What do you mean by metal-free?

Metal-free material is a kind of non-metallic material, which generally refers to materials that do not contain metal substances and are mostly used in buildings. Non-metallic materials are a kind of building materials with inorganic compounds as the main body, often referring to cement, lime, building glass, building ceramics, bricks, tiles, concrete and their products.

Other explanations

A building material with inorganic compounds as the main body, usually refers to cement, lime, building glass, building ceramics, bricks, tiles, concrete and its products. They have no metallic luster and are poor conductors of heat and electricity. Silica and silicate formed by metal oxides are the main components of crustal rocks and soil, and are important components of inorganic nonmetallic materials.

For example, 999 gold means no metal, which means that gold can be divided into two categories according to its nature: raw gold and mature gold. Raw gold, also known as raw gold, natural gold or barren gold, is unrefined gold mined from mines or river beds. All refined gold is called "cooked gold".

When other elements are added to cooked gold, the color of gold will change. Usually, the cooked gold containing metallic silver but not other metals is called "clear gold", that is, the gold without metal, while the gold mixed with silver and other metals is called "mixed gold". K gold is the representative of the color of mixed color gold.