This involves applying a liquid dye to the surface of a material and leaving the dye to “dwell” on the surface for a pre-determined period of time. The liquid can be either a color that is easily visible under normal lighting conditions or a yellow/green fluorescent color that requires special lighting conditions to be seen effectively.
This liquid dye enters into discontinuities that are open to the surface of the material through a phenomenon called “capillary action”. This capillary action takes place throughout the dwell time and the discontinuity retains this dye when the excess dye is cleaned from the surface. A type of developer is then applied to the surface of the material and the dye that is trapped inside the surface discontinuities is blotted back out on to the surface and forms an indication. This indication is then interpreted by a qualified interpreter.