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Glass, glaze and enamel

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Glass is not a crystalline but an amorphous material. Normally, melts crystallise on cooling into one or more crystalline phases in which the atoms assume an ordered, regular arrangement. Glass melts, on the other hand, do not crystallise on cooling, but retain their disordered liquid structure. They are therefore more or less solid liquids. A structural distinction is made between network formers and network transformers for the individual components in glass.

The absolutely most important network former or glass former is silicon oxide (SiO2), which organises itself in crystals to form regular chain or ring structures. However, the formation of crystalline structures during cooling takes time. When the melt cools, crystallisation is suppressed simply because there is not enough time for the molecules to arrange themselves regularly. Nevertheless, the individual building blocks cross-link with each other, just not in an ordered crystalline structure, but in a three-dimensional, but statistically disordered amorphous network.

In principle, glass can also be produced from silicon oxide (quartz) without the addition of other components, but the required melting temperatures are very high (over 2000 °C). Such high temperatures were technically impossible to realise in earlier times. The addition of so-called fluxing agents considerably reduces the melting temperature. Important fluxing agents or network converters in historical glasses are the metal oxides of sodium (Na2O), potassium (K2O), calcium (CaO) and lead (PbO). The effect of lowering the melting point is based on the fact that these metal oxides reduce the degree of cross-linking of the network formers. This also reduces the viscosity of the melt. Sodium oxide (Na2O), potassium oxide (K2O) and lead oxide (PbO) lower the melting point very significantly, whereas calcium oxide (CaO) only lowers the melting temperatures moderately, but makes the glass hard and resistant.

The most important raw materials for the production of antique glass are quartz sand and ashes from salt-loving plants or wood as a flux. A number of other substances used to be used as colouring agents, such as cobalt oxide (strong blue colouring) or copper oxide (blue colouring). Due to the large number of different components, historical glasses often have a very complex composition.

Special glasses are glazes, usually as a coating on ceramics and enamel as a coloured coating on metal.

Many questions regarding the origin, production technique and authenticity of glass cannot be answered by a purely art-historical or archaeological-typological approach. Fundamental problems such as the selection of raw materials or the allocation of the glass to specific production sites require precise knowledge of the composition of the glass. The question of the chronological classification or originality of glass often arises. Here, too, a chemical analysis can be helpful by comparing the often complex chemistry with glass typical of the period and at least allow a rough delimitation of the production period. With the technological development of glass production, various colouring elements, clouding agents, refining agents, decolouring agents etc. also change, which provide further approaches for the chronological classification or spatial allocation.

Sample composition

Depending on the issue in question, glasses, glazes and enamels are often analysed non-destructively using suitable methods (e.g. X-ray fluorescence analysis). No sampling is required for these analyses. Very small fragments are sufficient for analysing trace elements (e.g. LA-ICP-MS) or isotope measurements.