slags , including the harmful - substance - containing slags , are not directly melted but are neutralized and processed into eco - resource materials , so that the slugs can be effectively utilized to purify and revive a large area of polluted water without any burden on the environment . predetermined proportions of a non - recyclable harmful - substance - containing unused slag selected from by - product slags produced during the refining of a metal such as iron , copper , or aluminum from an ore , a porcelain clay for ceramics , and a mineral clay for ceramics , such as a quartz powder or an alumina powder , are kneaded to form a clay material with a suitable size . the clay material is burnt in a burning oven for ceramics at a predetermined temperature for a predetermined period of time so that a liquid glaze can sufficiently penetrate into the inner region of the clay material , thereby forming a biscuit clay material . a liquid glaze produced by kneading a powder made from a crushed special natural stone with a glaze for ceramics in a predetermined ratio followed by aging is applied , for example , to the biscuit clay material . as a result , a harmful - substance - containing unused slag is neutralized and used as an eco - resource filter medium .