Carlo Pellicani, after Carlo Buzzi, The Sacrifice of Cain
West façade, Milan Cathedral
The angel uses a stick to extinguish the fire on Cain's altar. In most other images of this episode there is no fire at all. The reason for God's rejection of Cain's offering, according to Procopius of Gaza, was that he selfishly kept back a goodly portion of what he had. That explanation is expressed here in the limp vegetables drooping over the edge of the altar and the jar that Cain holds tightly in his right arm.1
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Photographed at the site by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.