Black image from a thermographic imaging system

An article with a single imageable layer carried on a substrate, said layer comprising a polymeric binder, a combination of at least two leuco dyes, an acidic material, and nitrate salt, said leuco dyes being colorless compounds which when subjected to an oxidation reaction form colored products, said acidic material being present in said layer in a molar concentration of at least 0.2 times that of the nitrate ion in said nitrate salt, said nitrate salt having a cation which is nonreactive with said leuco dyes and said nitrate salt capable of liberating an oxidizing amount of HNO.sub.3 or oxides of nitrogen when heated to a temperature of no more than 200.degree. C. for 60 seconds, said layer being capable of providing an at least dark, stable image upon imagewise oxidation.