have been roughened using commercially available spray adhesives e g miranit manufactured by ed geistlich und sohne ag ch 8952 schlieren it is additionally recommended to spray the glued layer with a thin coating of clear lacquer for example zapon lacquer in order to increase its abrasion resistance in all cases it has been demonstrated by practical testing that safe ignition of the pyrotechnic layer is also ensured during operation i e during the read out of data and that even then no reconstructible data structures remain after ignition a preferred embodiment of the invention utilizing an inert metal substrate is shown in further detail in fig2 d and 2 e 0061 fig2 d shows a section through a nickel foam after having been rolled to a desired layer thickness of 0 5 mm its sponge like structure is designated by 44 with side edge surfaces 45 the metal foam is commercially available as a flat material in the form of strips with a thickness of 1 5 mm international nickel gmbh d 40211 dusseldorf it is brought to the desired thickness in the present case 0 5 mm by calendaring rolling and then being cut to the desired external form the pyrotechnic mass is produced in the same way as for the glass fibre non woven fabric although in this case the higher thermal capacity of.