Methods for verifying the authenticity of products

A method of verifying the authenticity of a product, comprising the following steps:

• I) providing a product with a tagged substrate comprising a covert, spectroscopically detectable security feature,
• II) recording a spectrum of the substrate by a spectroscopic method, and
• III) detecting the presence of the security feature by comparing the recorded spectrum with a library of spectra of tagged substrates,
• wherein the tagged substrate comprising the covert, spectroscopically detectable security feature is obtainable by a method comprising the following steps:
• a) providing a substrate, wherein the substrate comprises at least one external surface comprising a salifiable alkaline or alkaline earth compound,
• b) providing a liquid treatment composition comprising at least one acid,
• c) applying the liquid treatment composition onto at least one region of the at least one external surface to form at least one surface-modified region on or within the at least one external surface, and
• d) applying an opaque top layer over the at least one surface-modified region obtained in step c).