Final answer:
In brick manufacturing, the technique that involves firing bricks with variable air supply to change their face color is known as flashing.
Step-by-step explanation:
In brick manufacturing, the technique of firing brick units alternately with too much or too little air to vary their face color is known as flashing.
This method utilizes the variable conditions within the kiln to produce different surface effects and hues on the bricks, giving them a distinct, varied appearance. Unlike glazing, which involves a coating on the surface of pottery or ceramics, flashing occurs during the firing process itself and does not involve applying an external material. It should not be confused with sintering, which refers to the process of fusing particles together to form a solid mass, or with quenching, which is a rapid cooling process used in metalworking.