Chemical Analysis in Cultural Heritage

Luigia Sabbatini, Inez Dorothé van der Werf, Luigia Sabbatini, Inez Dorothé van der Werf

• Éditeur: De Gruyter
• Année: 2020
• ISBN:
  9783110456486
• Édition: 1
• Sujet: Tecnologia e ingegneria
• Sous-sujet: Scienza dei materiali

Part I: Analytical Techniques

1 Overview of materials in Cultural Heritage

Lorena Carla Giannossa

Inez Dorothé van der Werf

Pottery

Raw materials and production

• clay
• water
• firing process

Clay is an earthy, fine-grained material that combines one or more clay minerals with possible traces of quartz (SiO₂), metal oxides (Al₂O₃, MgO, etc.) and organic matter. Geologic clay deposits are mainly composed of phyllosilicate minerals and variable amounts of water trapped in the mineral structure. Surely, it becomes plastic and easily moldable when mixed with limited amount of water and rigid when dried and fired.

Clay is a rock originated from the alteration of minerals or other rocks. It can be divided in primary and secondary. The formers originate by minerals changing in situ, producing a rock with a chemical composition essentially unaltered with respect to the bedrock. The secondary ones are sedimentary rocks whose deposits were formed in places different from that of alteration, due to natural transport. Skeleton and matrix well mixed compose the clay. The skeleton is made of fragments of crystals of various kinds whose dimensions can reach up to 4 mm; whereas the matrix is made of clay minerals containing crystals smaller than 4 mm. Raw materials to produce ceramic artifacts are the plastic component consisting of clay materials, the inert or temper component consisting principally of silica and the glazing and fluxing component. The latter often concurs because fluxes favor the formation of a vitreous phase. Basically, they are mixtures of carbonates and feldspars. Inert, temper and flux materials are often added to clay to obtain a ceramic object with the desired characteristics.

The list of necessary steps in ceramic manufacturing includes discovering sources of raw materials, selecting those to be used, extracting and transport raw materials to the site of manufacturing (e. g. a workshop), preparing and shaping the ceramic paste, drying and firing the vase. Non-essential actions consist, for example, of adding decorations (engravings or stamps) or applying some kind of slip (glaze or enamel). Following possible crushing, grinding and purification procedures of raw materials, the next step is the mixing of raw materials with an appropriate amount of water to obtain the ceramic paste: an adequate workability is a mandatory pre-firing condition. Plasticity can be increased adding plasticizers or mix of clays owning different characteristics. On the contrary, it can be decreased by non-plastic additives (quartz, calcite, seeds, shell, crushed pottery, etc.).

Modelling

The actions aimed at modifying the shape of the unformed clay is defined forming or modelling. Of course, after the invention of the wheel, everything changed. Wheel function is turning the object incessantly, so working faces change, and pressure can be applied. Such a high technological advance increased hugely vessel production.

Drying and firing

Drying provides cohesion and mechanical resistance to the ceramic mixture, a slow evaporation of water, avoids deformations, breakages and possible explosions during firing step. During firing, process transformations, decomposition reactions, redox reactions and formation of new minerals occur. It involves the transformation of some of the constituent minerals into new ones (neoformation process), influencing the properties of the ceramic product. Clearly pottery characteristics will also depend on firing conditions and raw materials. For example, the reached temperature influences the permeability of the handcraft or depending on firing atmosphere, the object looks a certain color. The main methods of firing can be grouped into two categories: one in which the products are in close contact with the fuel (open fire) and the other in which the fuel and products are physically separated (kilns). The firing cycle is normally divided in three steps: (1) progressive heating of the dried material up to the selected temperature is reached, (2) a maximum temperature plateau for the time needed to carry out the appropriate ceramic characteristics and finally (3) the cooling phase.

Colors

The role of color is evidently aesthetic and can be intrinsic to the ceramic material or be produced by the addition of suitable dyes in form of oxides or specific salts. The color control was one of the great suc...