Glass roots

27th February 2004 at 00:00
Ray Oliver looks at different forms of glass and how they can be made - and even played.

Life without glass would be very different. There would be no celebratory clinking of glasses at the pub, no cool sunglasses on the beach and no chandelier in the hall. Its uses range from the decorative to the utilitarian and the very highest tech:from stained-glass windows and cheap paste gems, to wine bottles, windscreens and radiation-protective lead-glass screens; data and television soaps fan out across the world along fibre-optic networks.

Glass has been produced for thousands of years, and the ingredients you need to make it are very simple - sand, limestone and soda ash (sodium carbonate). Mix together and heat strongly. It is surely the world's easiest recipe.

Students will know about the three states of matter - solid, liquid and gas. But glass presents a bit of a problem. Crystalline solids have their own fixed melting points, which is the temperature where there is a change of state.

If you watch someone make glass animals or an expert glass-blower at work, it becomes clear that glass does not behave as other solids. The material softens over a range of temperatures, but it does not have a fixed melting point. This is because glass has a disordered internal structure, unlike the regular three-dimensional array of particles you find in crystalline solids.

Old Roman glass sometimes starts to crystallise - it devitrifies and become cloudy and brittle. The same thing happens with natural volcanic glass called obsidian. Volcanoes can blast obsidian high into the air. It cools and solidifies as it falls back to Earth in the shape of water drops. These are called Apache tears.

Glass-working experiment (key stages 34)

Students must be reminded that glass remains hot for a long time after heating.

Using thin glass rods or tubes, try the effect of a very hot gas flame.

Ordinary, soda glass colours the flame, giving the characteristic yellow sodium flame colour.

Try letting the glass sag under its own weight and join pieces together.

Students have a go at creating glass sculptures.

Some specialist glasses contain boron compounds, such as Pyrex glass, invented in 1915. These borosilicate glasses can withstand high temperatures and are used in ovenware. Borax itself provides a way to make a range of coloured glassy beads.

Glass bead production (KS34)

Make a 4mm-diameter loop at the end of a nichrome wire. Heat the loop in a gas flame and dip it into powdered borax. Reheat in the flame and repeat to give a clear glassy bead. While still hot, dip the bead into a few tiny grains of a transition metal oxide, such as manganese or copper oxide. Heat the bead again to dissolve the oxide and produce a beautiful coloured glassy bead.

If the beads are black, use less of the oxide. This technique is used by geologists to identify metals in ores.

Stained glass, from medieval cathedrals to Victorian front-door panels, has much in common with borax beads. Metal compounds are added to the molten glass to give the required colours - cobalt for blue, manganese for purple and antimony for yellow.

We use colours to sort glass at bottle banks. The crushed glass, known as cullet, is more valuable to glass-makers if each colour is separate. Pure silica sand gives clear glass. Most sand contains iron compounds as impurities, producing the familiar green and brown bottles. The green colour can be removed by adding "glassmakers' soap", the compound manganese dioxide, or by heat treatment.

Making soda glass (KS34)

This method gives a version of soda glass, avoiding the very high temperatures needed in a glass furnace. Soda glass - the type used for bottles and windows - is a mixture of sodium and calcium silicates.

Start with "water glass" - a concentrated solution of sodium silicate. Heat a 5ml sample until there is no further change. Cool and grind to a powder.

Mix with powdered limestone in the ratio 1 limestone: 4 silicate. Heat using a very hot flame until it fuses to give a glass bead. Extend the experiment by adding a little lead(II)oxide and fusing together to form lead glass. The high density and refractive index of lead glass make it ideal for cut glass. Lead crystal glasses make a distinctive ring when struck. You are not confined to glasses and bottles to make music. The glass harmonica was used by the composer Gaetano Donizetti in his opera Lucia di Lammermoor. Students can investigate the musical possibilities of glass like this.

The sounds of glass (KS23)

Try the effects of different sizes and shapes of glass container on both the pitch and loudness of a note. Generate the note by striking the container or blowing air over it. Extend by varying the nature and density of liquids in the containers. For example, try salt water or cooking oil.

Through a glass with difficulty

If you visit Tudor houses or upwardly mobile heritage centres, you will notice something about the window glass. The thickness and colour varies and images are often distorted. Making flat sheets of glass was a real problem for early glass-makers. The most successful method was the crown technique, developed in France in the 14th century. A bubble of hot glass was attached to the end of an iron rod. By spinning it rapidly and heating it in a furnace, the glass spreads out into a thin disc. When cool, the disc was cut into sections for windows. The thickened middle section, the bull's-eye, was not wasted. It was also used for glazing - an effect copied by curiosity shops everywhere.

An alternative source of flat glass was the cylinder method. A cylinder of glass was blown, slit open and flattened. All 300,000 panes for the Crystal Palace were made this way for the Great Exhibition of 1851. When the Crystal Palace was destroyed by fire there must have been a lot of broken glass. On thick glass you can see that the breaks have a sharp curved edge, known as a conchoidal fracture. With care, students can repair certain types of broken glass.

Mending broken soda glass (KS34)

If a test tube breaks near the top, it can be repaired. Use a file to score a line around the tube, about 1cm below the break. Bend a piece of wire around the tube then remove it and heat it until red hot. When you touch the scratch with the hot wire, a clean break is produced.

The sharp edges can then be flame polished. Heat the edge in a hot flame until it just starts to soften, giving a safe edge. The same technique can be used to remove sharp edges from damaged stirring rods and gas tubes.

WebsitesInformation about Pilkington glass can be found at

The Colour and Imaging Institute website has links to numerous glass sites About Glass Making features the history of glasswww.chrisfisherart.comHistory%20of%20Glass%20Making.htm

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