Etching is a process for producing images on metal plates that can be copied using a printing press. First, the metal plate is coated in a layer of wax. Next, an artist creates the image by scratching lines in the wax to reveal the metal. Finally, the plate is dipped in a liquid called the mordant, which eats away at the metal where the wax has been scraped away. This leaves an engraving in the plate that can pick up ink for printing. To break down the metal plate, the mordant should be composed of a(n) substance.