The method for making car number plates comprises applying a swellable material (b) to a support (a). A protective layer (d) is applied over the combination, which is then embossed and heated. An Independent claim is included for a number plate produced as...

The official document, e.g. a car licence disk, has text printed on it in an ink which irreversibly becomes visible when it is heated above a predetermined temperature. Suitable dyes for use in the ink are described in US4522429, US4632429 and US4867481.