How can the choice of acid affect an etching?

In etching, the choice of acid—or mordant—is a fundamental creative decision that dictates the character and precision of the final print. Different acids interact with metal plates in ways that alter the quality of the etched lines.12

• Nitric Acid: When used on zinc, this acid creates air bubbles that must be brushed away to prevent uneven lines. It also tends to "underbite," meaning it eats into the metal sideways as well as downward. Consequently, it is often favored for achieving a bold, rough, or expressive quality, particularly when working with dense textures.1
• Dutch Mordant (Hydrochloric Acid): This is generally preferred for copper plates when the artist requires delicate, controlled results. Its action is significantly more even and predictable than that of nitric acid, making it ideal for fine detail and precision work.1

Ultimately, the selection of the mordant is an integral part of the artistic process, as the biting action is considered an extension of the drawing itself.2