Lime and lime mortar

Lime is limestone burnt in a kiln until it gives off carbon dioxide, then slaked with water into a white putty or powder. Mixed with sand, it has made mortar and plaster for thousands of years, and it sets by taking carbon dioxide back from the air and turning into stone again.

What it is

Lime is a white binder made from limestone, chalk or shells, all forms of calcium carbonate. Burnt, the stone becomes quicklime; with water, quicklime becomes slaked lime, as a soft putty or a dry powder. Mixed with sand, slaked lime makes mortar to bed stones and bricks, and plaster for walls and floors.

Once in place, the mortar slowly takes carbon dioxide from the air and turns back into calcium carbonate, the substance it started from. Builders call this round trip the lime cycle, and the plate follows it from the kiln on the left to the wall on the right.

How it works

Burning. Above roughly 700 °C, and quickly at about 900 °C, calcium carbonate breaks down into calcium oxide and carbon dioxide, which leaves with the smoke. Pure limestone loses 44% of its weight, yet the lumps keep their shape and come out light, porous and chalky. Vitruvius noticed the loss and put it at a third.

In a traditional kiln the burner builds a vault of large stones over the fire channel, then fills the shaft with smaller and smaller pieces. Fire is fed through the opening at the foot for two or three days. Cato, writing about 160 BCE, gives the signs that the burn is done: the top stones are burnt through, the charge sinks at the bottom, and the flame comes out with less smoke. The kiln then cools for several days before it is emptied.

Slaking: the lumps swell, crack and fall apart while the water heats up.

Slaking. Quicklime reacts with water to form calcium hydroxide and gives off a great deal of heat. The water can boil, and the lumps swell and burst into a fine white mass. With plenty of water the result is lime putty; with just enough, a dry powder, the hydrated lime sold in bags. Putty improves with keeping: until the mid-nineteenth century it matured in a pit or a wooden box for several weeks or up to a year, kept covered so that the air could not harden it.

Setting. In a wall, the sand forms the body of the mortar and the lime fills the gaps between grains. As the water dries out, the calcium hydroxide takes carbon dioxide from the air and becomes calcium carbonate again. This carbonation starts at the surface and moves inward slowly. Mortar that dries too fast stays weak.

Pure lime mortar is usually softer and more permeable than the stones or bricks it binds, so a wall can settle a little and still dry out through its joints. Limestone containing clay gives a hydraulic lime, which also sets under water.

How to build

Buy lump quicklime from a supplier for building conservation, slake it, then mix mortar and lay a short garden wall of stones. With ready-made lime putty, skip the slaking and go straight to the mix.

Slake the lime. Half fill a steel tub with water. Wearing goggles, gloves and long sleeves, add the quicklime to the water a little at a time, never the water to the lime, and stand back as it heats. Stir with a long wooden paddle. Once cool, sieve it into a lidded bin, let the putty settle under a few centimetres of water and leave it for at least a few weeks.

Mixing by hand: the larry, a hoe pierced with holes, works the putty into the sand.

Mix the mortar. Use clean, sharp sand with grains of many sizes, one part of putty to three of sand by volume. Beat and chop the mix with a larry or a shovel for a long time, adding as little water as you can: long working makes lime mortar plastic and cohesive, while extra water makes it shrink and crack.

Lay and cure. Build only when it is at least 5 °C and rising, with no frost forecast for weeks. Dampen the stones so they do not draw the water out of the joints. Bed each stone in mortar and press the joints full. Keep the work damp and shaded under hessian for at least a week, misting it when it turns pale, and cover it with insulating sheets if frost threatens. Never dry lime work with a heater.

Safety. Quicklime and slaked lime are caustic: they burn skin and can blind. Wear sealed goggles, gloves, long sleeves and boots, and a dust mask for powder. If lime gets in the eyes, rinse at once with plenty of clean water, holding the lids open, for at least 15 minutes, and get medical help. Slaking can make water boil and spit, and quicklime that meets moisture has set wood and straw on fire: keep it dry, sealed and away from anything that burns, and never add water to it in a closed container. Do not build a kiln without local permission and experienced help, keep away from its top while it burns, and never climb onto or into a warm kiln. Keep children away.

History and evolution

The oldest burnt lime comes from foragers in the Levant: in Sinai, about 16,000 years ago, it seems to have helped fix small flint blades to their hafts. At Nahal Ein Gev II, near the Sea of Galilee, eight people were buried about 12,000 years ago under a layer of lime plaster some 40 cm thick, which means lime was already being made in quantity.

A plaster statue from ’Ain Ghazal: lime modelled over bundles of reeds tied with twine.

In the early farming villages that followed, from about 10,000 years ago, lime plaster covered floors and walls at many sites, and a kiln sunk into the ground at Nesher-Ramla in Israel dates from about 10,400 years ago. At ’Ain Ghazal, near Amman, people modelled 32 statues and busts in lime plaster over reed frames; calibrated radiocarbon dates place them around 7700 to 7500 BCE, while older publications give about 6700 to 6500 BC in uncalibrated years.

Roman writers left practical instructions. Cato’s kiln was 10 feet across, 20 feet high and 3 feet wide at the top. Vitruvius advised three parts of pit sand to one of lime, or two of river or sea sand.

After Rome, many mortars were lime putty and local sand, about one to three; others were hot mixes of quicklime slaked in damp sand, and recipes varied from region to region. Quicklime was slaked on the building site until the mid-nineteenth century. Portland cement, patented in Britain in 1824, gradually took over, and by the 1930s most American masons mixed it into their mortar. Conservation builders have gone back to lime for old buildings, because a hard, dense cement mortar can keep moisture in the masonry and speed the decay of the stone.