The choice of chemical used for etching depends on the specific metal being etched and the desired outcome. Here are some common chemicals used in metal etching:

1. Nitric acid: Nitric acid is commonly used to etch copper, brass, and bronze. It can also be used to etch stainless steel and other metals, but with varying results depending on the specific alloy.

2. Hydrochloric acid: Hydrochloric acid is commonly used to etch aluminum and zinc. It can also be used to etch brass and copper, but with less precision than nitric acid.

3. Ferric chloride: Ferric chloride is commonly used to etch copper, brass, and bronze. It can also be used to etch aluminum, zinc, and stainless steel.

4. Ammonium persulfate: Ammonium persulfate is commonly used to etch copper, as well as printed circuit boards.

5. Sodium hydroxide: Sodium hydroxide, also known as caustic soda, can be used to etch aluminum and some other metals.

These chemicals are typically used in a diluted form, as concentrated solutions can be dangerous to handle and can cause severe burns or respiratory issues. Proper safety precautions should always be taken when handling etching chemicals, such as wearing protective gloves, goggles, and clothing. It is also important to properly dispose of etching solutions, as they can be hazardous to the environment if not handled and disposed of properly.