a hard and durable surface with excellent post exposure wettability the density of the nitrided surface is preferably in the range from 5 mg ft 2 to about 30 mg ft 2 and a desirable thickness is in the range from about 10 u03bc to about 500 u03bc micrometers the treated arc grained aluminum sheet is coated with a radiation sensitive composition usually by the manufacturer of sensitized printing plates any suitable photosensitive layer may be used which after exposure followed by development and or fixing yields a surface with the desired image which is to be printed typically such coated layers contain silver halides but several others are used as described in 34 light sensitive systems 34 by j kosar john wiley amp sons new york 1965 for example colloid layers containing chromates and dichromates layers containing unsaturated compounds in which upon exposure the compounds are isomerized rearranged cyclized or crosslinked layers which can be photopolymerized in which upon being exposed monomers or prepolymers are polymerized optionally with the aid of an initiator layers containing o diazaquinones or condensation products of diazonium salts and still other layers which include electrophotographic layers that is those which contain an inorganic or organic photoconductor now referring to fig1 there is shown a photomicrograph of the typical surface of an arc cleaned sheet under conditions of electric power consumption within the aforestated range for arc graining the sheet but at the upper end thereof a visual comparison indicates.