Hematite is used to blacken iron. The process is odourless, can be done at room temperature and does not leave black fingers. The blackening process can be stopped at any time. Hematite can be used undiluted or diluted 1:3 to 1:5 with tap water to make a burnishing bath. To be sure that the surface is free of acids, the water running off should be tested for neutrality with an indicator paper.