aluminium of the present invention mainly consists of the following first an anodic layer is produced through a conventional anodizing process in sulphuric acid having a concentration ranging from 150 to 200 g l at a current density of from 1 to 2 a dm 2 corresponding to a potential of from 12 to 20 volts of direct current for treatment times of from 15 minutes to 1 hour once the anodic layer whose thickness can range from 3 to 30 microns has been produced by this process the aluminium piece is introduced in the colouring bath the colouring bath consists of a solution of metal salts having a ph above 0 8 among the metal salts to be used in this process are metal sulphates acetates and chlorides such as copper tin cobalt and nickel once the aluminium piece has been submerged in the colouring bath it is subjected to two electrolytic processes both with alternating current in the first electrolytic process the aluminium sample is subjected to an alternating current in which the effective voltage applied is from 7 to 35 volts and the peak voltage of the negative semi wave is always less than that of the positive semi wave and is from 2 to 15 peak volts the term alternating current will be applied here to any type of variable current between the positive and negative polarity having a positive and a negative cycle alternately and can.