A method of authenticating that a test polymer is an authenticatable polymer, wherein the authenticatable polymer comprises a substrate polymer and a thermochromic compound, the thermochromic compound having a first signal at a first temperature and at an authenticating wavelength, and a second signal at an authenticating temperature and the authenticating wavelength, the first and second signals being different, and the authenticating temperature being greater than the first temperature, the method comprising
- subjecting the test polymer to a stimulus sufficient to raise a portion of the test polymer to the authenticating temperature to create a heated portion,
- determining a test signal of the heated portion of the test polymer at the authenticating wavelength, and
- authenticating that the test polymer is an authenticatable polymer if the test signal is substantially the same as an authenticating signal of the authenticatable polymer.