowned humanist scholar Guarino da Verona. The subject provided artists with the opportunity to emulate the ancient poets who were revered as intellectual role models by Renaissance patrons and painters. Indeed, Guarino was enthusiastic about the idea, writing to Leonello in 1447 in praise of the concept and offering his learned opinion on how to bring the Muses to life in painting.

The surviving Muses from the series, which are now housed in collections across Europe, largely ignore his advice, making it difficult to identify them with any certainty. Our figure has been identified as Calliope, the muse of poetry. The branch of cherries may refer to justice, a principle with which Calliope was sometimes associated. A manuscript illumination in a Bible owned by Borso shows Justice as a woman seated on a throne framed by similarly ferocious-looking dolphin-like creatures, and features a cherub eating cherries. The dolphins, which were also a decorative feature of the Palazzo Schifanoia (the ducal palace in Ferrara) were clearly significant, though we do not know what they represent.

Infrared reflectography reveals that Tura painted this image over the top of another, partially completed design, possibly by Angelo da Siena. The original design also showed a woman seated on a throne but her pose was slightly different and – most significantly – the throne itself was far simpler: its only decoration was a row of plain columns, resembling the pipes of an organ, often leading to the idea that she originally represented Euterpe, the muse of music.

Analysis of the paint of the under layers shows that the original image was painted with egg tempera. The final image was painted using oil paint, which was not widely used in Italy in the 1450s – this picture is the earliest Italian oil painting in the National Gallery’s collection. Tura used two types of oil, preferring walnut, which had a less yellow tone for the lighter colours, ensuring they would retain their brightness. He used translucent glazes over more opaque paint layers to create a depth of colour impossible to achieve in tempera. The technique is flawless and suggests knowledge of Netherlandish art, particularly the work of Rogier van der Weyden, who perfected the technique. We know that the d’Este family owned a triptych by Rogier showing the deposition of Christ. There is no evidence that Rogier himself visited Italy but it is possible that other Netherlandish artists, perhaps trained by him, were at the Ferrarese court.