Which characteristic is NOT desirable in a penetrant?

High viscosity is not a desirable characteristic in a penetrant because it can impede the ability of the penetrant to flow into and fill small cracks and voids in the test material. Ideally, a penetrant should be low in viscosity to ensure that it can easily wet the surface and penetrate crevices effectively. This property allows the penetrant to be drawn into the defects by capillary action, increasing the likelihood of detecting discontinuities.

In contrast, high sensitivity, good contrast, and low surface tension are all desirable properties. High sensitivity ensures that even the smallest defects can be detected. Good contrast helps to easily distinguish between the penetrant and the background, making the indications of defects clearer to the inspector. Low surface tension allows the penetrant to spread easily and adhere to the surface, which is crucial for effective penetration into defects.