The surface of poly(tetrafluoroethylene-co-hexafluoropropene) foil was carbonized by a reaction with liquid alkali metal (Li, Na, K) amalgam at 25 °C. The carbonized layer is composed from n-doped elemental carbon and a stoichiometric amount of alkali metal fluoride. UV-Vis absorption spectra of the carbonized layers show features typical for carbons with supposed carbynoid structure. The plot of absorbance versus the preparation time is interpreted in terms of a two-step reaction mechanism, i.e. a fast chemical initialization followed by a slow electrochemical propagation. Long-term changes in the UV-Vis spectra are elucidated by a preconception of slow carbyne-to-graphene crosslinking taking place in the solid product. A phenomenological mathematical model of this process, similar to that describing long-term changes in electronic conductivity, is proposed.