Glasses are solid amorphous materials which transform into liquids upon heating through the glass transition. The International Commission on Glass defines glass as a state of matter, usually produced when a viscous molten material is cooled rapidly to below its glass transition temperature, with insufficient time for a regular crystal lattice to form ^[1]. The solid-like behaviour of glasses is separated from the liquid-like behaviour at higher temperatures by the glass transition temperature, Tg^[2]. The IUPAC Compendium on Chemical Terminology defines glass transition as a second order transition in which a supercooled melt yields, on cooling, a glassy structure^[3]. It states that below the glass-transition temperature the physical properties of glasses vary in a manner similar to those of the crystalline phase. Moreover, it is deemed that the bonding structure of glasses has the same symmetry signature in terms of Hausdorff-Besikovitch dimensionality of chemical bonds as for the crystalline materials^[4]. 

Introduction

Glass is one of the most ancient of all materials known and used by mankind. The natural glass, obsidian, was first used by man thousands of years ago to form knives, arrow tips and jewellery. Manmade glass objects from Mesopotamia have been dated as early as 4,500 BC and from Egypt from 3,000 BC. The high chemical resistance of glass allows it to remain stable in corrosive environments for many thousands and even millions of years. Several glasses are found in nature such as obsidians (volcanic glasses), fulgarites (formed by lightning strikes), tektites found on land in Australasia and associated microtektites from the bottom of the Indian Ocean, moldavites from central Europe, and Libyan Desert glass from western Egypt. Some of these glasses have been in the natural environment for about 300 million years with low alteration rates of less than a millimetre per one million years. For example, the natural glass obsidian is formed when lava erupts from volcanoes and cools rapidly without sufficient time for crystal growth. The composition of a typical California obsidian is (wt%) 75SiO₂ 13.5Al₂O 1.6FeO/Fe₂O₃ 1.4CaO 4.3Na₂O 4.5K₂O 0.7MnO. Obsidian glass edges can be extremely sharp reaching almost molecular thinness and was known for its ancient use as knives and projectile tips. Tektites are other natural glasses, typically up to a few centimetres in size, which have most probably been formed by the impact of large meteorites on Earth's surface which melted the Earth's surface material resulting on cooling in glass. The age of tektites found in Czech Republic, moldavites of typical composition (75-80)SiO₂ (9-12)Al₂O (1-3)FeO/Fe₂O₃ (2-3)CaO 0.3Na₂O 3.5K₂O, is assessed to be ~15 million years^[5]. 

Glasses are irreplaceable in the day-by-day life with important technological, medical and scientific applications including physics, chemistry, biology, geology as well as artistic and decorative uses. Glasses are typically formed on enough rapid cooling of molten materials therefore, following Michael Faraday, the glass can be defined as a solid solution of different substances one in another. Glasses are also formed and occur naturally for example volcanic glasses such as obsidians.