Methods for producing coatings on substrates are provided. These methods comprise the steps of introducing the substrate in a photo-initiated chemical vapor deposition reactor, introducing a gas precursor in the reactor, irradiating said gas precursor with UV radiation at a given wavelength, thereby at least partly photodissociating the gas precursor, until the coating is formed. In one method, the gas precursor is a mixture comprising carbon monoxide and hydrogen. In another method, the pressure in the react or is between about 0.75 and 1.25 atm and the gas precursor has an absorption cross section of about 5×10−16 cm2/molecule or less at said given wavelength. In another aspect, the substrate is ash.