For tiny cracks, pores or other surface glitches that are hard to detect by the human eye, dye penetrant testing is used for both ferrous and nonferrous materials. In this method, a colored dye solution is applied to the surface of the casting. The dye, which is suspended in penetrating oil, will find its way into the surface defects. When a special developer is applied, the defects are clearly indicated.
A similar method involves fluorescent powder suspended in penetrating oil. Again, the solution penetrates the defects, so when the casting is dusted or sprayed drying powder, the solution is drawn from the defect and glows under an ultraviolet light where defects have occurred. Fluorescent powder testing only detects surface cracks and flaws but is more effective and economical than radiographic testing.
In general, dye-penetrant techniques identify defects on the surface of the casting and do not detect internal porosity or shrinkage that is not open to the surface. But it can detect rounded indications for porosity or gas on the casting surface.