chemical characteristic the concentration of the tag in the ink is required to be sufficient to be detected and identified by methods such as mass spectroscopy or gas chromatography when the tag has a unique physical characteristic or characteristics sufficient microparticles must be present in the printed impression as to facilitate detection and identification of the particles and when a mixture of microparticles is used as a tag sufficient of each kind of microparticles must be present in the printed impression to facilitate detection and identification of each kind of particle as described hereinbefore the identification tag is unique to a specific franking machine but if desired the tag may be unique for a group of franking machines for example franking machines operated by a single user franking machines operated in a specific postal region franking machines manufactured or supplied by a specific manufacturer or supplier or any other desired group of franking machines the ink supply for franking machines needs to be replenished relatively frequently particularly when there is high volume use of the franking machine accordingly where it is desired to provide unique identification of each franking machine the logistics of providing a replenishment supply of ink which is unique for each machine may be unacceptably expensive in order to reduce this cost the franking machine may be provided with a normal non specific ink supply 21 and a supply 23 of an identification tag as shown in the drawing the identification tag is a fluid substance or is carried in a fluid substance if desired the fluid substance carrying the identification tag may be ink of.