Liquid penetrants are among the oldest nondestructive testing methods and are historically contemporary with the determining of cracks by the sonic ringing of car wheels; perhaps the earliest penetrant inspection was the observation by blacksmiths of the stain caused by quenching liquids seeping out of quench cracks. Another early technique was the oil and whiting method which involved coating the test object with a light oil, removing the surplus oil, and applying a thin coat of chalk or whiting; the object was then hit with a hammer, thus forcing the liquid out of the cracks and staining the dull white-chalk coating. In later years, with the advent of “black light,” it was observed that oil fluoresced a pale blue color, and the use of “black light” illumination accentuated the presence of small indications caused by the oil retained in the surface discontinuities. The light blue fluorescent color, natural to the oil, was inadequate for good inspection; consequently, a yellow-green fluorescent dye was added to the penetrating oil and the resulting brilliant indication exhibited an extremely high contrast against the dark purple background. The use of oil, or an oil treated with the fluorescent dye, made an excellent inspection system but it was limited by the necessity of using “black light.” The welding industry, in general, was hampered by this limitation on the use of liquid penetrants for field inspection, and the next development was the substitution of a brilliant red contrast dye for the fluorescent dye.