Abstract: An improvement in the method of inspecting a test piece for surface flaws in which an oily based dyed penetrant is applied to the test piece and subsequently the test piece is treated with an emulsifying agent to remove excess penetrant from its surface. The improvement is concerned primarily with using an aqueous soap solution as the emulsifying agent, the soap being a water soluble compound of a fatty acid containing from 12 to 20 carbon atoms. The emulsified penetrant is removed by rinsing with an aqueous solution, the emulsified penetrant, residual soap and rinse water are collected and then the collected liquid is reacted with an aqueous solution of a water soluble calcium or magnesium salt to thereby form a readily filterable curd containing the oily penetrant and a substantially dye-free supernatant liquid layer which can be totally separated from each other.