What primary characteristic must a penetrant possess for effective testing?

For effective penetrant testing, the primary characteristic required is excellent wetting ability. This is crucial because the penetrant must adequately spread over and fill the surface discontinuities of the material being tested. Good wetting ensures that the penetrant can efficiently seep into tiny cracks or voids, allowing for a more accurate assessment of the component's integrity. Without exceptional wetting ability, the penetrant might not properly adhere to the surface or reach deeper flaws, compromising the accuracy of the detection process.

The other characteristics, while they may have their own importance in specific contexts, do not directly impact the primary function of the penetrant in terms of its ability to reveal defects. High viscosity could hinder the penetrant's ability to flow into fine cracks. Low toxicity is valuable for safety but doesn't influence the penetrant's effectiveness in defect detection. Bright color can aid in visibility during inspection but is secondary to the penetrant's functional ability to accurately highlight flaws.