What chemical treatment can darken copper and brass?

The chemical treatment that can effectively darken copper and brass is potassium sulfide. This compound reacts with the surface of the metals, leading to a darkening effect that enhances the aesthetic qualities of these materials. When potassium sulfide interacts with copper or brass, it forms copper sulfide, which is responsible for the darkening. This technique is often employed in metal finishing and patination processes to give a desired aged look to the metal.

Other substances mentioned, such as ammonia, vinegar, and copper sulfate, do not achieve the same darkening effect on copper and brass. Ammonia can clean metal but does not darken it significantly. Vinegar may cause some etching or surface alteration but typically results in a green patina rather than a darkening effect. Copper sulfate can be used in patina processes but is not known specifically for darkening metals like potassium sulfide. Thus, potassium sulfide stands out as the most effective chemical treatment for achieving a darkened appearance on copper and brass.