Plaster through the ages: from lime plaster to gypsum

Photo: a plasterer at work, smoothing fresh plaster on a wall with a trowel and a hawk. Adrian Pingstone, Wikimedia Commons, public domain (cropped)

Building facts #9 · Every Monday and Thursday one building topic, examined in depth  

Behind almost every smooth wall is a material that people have made for more than 9,000 years. Burn a stone, add water, spread it on a wall, and it turns back into stone. That simple trick gave us plastered floors in the Stone Age, painted walls in Rome, richly decorated ceilings in Dutch canal houses and the gypsum plaster of today. In this ninth edition of Building facts we look at one topic: the history of plaster, from lime plaster to gypsum, and the chemistry that makes it work. (Missed edition #8? Read the history of the spirit level and the plumb line.)

What is plaster, and what is the difference between lime and gypsum?

Plaster is a building material that you mix with water into a paste, spread on a wall or ceiling, and that then hardens. In English, plaster usually means the material for interiors, render is the word for outside walls, and stucco is the name for exterior wall cladding and decorative work. The Dutch use one word for both the material and the craft: stuc, and the person who applies it is a stukadoor.

There are two classic binders, and they harden in completely different ways. Lime plaster is a mix of slaked lime and sand that slowly takes carbon dioxide from the air and turns back into limestone. Gypsum plaster is burnt gypsum powder that takes up water again and sets in less than an hour. A third binder, cement, arrived much later. In short: lime hardens with air, gypsum hardens with water.

How old is plaster?

Much older than the pyramids. Around 7500 BC, the people of 'Ain Ghazal, in present-day Jordan, used lime mixed with crushed limestone to plaster walls, floors and hearths on a large scale, sometimes with red patterns painted with the fingers. Around 7200 BC they also made large human figures by modelling lime plaster over frames of reed and twine. That these statues still exist says a lot about how durable lime plaster is. In Çatalhöyük, in present-day Türkiye, the houses had plastered interiors too.

One of the most striking finds comes from Jericho. There, between about 8000 and 6000 BC, people covered human skulls with plaster to model a face, and set shells in the eye sockets. The first examples were found by the archaeologist John Garstang in the 1930s, and more by Kathleen Kenyon in the 1950s.

In Egypt, gypsum was burnt in open fires, crushed into powder and mixed with water. The Egyptians used this plaster as mortar between the blocks of the pyramids and as a smooth finish on walls. Lime was used in pyramid chambers too, from around 2000 BC.

How does lime plaster harden?

Lime plaster goes through a full circle, from stone to stone. It starts with limestone (or shells), which is calcium carbonate. Heat it above about 850 °C in a kiln and it splits into quicklime (calcium oxide) and carbon dioxide, which escapes. Add water and the quicklime 'slakes' into calcium hydroxide. That reaction gets hot by itself; the Roman writer Pliny already noted that 'once it has been burnt, its heat is increased by water'.

Mixed with sand and water, the slaked lime becomes plaster. On the wall, it slowly takes carbon dioxide from the air and turns back into calcium carbonate, the same material as the limestone it came from. This is called carbonation. It takes time: lime plaster needs moisture to set and must not dry out for the first few days, and research has shown that carbonation is still not complete after a year.

Wet lime is caustic, with a pH of about 12, so plasterers protect their skin and eyes. Once the plaster is dry, the pH drops to about 8.6. The high pH has a useful side effect: it works as a fungicide, which helps keep mould off lime plaster. To make the plaster stronger, old plasterers mixed in horse hair, and later ox hair, as reinforcement.

How did the Romans plaster their walls?

With patience and many layers. The architect Vitruvius describes the work in Book VII of his De architectura. The lime had to be slaked for a long time before use. If it was used fresh, he warns, unburnt particles would cause blisters that break the smooth surface. After a rough first coat came at least three coats of sand plaster, followed by three coats of marble dust plaster. 'Thus, with three sand coats, and the same number of marble-dust coats, the wall will be rendered solid, and not liable to cracks.'

Vitruvius compares plaster with a mirror: a thin silver plate gives a weak reflection, a thick one a brilliant reflection. A thin layer of plaster cracks and decays, while thick, well-polished plaster shines. He also explains why Roman wall paintings kept their colours: 'Colours, when used with care on damp stucco, do not fade.' That is the fresco technique: the pigment is painted onto the wet lime and becomes part of the wall when the lime hardens.

A century later, Pliny the Elder wrote that the old building laws did not allow any contractor to use lime putty less than three years old. 'Consequently, old plaster work was never disfigured by cracks.' He also mentions that gypsum must be used immediately after mixing, because it sets very quickly, and that it was used for moulded figures and festoons in architecture.

How does gypsum plaster set so fast?

Gypsum is a soft mineral (hardness 2 on the Mohs scale) that contains water in its crystals: calcium sulphate dihydrate. Heat it to between about 120 and 180 °C and it loses three quarters of that water as steam. What remains is a powder, the hemihydrate, better known as plaster of Paris. Add water and the powder takes the water back and becomes gypsum again.

That is why gypsum is so fast. Setting starts about 10 minutes after mixing and is complete in about 45 minutes, with a slight expansion. That small expansion is useful: plaster fills a mould or a gap neatly. Over the next 72 hours the crystals grow into an interlocking mass of fine needles, and the plaster gets harder and stronger. The reaction gives off heat: in large volumes, setting plaster can get hotter than 60 °C, so never put your hand in a bucket of setting plaster. Heat gypsum too far, above about 250 °C, and you get 'dead burned' plaster that no longer takes up water.

The name plaster of Paris comes from the large gypsum deposit at Montmartre, the hill in the north of Paris, where it was originally made. Plaster also turned out to be a good fire barrier: when it burns, it releases only water vapour. A year after the Great Fire of London, an ordinance of 18 August 1667 under Louis XIV made it compulsory in Paris to cover the wood of houses with plaster, inside and out, so that fire could not spread so easily. By the 18th century, thanks to its underground gypsum deposits, Paris had become the 'city of plaster'.

Where do the words stuc, stukadoor, gips and pleister come from?

The Dutch words for plaster tell a story of travel. Stuc was borrowed from French or Italian stucco and is first recorded in Dutch in 1604. The Italian word itself goes back to a Germanic word *stukki, 'piece' or 'crust', the same root as the Dutch stuk. So the word travelled south and came back.

Stukadoor first appears in 1750, in a rule stating that no stukadoor or artisan was allowed to plaster flat walls, other than ceiling work. It comes from Spanish estucador or Italian stuccatore. Gips comes via Latin gypsum from Greek gúpsos, possibly of Semitic origin; as a building material, gyps is recorded in Dutch in 1573.

The oldest of the four is pleister. Already in 1285 a Middle Dutch text says that 'no plaster is found that is so white' (menne vint so witte ghene plastre). It goes back, via Old French plastre, to Latin emplastrum and Greek émplastron, a salve that you 'smear on'. That explains a fun double meaning: in Dutch, as in English, pleister or plaster is both the material on your wall and the strip on your finger.

How did plasterwork develop in the Netherlands?

Lime has a long history here. The Romans brought the burning of shell lime to the Netherlands around 75 AD. The first records of shell lime kilns in the county of Holland date from the 14th century, and Hasselt in Overijssel has had shell lime kilns since 1504. Shells were burnt at more than 1000 °C in kilns at more than 100 locations, mostly along waterways and the coast where shells and peat were easy to bring in. In South Limburg, local limestone was burnt instead; the last kiln there, in Ubachsberg, closed in 1969. The last shell lime kilns, in Hasselt and Harlingen, closed at the end of the 20th century.

Decorative plasterwork flourished in the 18th century. Italian stuccatori worked in Amsterdam, The Hague, Delft and Leiden, for example David Buoni in Huis Dedel in The Hague and Giuseppe Bollina in the Fundatie van Renswoude in Delft. Gradually, Dutch stukadoors mastered the craft too. Many Amsterdam canal houses have richly decorated stucco in their corridors, staircases and garden rooms, such as Herengracht 475 by Jan van Logteren. In the north, Johan Bernhard Logeman decorated houses in Groningen, among other places.

In the 19th century demand for decorative plasterwork in ordinary homes grew enormously, and from the end of that century more and more ornaments were prefabricated. After the Second World War, smooth walls became the norm and demand for decorative stucco fell sharply. The stukadoor's tools tell the same story: the raapbord (hawk), the spaan (trowel) and the rei (straightedge) are still used today. Plaster used to be mostly cement, lime and sand; now most plastering is done with gypsum and spray plaster.

When did gypsum take over from lime?

Gradually, over many centuries. From the mid-13th century, gypsum plaster was used for internal and external plastering, with hair as reinforcement and additives to help it set or make it easier to work: malt, urine, beer, milk and eggs. In the 16th century, stuccoists in Bavaria invented scagliola, a mix of gypsum plaster, animal glue and pigments that imitates coloured marble.

In the 19th century, modified gypsum plasters such as Keene's cement appeared for interior walls. Outside, stucco changed too: from about 1860, most exterior stucco was made mainly with Portland cement, mixed with some lime.

The biggest change came with the board. In 1890, Augustine Sackett and Fred L. Kane invented the Sackett Board: layers of plaster between four plies of wool felt paper. Between 1910 and 1930 it evolved into the plasterboard we know, with paper facings and wrapped edges. Plasterboard also resists fire well, thanks to the same chemistry: the water bound in the gypsum crystals evaporates when heated, and while that happens, it draws energy from the fire and keeps the board at a steady temperature.

Lime didn't disappear, though. In Venetian plaster techniques such as marmorino, marble dust is still added to lime plaster for a polished, marble-like look, not unlike Vitruvius' marble coats. Working on old walls? Read lead paint and old plaster: working carefully, and for repairs, how to repair plaster.

What are some fun facts about plaster?

• Pliny tells of a temple at Elis where the plaster was worked with milk and saffron. Centuries later, if you wet your thumb and rubbed the wall, it still smelled and tasted of saffron.
• The plaster cast for broken bones is a Dutch invention: military doctor Antonius Mathijsen came up with it in 1851 in the military hospital in Haarlem.
• In Dutch, the same word pleister has meant both wall plaster and a plaster for a wound since the 13th century.
• Gypsum also grows as giant crystals: selenite crystals can be up to 12 metres long.
• Lime plaster in old houses was often reinforced with horse hair, and a good recipe used about 1 pound of hair to 2 or 3 cubic feet of coarse plaster.
• The Romans believed thick plaster could shine like a mirror: Vitruvius wrote that well-polished, thick plaster reflects images.

What do you need for plastering today?

The chemistry hasn't changed: gypsum still sets with water and lime still hardens with air. What has changed is that the powder comes ready-made in a bag and the tools are lighter and more precise. Below are a few products in which you can recognise this history. Want to know the steps? Read how to plaster a wall, and for the tools for filling and sanding, putty knives, sanding and scrapers.

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Frequently asked questions about the history of plaster

Building facts takes one building topic under the magnifying glass every Monday and Thursday. This is edition #9. Read the previous edition too: the history of the spirit level and the plumb line, and edition #1: the history of the brick. Fun link: for centuries, the brick wall was only half the job; then the plasterer came.