the steel on the shelf was aged using muriatic acid (very carefully) and then waxed with annie sloan clear and dark waxes to give it a more authentic look (oliverandrust.com)
For example, a weak solution of muriatic acid is used for some surfaces, but it might damage others. (thebalance.com)
Use a solution of one part muriatic acid to 10 parts water (wear goggles when mixing). (homebuyinginstitute.com)