When testing for shallow discontinuities using a post-emulsification penetrant, emulsification times should be long enough to:

When using a post-emulsification penetrant for testing shallow discontinuities, emulsification times should be sufficient to effectively mix the emulsifier with the excess penetrant. This is important because the emulsifier serves to develop a visible contrast between the penetrant that has seeped into any defects and the surrounding material. A proper emulsification allows the excess penetrant on the surface to be dispersed and helps to formulate a cleaner, clearer substrate for defect detection.

Long emulsification times ensure that the emulsifier can adequately interact with the penetrant, leading to better washing and a clearer indication of any discontinuities present in the material. This step is critical, as incomplete emulsification can lead to false positives or negatives in testing results due to residual penetrant interfering with the interpretation of the test.

The other options, while related to the process, do not specifically address the critical role of emulsification in mixing the emulsifier with the excess penetrant. They may suggest actions that occur during the testing process, but do not focus on the necessity of proper emulsification times, which is crucial for accurate assessment of shallow discontinuities.