Getting plastered

5th December 2003 at 00:00
An ancient material for decorating buildings has many uses in the science lab. Ray Oliver suggests some experiments.

You have to feel sorry for plaster of Paris. All it is good for is casts at the fracture clinic, friable garden gnomes or moulds for dentures. But it did have its glory days. Made from the mineral gypsum, once quarried in Montmartre, plaster of Paris was introduced into England by Henry III, in the 13th century. By the 18th century, Italian architects were working it into an exuberant frenzy in the rococo buildings of Naples and Sicily.

Scagliola (artificial marble), and the more familiar stucco used in decorative mouldings, are both derived from plaster of Paris. Stucco is prepared by mixing the plaster with a solution of glue or size. If you also include fresco work, you realise we have misjudged this prosaic material.

Where does it all come from?

Imagine that a new volcano has just blocked the Strait of Gibraltar. The Mediterranean would start to dry up. As the last of the brackish water steamed away, we would be left with a white desert of different mineral salts. Among these are the mineral gypsum and its anydrous form, anhydrite, which are used in the preparation of plaster of Paris.

In cliffs of clay soil, you can sometimes find beautiful glassy crystals of gypsum as long as your hand (this is not a fisherman's tale). In this form it is called selenite, from the Greek word for the Moon (Selene), for its claimed resemblance to moonlight. Selenite crystals are companionable, often found in twos known as arrowhead twins.

All these minerals share the same chemistry: they contain calcium sulphate.

When gypsum is heated, it loses water from within the crystals. If water is added again to the dry powder, the chemistry reverses and the plaster sets hard. As anyone who has worn a plaster cast will know, plaster releases heat as it sets. A network of crystals forms, holding the material together.

Activity 1

Cast of a coin

Make a paste by adding plaster powder to water and pour it into a shallow container such as the lid of a jam jar. It starts to set immediately.

People discovered long ago how to slow down this process by adding a retarder. You have quite a choice: dried blood, sawdust and cold tea all work well. This gives you more time to complete the cast. As with all additives, different groups can experiment with different proportions of retarder and plaster.

Press a coin or medal into the smoothed paste. Leave for an hour before removing it. A finely detailed cast will be seen. Rub carbon dust into the cast to enhance the detail.

Try making a reproduction of the coin by filling the cast with melted wax or sealing wax.

Getting larger

Victorian engineers were very keen on cast iron. The decorative ironwork in 19th-century stations shows the artistic and engineering possibilities of this material. The secret is a simple one. Cast iron expands slightly as it sets. This gives a very faithful copy of the mould. Plaster is the same, it expands as it sets.

But there is a second property shared by plaster and cast iron. They are both very brittle. Slam a car door made of cast iron and you would be showered with shrapnel. How can plaster be rendered less brittle?

Activity 2

Load it until it snaps

Eat all the chocolates or biscuits in a long narrow pack to release the plastic liner. This makes a good mould for plaster beams. Make a selection of beams, each with a different strengthening agent (along the mould, stretch strands of dried grass, hair, dried spaghetti or fibres from string or nylon rope). After setting, place each beam across two wooden blocks, like a bridge. Add weights to the centre until the beam snaps.

When concrete is used for roads and bridges, it is reinforced with steel strengthening bars. Luckily, steel and concrete expand at similar rates as they warm up. But the concrete might split if the steel rods corrode, as famously occurred at "Spaghetti junction" in Birmingham, where the use of salt on the roads exacerbated the corrosive effect of water entering the concrete. If water cannot be excluded, then stainless steel reinforcing rods should be used.

Plaster art

Old-fashioned blackboard chalk may look like it comes from the white cliffs of Dover, but it does not. The chalk cliffs are formed from calcium carbonate, the same material we find in limescale, seashells and stalactites. All of them fizz when added to acids such as vinegar.

Blackboard chalks are made from gypsum, the source of plaster, and they obstinately refuse to fizz when added to acid. In addition to using chalks for drawing, there are other ways to exploit plaster as an artistic medium.

Activity 3

Engravings and sculpture

Make a slab of plaster about 2cm thick by pouring the paste into a cooking tray or shallow tin. You can make the plaster harder by mixing it with a hot solution of alum instead of just cold water. Alum (aluminium potassium sulphate) is the material favoured by dyers as a mordant. It makes dyes stick fast to fabrics.

Once set, the block can be engraved by scratching and then used as a printing block. Most children will have tried making plaster figures of fairytale characters, using flexible latex moulds. This idea can be extended by preparing a block of plaster mixed with sand. Use about 20 per cent dry weight sand to plaster - different groups could investigate the best proportions. The resulting material is sufficiently robust to be carved using a penknife blade.

Cementing a relationship

Gypsum cements were in widespread use by the 14th century in England. At Windsor Castle, plaster of Paris was used for decorated chimney pieces and wall plaster. But these plasters are not strong enough for use outdoors.

The most famous development was the invention of Portland cement, which is able to withstand weathering; it was alleged to resemble the limestone that was quarried on Portland Bill. Joseph Smeaton started the research in 1756, trying to develop a cement strong enough to build the Eddystone lighthouse.

But Smeaton's final design used both real Portland stone and granite. It was completed in 1759 and survived for over 100 years.

A generation later, in 1796, James Parker produced a hydraulic cement that could harden under water. He used a clayey limestone from the banks of the River Thames.

Further work on burning limestone and clay together led to the first patent for Portland cement, issued in 1824.

It is just the luck of local geology that determines where cement can be made. Near Rugby, the local rock is called a cementstone since it contains a mixture of limestone and clay in just the right proportions. On the banks of the lower Thames, chalk and river mud are available close to the city.

Would London have grown so vast without this accident of nature providing a source of cement?

Lesson ideas

Art amp; Design

* Research the use of gilded plaster, for example in picture frames.

* Design and make a plasterwork toy figure or chess piece.

* Find examples of the decorative use of alabaster, a variety of gypsum.


* Locate some of the world's gypsum deposits on a map and relate them to the palaeoenvironments in which they formed.

* Find out about salt deserts, gypsum desert roses and the formation of salt flats.


* Find chemical tests for the presence of dissolved gypsum minerals (calcium sulphate) in tap water.

* Investigate the effect of this hard water on soap solutions.

* Devise an experiment to measure weight and heat changes as plaster sets solid.


* Research the development of Roman cement or the use of plaster in the decoration of Roman villas.

* Find out about the series of lighthouses on the famous Eddystone rock near Plymouth, and the connection with the shipwreck of the Winchelsea in 1703.

Further reading

* A History of Western Architecture by D Watkin, Laurence King, 1996.

* The Batsford Encyclopaedia of Crafts by H E Laye Andrew, Batsford, 1978.

* A Field Guide to Rocks amp; Minerals by FH Pough, Houghton Mifflin, 1976.

* A New Geology by M J Bradshaw, Hodder, 1975.

* Man the Builder by J P M Pannell, Book Club Associates, 1977.

* Roman Art by S Walker. British Museum, 1991.

* The Shadow of Vesuvius - Pompeii - JAD79. R Trevelyan. Folio Society, 1976.

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