Oil paint does not rot or expire. Certain pigments fade or darken when exposed to light. These do not change within the tube. If air gets to the paint it will oxidize, drying out. In some cases the dryer added in some paint brands to speed up the drying time may trigger a reaction causing the paint to harden within the tube over time. Dryers generally rely on a reaction to oxygen which points to a fault of the tube itself. Some paints contain lead which can harden in the tube over time.
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