The pigments are substances of both natural and artificial origin used to change the color of a material.
Their main characteristic is that they are chemically stable, so it means that they are inert towards atmospheric agents and towards water. They have a particular hiding power and very high opacity, especially with the presence of a specific substratum.
The pigments are divided in 2 categories: organic and inorganic.
Inorganic: they are colored substances in whose chemical composition carbon does not appear, if not in elemental form (carbon black) or carbonate. It is mainly about oxides, salts or silicates. Unlike these, the organic ones have a significant better coloring and covering power and extremely bright and clean hues. They have considerable stability to acids and alkali, sometimes even to heat and being chemically stable and lacking of heavy metals, they are not harmful to the human body, nor to ingestion or contact with the skin.
In inorganic pigments, the color arises from electronic transitions completely different from those responsible for the color of organic pigments. For example, they may involve charge-transfer transitions between the binder and the metal, as it happens in lead chromates, or between two metal atoms that are in a different state of oxidation. The inorganic pigments show a high intrinsic opacity that can be attributed to the high refractive index caused by the compact arrangement of atoms in the crystalline structure.
One type of inorganic pigment is Hematite. The name derives from the Greek "aima" that it means blood, and it is the most common iron ore, it has a dull red color. It is abundant in many rocks and in terrains to which it gives the red color. The high stability of the pigment means that it is the point of arrival of the alteration chain of the other iron oxides and also indicates that the pigment that is obtained is durable and reliable. Easily it was the first pigment used, in fact in the prehistoric caverns there are rock paintings made with red ocher.
The chemical preparation of these pigments is often carried out in an aqueous solution from which the pigment can be precipitated directly into its proper physical form. In other cases, high-temperature solid-state reactions are used, or gaseous-phase processes are made especially for prepared in huge quantities.