What is to be done in the event of metallic abrasion?

Black lines on ceramic can be caused, e.g. by metallic abrasion from jewellery. These marks can be removed by using Etolit or Stahlfix. To do so, apply the cleaning substance on a dry cloth und rub the dry, dirty area. These cleaning agents, however, can damage the ceramic surface if used too often. Therefore, only treat the dirty area and not the entire basin.