Glass and Enamel

A short history of glass

Natural glass (such as obsidian from volcanoes) was used for blades in the Stone Age. People learnt to make glass about 4,500 years ago in West Asia and Egypt, first for beads. Glassblowing started about 2,000 years ago and made cheap bottles. Windows and mirrors followed. In the 1950s the float process made big, flat, clear sheets. Today glass is also used for optical fibres and phone screens.

Raw materials and making glass

Common soda-lime glass batch:

• Silica (sand), about 72%: the glass-former.
• Soda ash (sodium carbonate), about 14%: lowers the melting heat.
• Limestone (calcium carbonate), about 10%: makes the glass hard and stops it dissolving in water.
• Others, about 4%: alumina, magnesia, colours. Broken old glass (cullet) is added to save energy.

Steps: weigh and mix, melt at about 1500 °C, remove bubbles, shape (blow, press, draw or float), anneal (cool slowly from about 550 °C), inspect and cut.

Glass: what is special about it?

Most solids are crystals with atoms in neat rows. In glass the atoms are in a jumble, like a frozen liquid. This is called an amorphous solid. So glass has no sharp melting point: it softens slowly as it heats. That is why it can be blown and drawn.

• Soda-lime: windows and bottles.
• Borosilicate: lab ware and oven dishes; it hardly expands when hot, so it resists cracking.
• Toughened: heated and cooled fast on purpose, so it breaks into small blunt pieces.
• Lead crystal: bright and heavy, for decoration.

Glass is transparent because it has no crystal edges or pores to scatter light.

Enamel

Enamel (vitreous enamel) is a thin glass coat fused onto metal. First, a frit (glass melted and ground to powder) is mixed with water and clay. It is sprayed or dipped on clean steel, iron or aluminium. Then it is dried and fired at about 750-850 °C. The powder melts into a smooth, shiny skin that sticks to the metal.

It makes the metal rust-proof, easy to clean and colourful. It is used on cookware, bathtubs, water-heater tanks, signboards and jewellery. If the coat chips, the metal underneath can rust.

Try it

Try it at home. Look at a glass bottle in strong light and find the seam line. Also look at a chipped enamel mug: see the thin white glass layer and dark metal under it. In the 3D, drag Heat and find where the glass is stiff, soft and liquid.

Key formulas and definitions

• Glass batch (soda-lime): about 72% silica + 14% soda ash + 10% limestone + 4% others
• Melting about 1500 °C; working (soft) range about 700-1100 °C; annealing from about 550 °C
• Soda ash: Na2CO3 -> Na2O + CO2 (106 g gives 44 g of CO2)
• Enamel = glass frit fused on metal at about 750-850 °C

Worked examples

Common mistakes

• Calling glass a liquid that flows very slowly. It is an amorphous solid.
• Cooling glass fast to save time. It traps stress and breaks later.
• Thinking enamel is paint. It is a fused glass coat.
• Saying glass melts at one sharp temperature. It softens over a range.