Abstract: The invention relates to an indicator, which comprises a previously enamelled ceramic part, on which reactive inks that react with the temperature are deposited, displaying a series of variations on the surface thereof. The indicators have two types of reactive inks, which act as follows: • by surface change: the indicators have a predetermined printed surface which is known prior to firing and the size of said surface varies after firing in accordance with the temperature 1 time to which the indicator was exposed; Or • by colour change: the indicators have a predetermined surface printed with an ink which develops a specific colour in accordance with the temperature to which the indicator was exposed. The reactive inks used are advantageously lead-compound inks coloured with a cobalt ceramic pigment, or compounds of copper, nickel or iron, among others.