What primary characteristic must a penetrant possess for effective testing?

For effective penetrant testing, the primary characteristic that a penetrant must possess is excellent wetting ability. This property ensures that the penetrant can easily spread across and fill the surface cavities or defects in the material being tested. Excellent wetting ability allows the penetrant to make good contact with the surface irregularities and permeate into any cracks or openings, which is crucial for detecting defects.

While factors like viscosity, toxicity, and color can play roles in the overall usability and applicability of penetrants, they do not directly impact the main function of the testing process as effectively as wetting ability does. High viscosity could hinder the ability of the penetrant to flow into tight spaces, while low toxicity and bright color may be beneficial for safety and visibility, respectively, but they do not contribute to the fundamental purpose of ensuring that discontinuities are identified through successful liquid penetration.