Inspectors have to decide the type of penetrant to use depending on the part they will inspect.The industry and military specifications that control penetrant materials and their use, all stipulate certain physical properties of the penetrant materials that must be met. Some of these requirements address the safe use of the materials, such as toxicity, flash point, and corrosiveness, and other requirements address storage and contamination issues. Still others delineate properties that are thought to be primarily responsible for the performance or sensitivity of the penetrants. The properties of penetrant materials that are controlled by AMS 2644 and MIL-I-25135E include flash point, surface wetting capability, viscosity, color, brightness, ultraviolet stability, thermal stability, water tolerance, and removability.

More information on how some of these properties can affect penetrant testing can be found by following these links.

Surface Energy
Density or Specific Gravity
Color and Fluorescence Brightness
Dimensional Threshold of Fluorescence
Ultraviolet and Thermal Stability