Vittore Carpaccio
St. Thomas Aquinas with SS. Marc and Louis of Toulouse

Three saints engaged in conversation. On the far left stands Saint Mark – the patron saint of the city of Venice. In the centre, St Thomas Aquinas sits enthroned at a writing desk, with the son of the altarpiece's donor kneeling at his feet. And on the right is Saint Louis of Toulouse wearing the regalia of a bishop. 

You're probably wondering what these figures have in common. From a theological point of view, not very much. They lived at different times, so they could never have met. But they share the given names of an important Venetian family – the Dragans. Tommaso Dragan and his brother-in-law Marco were from a generation that had revolutionised glass-making. In the late 15th century, new manufacturing techniques and the importation of high-quality raw materials across long distances meant the local glassworks could now produce especially high-quality, pure white glass – a luxury product known as crystal glass, or simply crystal.

That's actually referenced on the page of the bible Louis is holding. It's from the Book of Revelation: 

"Then he showed me a river of the water of life, clear as crystal, coming from the throne of God and of the Lamb."

However, when Carpaccio delivered the altarpiece as per his client's order, there was probably a minor upset. Take a look at the clouds moving in front of St Thomas's throne-cum-desk and forming a second level to the composition: The angels holding a length of red cloth and the two red putti heads appear to be later additions to the altarpiece... And indeed: a recent examination using infrared photography revealed an underdrawing. That infrared photograph is on display as the second image at this stop. Above the heads of Saint Mark and Saint Louis, you can see an architectural feature with an arch and round panes of glass, what's known as bull's-eye panes. That was the kind of glazing used in windows at the time.

Carpaccio had obviously misunderstood. The up-and-coming Dragan family hadn't made its fortune by producing commonplace bull's-eye panes, but by making gilded and enamelled crystal glass vessels! The Dragans were presumably outraged, rejected the painting and forced Carpaccio to hurry up and rework it.

The piece of music selected to accompany this altarpiece is by Alexander Demophon Venetus, and it addresses the Virgin Mary directly: "Volgi gli occhi, o madre pia" – "Turn your eyes, O pious mother". If you'd like to listen to the whole piece, you can access it on the second player.