Ice shelves slow the flow of Antarctic glaciers into the sea and help limit their contribution to sea level rise. Their stability depends on how different types of ice respond to stress. We collected a 37 m ice core from the Brunt Ice Shelf and carried out fracture experiments to measure ice strength. We found that refrozen surface melt strengthens the ice, while salty seawater weakens it, affecting how cracks grow and how ice-shelf break-up should be modelled.