Seven kinds of convexity

An Oil-Base Water-Absorbing Fluorescent Dyestuff Carrying Fluid used for testing metal which not only locates surface defects such as cracks but distinguishes between flaw depths and, consequently, flaw relevancy by means of combining different color water-soluble and oil-soluble fluorescent dyestuffs is disclosed in this invention. A process step in the use of this fluid is water washing of the surface to remove fluid which has not become entrapped in a surface discontinuity. During this step, fluid entrapped in shallow defects absorbs a relatively high percentage of water which activates the water-soluble dyestuff while fluid in deep defects remains substantially water free and the water-soluble dyestuff remains substantially inactive. Thus, shallow defects fluoresce the color of the water-soluble dyestuff and deep defects fluoresce the color of the oil-soluble dyestuff.