be highly visible. In the oil-and-whiting days, it was found that used or dirty oil was much more visible than
clean machine oil. Present penetrants obtain visibility by having highly colored dyes dissolved in the
penetrating vehicle or oil. The type of dye materials provides one means of classifying penetrants.
DOD prohibits the use of visible-dye penetrant on aircraft, engines and
missiles, except for those parts with specific engineering approval.
Visible-dye or color-contrast penetrants contain a red dye dissolved in the penetrating oil. The visibility is
further enhanced during the penetrant process by the application of a layer of white developer. The white
developer provides a high contrast background for the bright red penetrant when viewed under natural or
DOD prohibits the use of water washable dye penetrant on aircraft, engines,
and missiles, except for those parts with specific engineering approval.
Some chemical compounds have the capability of emitting visible light when exposed to near-ultraviolet light
(UV-A, energy with a wavelength of 320 to 400 nanometers), commonly called black light. This property is
termed f luorescence, and the materials are called f luorescent (see paragraph 18.104.22.168.6). Very small
quantities of f luorescent penetrant will emit highly visible indications when exposed to black light.
Dual Mode (Both Visible and Fluorescent) Penetrant.
DOD prohibits the use of visible / f luorescent (dual mode) penetrant on
aircraft, engines, and missiles, except for those parts with specific engineer-
Dual mode penetrants contain dye materials that are reddish in color under white light and f luorescent
under black light. However, the intensities of the visible red color and the f luorescent color (usually orange)
are less than the individual visible dye and f luorescent penetrants. The brilliance of the visible color and
brightness of f luorescence are less than that obtained with the single mode visible-dye and f luorescent-dye
Methods of Penetrant Removal.
Penetrant materials are manufactured or formulated for specific removal methods. The removal method
provides another means of classifying penetrant materials. Each removal method has advantages and
disadvantages, which are covered in later paragraphs.
The usual liquid base or vehicle for a penetrant is petroleum oil, which is insoluble or immiscible in water.
This means that the penetrant cannot be removed with water. However, there are chemical compounds
called emulsifiers that can mix with oil vehicles to form a mixture that can be removed with water. When
emulsified, the oil-based penetrant then can be removed with water. The chemical compound forming the
emulsifiable mixture is called an emulsifying agent