How are penetrants classified?

• A. By their color.
• B. By their method of removal.
• C. By their penetrant viscosity.
• D. By their application method.

Answer: (B)

Penetrants are categorized by how the residue is removed after development. The standard grouping is solvent-removable, water-washable, and post-emulsifiable, each requiring a different removal step and each with its own handling and surface compatibility considerations. While appearance (color vs. fluorescence) and viscosity affect visibility and penetration, they don't define how the penetrant is classified. Therefore, the removal method is the basis for classification, making the correct answer the method of removal.