Discovered by the Egyptians and perfected by the Romans, glass is one of the oldest of man- made materials. For most of its long history it was a possession for the rich – as glass beads, ornaments, and vessels, and, of course, glass as glaze on pottery. Its use in 🪟 started in the 15th century but it was not wide spread until the 17th. Now, of course, it is so universal and cheap that – as bottles – we throw it away. Glass is a mix of oxides, principally silica, SiO2, that does not crystallize when cooled after melting. When pure it is crystal-clear, but it is easy to color by adding metal oxides.
Glass is formed by pressing, blow molding, centrifugal casting, drawing or rolling, and must be cooled at a controlled rate to avoid residual stresses. It can be strengthened by rapid air cooling of the surface – known as “tempered” glass; tensile stresses build up inside the glass, creating compressive stresses in the surface, increasing the impact strength by a factor of 4. Electric light bulbs are molded on a machine that converts a fast moving ribbon of glass into as many as 10,000 bulbs per hour. Glass is joined by glaze (melt) bonding, clamping, or adhesives. A silver coating gives nearly 100% reflection of visible light – this is used for most mirrors. A gold mirror coating can reflect 90% of ir radiation.
Adding metal oxides produces colored glass.
- Nickel gives a purple hue
- cobalt a blue
- chromium a green
- uranium a green-yellow
- iron a green-blue
The addition of iron gives a material that can absorb wavelengths in the infrared range so that heat radiation can be absorbed. Colorless, non-metallic particles (fluorides or phosphates) are added from 5 to 15% to produce a translucent or an almost opaque white opalescence in glass and glass coatings. Photochromic glass changes color when exposed to uv (it sometimes fades when heated); photosensitive glass turns from clear to opal when exposed to uv or heated. Filter glass protects from intense light and uv radiation – it is used in visors for welding. Dichroic coatings and thin laminations give color modulation.