Tourmaline is used to colour brass and copper brown. The process is odourless, can be carried out at room temperature and does not leave black fingers. The browning process can be stopped at any time. You can use Tourmaline undiluted or diluted 1:3 to 1:5 with tap water to prepare a burnishing bath. To be sure that the surface is free of acids, the draining water should be tested for neutrality with an indicator paper.