Natural glasses

GLASS HISTORY

GLASS

TEXTS FOR READING

Glass, existing for millions of years, has fascinated and attracted much interest both scientifically and technologically. For long, glass was considered the “fourth state of matter” before the realization of its “liquid-like” structure. Glass appears on cooling down a liquid continuously until its viscosity becomes so high that it freezes to a glassy state. This happens at some range of temperatures that depends on thermal history. This lets glass be amysterious material, since the way it is prepared may change its properties. These theoretical considerations did not however prevent the development and use of glass over the centuries. Empirical knowledge and know-how have developed through the ages so that, nowadays, glass is a commonly available material. The application of glass science to the improvement of industrial tools, so called Research and Development (R&D). Glass has been employed in many forms to fabricate glazing and containers for centuries while it is now entering new applications that are appearing in nanotechnology (fibres and displays). Many qualities make glass attractive since it is transparent, chemically inert, environmentally friendly and keeps very strong. In fact, no other materials being mass-produced have shown such qualities over so many centuries. Nowadays glass offers recycling opportunities and allows for tailoring new and dedicated applications.The glass industry has been developing considerably in efficiency in terms of production rate and quality over the past few decades. Studying glass history in terms of raw materials and process offers an interesting perspective and understanding of how the glass industry progressively developed. These progresses were achieved because of further knowledge on materials properties and tools. The evolution in technology varies much according to the area of application.

Natural glass has existed since the beginnings of time, formed when rocks melt as a result of high-temperature phenomena such as volcanic eruptions, lightning strikes or the impact of meteorites, and then cool and solidify rapidly enough so that a liquid-like structure can be frozen in glassy state. Rocks can be classified according to the way they form either from sediments or eruptions. The compositions of volcanic rocks do not differ significantly from sedimentary ones contrary to their respective thermal histories. It is well-known that glass formation is a kinetic phenomenon. Most well-known natural glass is obsidian that is believed to be one of the first glasses that appeared on earth at least 40 million years ago. Such an impressive age demonstrates that the vitreous state may be resistant in the long term against devitrification and water corrosion. Glass utilization would have started when stone-age people became aware of the strength and sharpness that could be obtained from obsidian. The formation of natural glasses may happen under three main conditions, the first two corresponding to a compromise between the glass composition (viscosity regime) and the cooling conditions.

Дата добавления: 2015-01-03