Tar hardens when lighter compounds evaporate, the surface oxidises, and the coating loses mobility. The process can be slow and uneven because tar is a complex mixture.
Tar does not harden like a simple glue or cement. It is usually a mixture of oils, resins, smoky compounds, and heavier residues. Some parts leave the coating, some react with air, and some remain as a softer internal mass beneath the surface.
The first hardening is often skin formation. The surface becomes less wet, less glossy, and less likely to mark. Underneath, the tar may still be soft. A thick coat can therefore look dry while still moving if pressed, warmed by sun, or trapped between two surfaces.
Air exposure matters. Thin coats harden more predictably because more of the material is close to the surface. Heavy layers, deep cracks, and tar trapped under boards or fittings can stay soft for a long time. Heat can reverse some of the apparent hardening by making the tar more mobile again.
Hardening should be understood as gradual settling and weathering rather than a clean one-time cure. That is why old tar can still soften in heat, smell faintly, or mark nearby materials years after application.