What are the three stages of flux when heated?

The three stages of flux when heated are indeed correctly characterized as powdery, milky, and watery. This distinction outlines the changes in the physical state and appearance of flux as it undergoes thermal activation.

Initially, flux starts in a powdery form. When it's subjected to heat, it begins to melt and transition into a milky appearance, which indicates that it is dissolving and becoming active. During this second stage, the flux is in a semi-solid state that allows it to flow and properly cover the metal surfaces to promote adhesion and protection against oxidation.

As the temperature increases further, the flux eventually becomes watery. This fully liquid form is critical for ensuring that the flux can effectively act as a medium for the brazing filler metal to flow and bond with the base metals being joined. The watery stage allows for better coverage and penetration into the joint, leading to a stronger and more reliable brazed connection.

The other options present varying understandings of the flux stages or refer to terms not typically associated with the flux process during heating. The descriptions in the correct choice encapsulate the transformations that are vital in the brazing process, emphasizing the importance of these physical changes to achieve successful metallurgical bonding.