How do you date a painting?
Not directly. Red ochre has no carbon in it to measure. But water seeping down a cave wall leaves a thin film of calcite, the same mineral as limescale in a kettle, and over thousands of years the films build up into a crust on top of the paint.
Calcite takes in a little uranium from the water as it forms, and almost no thorium. Uranium decays into thorium at a known, steady rate, so measuring the two shows how long ago a layer formed.4 Scientists scrape a tiny sample from the layer closest to the paint.
That is why every date on this page says at least. The paint was there before the crust began. It could be older, but not younger.