Thoughtful Living: putting proper plaster (and mortar) in the limelight

Between 1983 and 1985, archaeologists digging the ʿAin Ghazal site, near Amman in Jordan, discovered 15 statues. They were remarkable for many reasons – not just because they were made between 6250 and 7500 BCE, neither that they were some of the earliest large-scale representations of the human body ever found. The figures also revealed something new: that the Neolithic people of Mesopotamia knew how to make lime plaster – and that this development altered the course of human civilisation. As plasterer Patrick Webb puts it in an article for Traditional Building magazine: “Mankind’s ability to leave the metaphorical cave, raise a shelter of stones or reeds and coat that shelter with an earthen plaster enabled him to create the cave wherever he desired.”

Employed in the construction of the pyramids of Giza, used by the Aztecs and in ancient China, then later refined by the Romans, who blended powdered marble into their mortar to make it silky smooth, lime was the go-to building material for millennia as it is both easy to produce and to use. It was used as a mortar and a coating, internally and externally, for moulding and ornament. Our language reflects this flexibility: the etymology of plaster lies in the Classical Greek, coming from both emplassein, meaning to mould or form, and emplastron, daub or salve.

But then, in the 1820s, the face of construction would change dramatically once again for the first time since 6250 BCE, when Joseph Aspdin developed Portland cement, the clay-heavy binder that when mixed with sand or aggregate becomes concrete. By the 1930s it had virtually taken over as the modern building material. Today, it’s consumed across the world at 150 tonnes per second and its production is responsible for between five and eight per cent of global CO₂ emissions. To put this number into perspective, the aviation industry emits four per cent. In contrast, lime mortar and render – which absorb CO₂ and convert it into calcium carbonate – is considered carbon neutral.

People are beginning to take note of the concrete problem, but given that it’s the second most consumed material on earth (after water) there’s some way to go. With all this in mind, we spoke to two of the country’s most loved lime experts to dispel any myths surrounding this ancient, beautiful and wonderfully usable material.

What is lime?

Lime is a binder made from calcium-rich materials, normally limestone or chalk. As well as being traditionally used to make mortar, plasters and renders, it’s also the basis of natural limewash for decoration. “Building limes are made by heating those materials to create quicklime, or calcium oxide,” explains Tom Balch, director of Rose of Jericho, a Dorset-based company that produces lime mortars and traditional paints. The firm also consults on historic buildings.

“The volatile quicklime is then mixed with water – in a process called slaking – to make lime: calcium hydroxide. This is a highly exothermic reaction, giving off a lot of heat and steam.” If slaked with an excess of water, you get lime putty, which was traditionally used to make the best lime plaster. If slaked with just enough water, you get hydrated lime powder, also known as builders’ or caustic lime, which is much less volatile and can be mixed with water and aggregates such as sand to make mortar. “Mortar can pretty much be used in evey part of a whole building,” says Clémence Caro of Anglia Lime Company, “from roofing and plastering too rendering and flooring.” Mix the lime powder with natural-earth pigments, however, and you’ll end up with limewash, Tom explains. One thing to note is that slaked lime in any form is caustic and can cause burns to skin.

What can it do for historic buildings?

Lime’s permeable nature is its superpower. Its open-pore structure means moisture doesn’t get trapped. Just as a wet wall is an inefficient wall (as we learned when speaking to SPAB’s Matthew Slocombe for our piece on energy efficiency), a dry one could help you save on your energy bills, says Tom.

On the flip-side, the thing that historically made cement so desirable – its hardness – is also its achilles heel. Becoming brittle over time, it is liable to crack, letting in water that then causes its surroundings – bricks, for instance – to disintegrate. Trapped moisture, Clémence explains, “is one of the most common reasons for decay and infestation, such as woodworm in timber”. A good – if rather tragic – example of this is Charles Rennie Mackintosh’s landmark Hill House in Helensburgh, Scotland. Built with Portland cement in 1904, by 2017 the entire structure was at risk. It has since had to be enclosed by a box of porous mesh, designed by Carmody Groarke, to avoid any further disintegration.

Unlike cement, lime “acts sacrificially, enabling the safe passage of moisture and salts through the lime, rather than through the stone or brick,” she explains. “The lime remains softer than the stone it surrounds, protecting the valuable historic fabric.” Lime is also flexible in a way that Portland cement isn’t; given that historic buildings often have quite shallow foundations and are prone to movement due to ground conditions and changes, lime – which can accommodate these changes – is the ideal building material.

What can it do for the environment?

“The production of lime generally uses less energy than cement,” Tom explains, “as it’s burned at lower temperatures.” Furthermore, as we’ve mentioned, lime absorbs CO₂ as it hardens and reverts to calcium carbonate, offsetting its production in near equilibrium. This is known as the lime cycle. As a consequence, Clémence tells us, lime has become the perfect eco-material. It’s not just a solution for heritage conservation, but for new-build construction as well.

What are the misconceptions about lime?

“Lime is still perceived as an old-fashioned material that many don’t know anything about,” says Clémence. “People think it’s hard to use and are scared of it.” In an attempt to demystify it, Anglia Lime produces mixes that can be used straight from the tub, or just need combining with water. “With no margin for error”, these products aren’t just designed for professionals but homeowners too.

Tom is also keen to stress that despite the idea that cement is ‘harder’ than ‘flexible’ lime, the latter isn’t exactly feeble. “It has proved over millennia to be durable and appropriate for use with a variety of building materials in traditionally constructed buildings,” he explains. Just look at those pyramids.

Further reading

SPAB’s advice on lime can be read here.

The Building Limes Forum is an organisation dedicated to spreading the word about the appropriate use of lime in the repair of historic buildings and in construction of new ones. For more information and to become a member of its community of practitioners and enthusiasts, click here.