Acceptable materials shall be marked by means of a low stress (dot, vibratory or 'U') hard stamp or indelible, non-deleterious ink or paint (i.e. free of low melting point metals such as zinc, lead, copper etc.), at or adjacent to each PMI sample point on each component or weld. For 'sensitive' materials such as gaskets, RTJ rings, thin-walled pipe etc., identification shall be by indelible, non-deleterious ink or paint.