Mud: Its melting, opacifying properties and coloration depend on the presence of certain metal oxides
Mud was the first raw material used by Homo sapiens. Used to build houses, utensils, containers and the first artistic forms, either for anthropomorphic representations or to model zoomorphic figures.
The need to transport food or store it means that durable and more practical containers are built instead of the baskets or baskets used by hunter-gatherers.
The birth of ceramics dates back to the Neolithic period, around 10,000 BC.C., which was when the first containers for storing food and water appeared.
The next step was to decorate his creations with different chromatic nuances.
The firing of the clay allows to achieve more durable, resistant and waterproof pieces, whose shape can be modified at will. This process not only allows to manufacture vessels, but also to build other objects, such as stamps for stamping that could serve as an amulet or to mark properties, ornaments, etc.
Being a soft material, it is also easy to decorate, either by printing, incision, painting, etc.
The appearance of ceramics marks a decisive stage in human evolution and a turning point in the process of neolithization.
In the pottery manufacture several elements intervene. Clay, the main one, can be mixed with other materials such as sand, waste, hair, etc., which determines a very different result.
Cooking, perhaps the most important part of the process, depends on the conditions in which it is carried out, especially the amount of oxygen, which will determine the final color of the piece, from red -with oxidation- to gray or black -reduced-.
The shape and size of the piece could be determined by hand, molding it and adding pieces as handles, balls, rings, sheets, etc.
Clay could also be pressed into molds or molded into a slow lathe (from 4500 BC.C.) or fast (from 2000 BC.C.).
Finally, clay could be enamelled, painted, polished, inlaid or, from 1500 BC.C., glazed. The different resulting ceramic types determine the existence of different archaeological traditions, being very useful for the study of the cultures of the past.
Etymologically the word ceramic comes from the Greek 'keramiké', feminine form of 'keramikós', which means 'burned substance'. With this term was designated the neighborhood of the potters of ancient Athens, the artisans in charge of making objects from baked clay.
Metal oxides and decoration
The ceramics industry, from a chemical point of view, is based on binary combinations between oxygen and metallic elements, such as copper, manganese or lead. The properties of materials are dictated by the types of atoms present and by the bond that is established between them. They generally have a combination of ionic bonds, between a metal and a nonmetal, and covalent bonds, between nonmetallic elements. They are responsible for the hardness, the high melting point, the low thermal expansion and the excellent chemical resistance.
Metal oxides involved in the composition of ceramics can be grouped into three categories: fluxes, opacifiers and dyes. The first are those that allow to lower the melting point of the mixture and get all the components to melt into the cooking. There are basically two types, those that act as fluxes at low temperature (lead, lithium, potassium and sodium) and those that act at high temperature (calcium, barium, magnesium and strontium).
The next group would be formed by metal oxides that serve to opacify, that is, to eliminate the natural transparency of the glazes and obtain opaque covers. In this group we find the oxides of zinc, titanium, tin and zirconium.
Last but not least, there would be the metal oxides that provide color and are those that are present in the oldest ceramics, since they were the first to be used to decorate the surface of the vessels. Among them are iron oxide, which generates reddish and brown tones; copper oxide, responsible for greens and turquoises; cobalt oxide, for bluish tones, or manganese oxide, when what you want to achieve are earth or violet colors.
The use of manganese, copper and tin oxide was the decorative basis of the famous caliphal ceramics, and it was with these chemical substances that their well-known green colors (manganese oxide) were achieved on a whitish base (tin dioxide). The manganese green of the caliphal ceramics, from the workshops of Medina Azahara, managed to impose itself on the tastes of the time.
In al-Andalus the technique of sgraffito was also developed, which consisted of the covering of ceramic objects, mostly small jars, with a layer of manganese oxide, which was 'scratched' with a burin to conform the different decorative motifs (vegetable, symbolic, zoomorphic, figurative, geometric or epigraphic forms).