15.1 Introduction

Ceramics are universally used in industry because of their very desirable properties – high compressive strength, reasonable tensile strength, ease with which they can be shaped, resistance to chemical attack and weathering (in many circumstances they can last indefinitely), excellent electrical resistance and great stability when exposed to high temperatures. Vessels can be made that can hold molten metal, tiles that can form the walls of furnaces and ceramics can be tailored for use in aeroplanes, launch platforms and rocket motors.

This chapter initially describes some of the relevant materials not yet covered and then a selection of diverse applications.

15.2 Glass

The earliest known glass was the black, translucent obsidian, the volcanic glass that our forefathers used as tools tens of thousands of years ago. Later it was used for jewellery and mirrors. In Uruguay there are strange tree-like structures formed by lightning striking the sand dunes forming columns of glass that are now exposed by erosion. Glass is not strictly a ceramic, but for scientific uses is often classed as a sub-set, especially in considering electronic ceramics covered in Chapter 16. Glasses are vitreous materials in which the atoms do not have the long range order that is characteristic of crystals, they do not have planes of easy cleavage that are also a feature of crystals and do not have such a well-defined melting point. Various substances can be prepared in the glassy vitreous state by cooling their liquid phase in a controlled manner to a low temperature forming a “supercooled liquid”. Glasses so formed are strictly unstable, but can exist as a useful material at room temperature for hundreds of years with only infinitesimal changes taking place in their properties.

Glasses do not have a sharp melting point but soften and become less viscous over a broad range. At room temperature glasses have a high electrical resistance but can become conductive when molten.

Most commercial glasses are based on silica, usually using sand that can be 99% silica. Pure silica glass is costly to make and is used for special applications such as for rods, tubing, laboratory ware and high temperature windows. It has a very small thermal expansion coefficient and can be heated to 550 ºC (red heat) and plunged into cold water without cracking. As a comparison, the Standard European ISO test for laboratory ceramic crucibles is to heat to 250 ºC for 15 minutes then plunge into water at 20 ºC, with no sign of cracking.

Glass blower, 1933 - courtesy Daily Herald/SSPL

Mass produced window and bottle glass is “soda-lime-silica” glass, which is the glass first produced in the Middle East, and is made of 70% silica, 15% lime (CaO) and 15% soda (Na2O). It is easy to make and inexpensive, but has a thermal expansion 20 times that of silica glass. If the lime content is replaced by lead oxide, its refractive index is increased and it has exceptional brilliance and lustre. Such “lead crystal” glass is used in tableware, artware and optical systems, as well as electrical applications such as lamp and electronic valve envelopes. Adding a few percent of boric oxide to silica glass can make its properties vary significantly. Borosilicate glass can have a coefficient of thermal expansion around a quarter of lime glasses so has a greater resistance to thermal shock. Corning introduced this glass in 1915 with the name “pyrex” that was used for cooking utensils and industrial piping, but has long been superseded. Complex glass shapes such as chemical wares can be made by pressing the powdered constituents in a mould and then firing them; however, the malleability of molten glass makes it ideal for blowing vessels and complicated shapes.