Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell," excess surface penetrant is removed and a developer is applied. This acts as a "blotter." It draws the penetrant from the flaw to reveal its presence.
A very early surface inspection technique involved the rubbing of carbon black on glazed pottery, whereby the carbon black would settle in surface cracks rendering them visible. Later it became the practice in railway workshops to examine iron and steel components by the "oil and whiting" method. In this method, heavy oil commonly available in railway workshops was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged. After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated and stroked with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating.
This method was in use from the latter part of the 19th century through to approximately 1940, when the magnetic particle method was introduced and found to be more sensitive for the ferromagnetic iron and steels.