 These failures are usually the result of poor commissioning practice although a significant number are initiated by flux left in the bore after assembly of soldered joints.Prior to about 1970 the most common cause of Type 1 pitting was carbon films left in the bore by the manufacturing process.Research and manufacturing improvements in the 1960s virtually eliminated carbon as a cause of pitting with the introduction of a clause in the 1971 edition of BS 2871 requiring tube bores to be free of deleterious films.