It depicts an old woman, who seems to be in the act of emptying a chamber pot into the street, not an uncommon practise in days gone by. However, the vessel she is holding is not a chamber pot, but a mortar, as in mortar and pestle.
The relief marks a critical moment in Venetian history when the action of a little old lady, by the name of Giustina Rossi, saved the Most Serene Republic from a coup d'etat.
It was June 15th, the Feast Day of San Vio (Saint Vitus), in the year 1310, and Bajamonte Tiepolo, a disgruntled nobleman bent on seizing power, was marching with his troops along the Merceria in the direction of the Doge's Palace. Just before the insurrectionists burst into the piazza they paused. This would prove to be a fatal mistake.
Signora Rossi, curious to see what all the fuss was about, opened her window and looked out. She had been busy grinding herbs and had taken her mortar with her to the window. As she leaned out she dropped the mortar (no one knows whether it was by accident or design) onto the head of Tiepolo's standard-bearer, killing him outright. Tiepolo immediately took fright, retreated and the rebellion was over. And all thanks to a little old lady.
The Republic asked how they could reward her and she asked for two things. The freezing of her rent and the right to hang the banner of St Mark on the Feast Day of St Vitus. She was granted both requests.
The building in which Giustina Rossi lived was knocked down in the 19th century and the relief was erected in her memory.