Modern Methods for Analysing Archaeological and Historical Glass

Koen H. A. Janssens, Koen H. A. Janssens

• Publisher: Wiley
• Year: 2013
• ISBN:
  9781118314203
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

The first scientific volume to compile the modern analytical techniques for glass analysis, Modern Methods for Analysing Archaeological and Historical Glass presents an up-to-date description of the physico-chemical methods suitable for determining the composition of glass and for speciation of specific components. This unique resource presents members of Association Internationale pour l'Histoire du Verre, as well as university scholars, with a number of case studies where the effective use of one or more of these methods for elucidating a particular culturo-historical or historo-technical aspect of glass manufacturing technology is documented.

1.1

What is Glass?

An Introduction to the Physics and Chemistry of Silicate Glasses

José-María Fernández-Navarro1 and María-Ángeles Villegas2

1Instituto de Óptica Daza de Valdés CFMAC, CSIC, Madrid, Spain

2Instituto de Historia, CCHS, CSIC, Madrid, Spain

1.1.1 Introduction

The purpose of this introductory chapter is to present the basic concepts of the glassy state as well as the main properties of glasses that are different from those of other materials. There are two questions of theoretical interest to which many authors have paid special attention. These questions are the following: on the one hand, the establishment of structural models describing the behaviour of different kind of glasses and, on the other hand, the knowledge of the conditions that a substance must meet in order to obtain a glassy state (i.e. geometrical-structural factors, thermodynamics, kinetics, chemical bonding, etc., on which the ability to form a stable glass depends).

In this chapter only the main properties of glasses will be considered: viscosity and thermal expansion coefficient, affecting glasses during their heating and cooling, and the most important properties for their application and use, such as mechanical behaviour, optical transmission, reflectance and chemical durability.

The concept and meaning of the word glass depends on the corresponding context. In colloquial language the word glass is used to name objects made in this material (goblets, ophthalmic lenses, table vessels, etc.). In the scientific and technical world, the word glass is used for a wide series of materials with very different chemical composition, having all the fundamental physical and chemical characteristics that define the glassy state.

A material can be obtained in a glassy state through condensation from a gaseous phase, via cooling or polymerisation from a liquid phase or by disordering a solid phase. All of these pathways yield a non-crystalline structure. The most commonly used manner of obtaining a glass is the cooling of a liquid phase.

Throughout history (and also nowadays) most glasses have been produced by the reaction of their components at high temperature, followed by melting and cooling of the resulting liquid phase under controlled conditions in order to avoid crystallisation.

1.1.2 Fundamentals of the Glassy State

1.1.2.1 Transition Temperature

When the cooling of a liquid is plotted versus the variation of a first-order parameter such as its specific volume, two different possibilities could occur (Figure 1.1.1). In the most common case, when the melting point is reached, both the liquid phase and the solid phase co-exist under equilibrium conditions and crystallisation suddenly takes place, accompanied by volume shrinkage. Below such temperature, cooling continues under equilibrium conditions and the volume of the solid phase diminishes according to its thermal expansion coefficient.

Figure 1.1.1 Variation of the specific volume of a glassy and a crystal phase of the same composition as a function of temperature.

The other possibility is as follows: the temperature of the liquid drops below its melting temperature (Tf) without any crystallisation. In this case a supercooled liquid is obtained, which is under metastable equilibrium inside the visco-plastic range. During the cooling, the viscosity significantly increases and progressively hinders the mobility of the atoms in the melt in such a way that they cannot order themselves in a crystalline symmetric sequence. Below the transition temperature (Tg) the material reaches a thermodynamic non-equilibrium state and becomes rigid and brittle (glassy state). Thus, the transition temperature is the limit between the supercooled state and the glassy state. It is important to note that this transition temperature is not strictly constant, since it depends on the cooling rate. Independently of the glass composition, the Tg value corresponds to a viscosity in the 1012.0–1012.5 Pa.s range. When cooling takes place quickly, the transition temperature slightly shifts towards higher values, and vice versa. If a glass is re-heated (annealed) in its supercooling range (Tf – Tg), the former process occurs reversibly if the heating rate is the same as the previous cooling rate.

The higher the difference between the melting temperature and the transition temperature (i.e. the wider the supercooling range (Tf – Tg), the higher the thermal stability of the glass; in other words, the higher its tendency to form a glass.

1.1.2.2 General Characteristics of Glasses

Some of the main characteristics of glasses can be deduced from their behaviour during cooling. The most important is that glasses do not have a crystalline structure, due to the strong increase in viscosity that takes place during cooling, thereby hindering the regular ordering of the components. This implies that, from the thermodynamic point of view, glasses are frozen in an unstable state and therefore have an internal energy that is higher than that of the corresponding crystalline phase with the same composition. That is why glasses show structural relaxation phenomena under some thermal conditions, as well as reversible transition from the glassy state to the liquid state, without the appearance of crystalline phases.

1.1.2.3 Definition of Glass

When taking some of their properties into account, glasses have been defined in different ways. Many definitions are only based on a few characteristic aspects and, thus, are not satisfactory. Since glasses do not have an ordered structure, they can be considered as non-crystalline solids. However, from the point of view of their physical and chemical behaviour, glasses should be considered as supercooled liquids frozen in a rigid state. Both definitions are correct, but the first is more appropriate. Nevertheless, there are non-c...