Lime

Every practical rule about lime follows from the chemistry, and the chemistry is three reactions.

1. Burning (calcination): CaCO₃ → CaO + CO₂. Needs about 900 °C sustained through the whole lump. 44% of the mass leaves as gas, which is why well-burnt lime feels noticeably light.
2. Slaking (hydration): CaO + H₂O → Ca(OH)₂ + heat. About 1.2 MJ of heat per kg of quicklime — enough to boil water and throw it out as steam. The solid roughly doubles in volume.
3. Setting (carbonation): Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Slow — weeks at the surface, months to years deep in a thick wall. It needs air and a little moisture; lime sealed away from air, or kept bone dry, does not set.

Lime comes from calcium carbonate, and calcium carbonate is common: limestone, chalk, marble, and the shells of molluscs and coral. Coastal peoples without limestone burned oyster, cockle and mussel shells, which are about 95% calcium carbonate and burn more easily than rock.

The acid test. Put a drop of acid on a fresh surface. Calcium carbonate fizzes as it releases CO₂. Vinegar gives a weak, slow fizz you may need to watch closely; a 10% hydrochloric acid gives a vigorous one. Dolomite — magnesium-rich limestone — fizzes only faintly unless it is powdered first; it burns to a lime too, but a slower-setting and more variable one.

Purity matters for what you get:

• Pure limestone and chalk give a fat, air-setting lime — it sets only by carbonation and makes the best plaster and limewash.
• Limestone containing clay gives a hydraulic lime — it sets partly by reaction with water and hardens even in damp conditions. Grey or buff stone that does not fizz uniformly is often clayey.

Break the stone to lumps of roughly fist size, 10–15 cm. Smaller pieces pack too tightly and choke the kiln draught; larger ones stay raw at the core.

The construction is in the lime kiln build. What matters here is the burn.

Calcination only happens where the stone itself reaches about 900 °C, and it moves into each lump from the outside at a rate set by heat and time. A draw kiln burned continuously with wood or charcoal typically holds the charge at heat for 2–3 days; a single-batch flare kiln burns similar times and then must cool before it can be drawn.

Three states come out of a kiln:

| | | |---|---| | Well burnt | Lighter than the stone that went in, chalky, and slakes vigorously within minutes when wetted | | Underburnt | A hard grey stone core inside the lump. Drop a broken piece in vinegar: the core fizzes, because it is still carbonate | | Overburnt ('dead burnt') | Dense, glassy or shrunken, slakes only slowly or not at all. Too hot for too long — the particles have sintered |

Sort the output. Underburnt cores go back in the next charge. Overburnt lumps are discarded from plaster work, because they slake weeks later inside a finished wall and blow pits out of it.

There are three ways to slake, and they produce different materials.

1. Lime putty — for plaster, limewash and the best mortar. Half-fill a steel or masonry tub with water. Add the quicklime to the water, a shovel at a time — never pour water onto a heap of quicklime, which boils the first water instantly and throws hot caustic paste into the air. Keep the lumps covered with water and stir with a long-handled rake while it boils. Add water as it thickens. When it has stopped reacting, run the milk of lime through a sieve — about 1 mm — to remove unburnt grit. Let it settle; pour off the excess water. Store the putty under a layer of water, sealed from air, for at least three months before using it for plaster. It improves for years. Plaster made from under-matured putty contains particles that slake later and blister the finish.

2. Dry hydrate — for storage and transport. Sprinkle just enough water on quicklime — about a third of its weight — to slake it to a dry powder. It does not store as well as putty and makes a less workable mortar, but it can be bagged.

3. Hot-mixed mortar — the traditional building method. Mix quicklime lumps directly with damp sand, and slake the whole heap together. The heat drives the reaction through; the mortar is used after it cools, or within days. It is strong, fast and was used for most historic masonry. It is also the most hazardous: the whole heap is hot and caustic, and lumps can pop as they slake.

Mortar — 1 part lime putty to 3 parts sand, by volume. The ratio is not tradition for its own sake. Clean, well-graded sand has about one-third empty space between its grains; the lime's job is to fill that space and coat the grains. Less lime and the mortar is weak and crumbly; more and it shrinks and cracks. Use sharp, well-graded sand — a mix of grain sizes, washed free of clay and salt. Beach sand carries salt that draws damp and blooms out of the wall; rinse it thoroughly.

Plaster goes on in thin coats, each 10–15 mm at most, onto a wall that has been dampened. A coarse first coat with more sand; a finer finishing coat with more lime and fine sand. Let each coat stiffen before the next, and keep it damp — mist it with water for several days — so it carbonates rather than drying out.

Limewash is lime putty thinned with water to the consistency of milk, brushed on in several thin coats — three to five — onto a damp surface. A thick single coat flakes. It is cheap, it lets walls breathe, and at pH 12.4 when fresh it is a disinfectant: whitewashing dairies, animal houses and hospital wards was a real sanitary measure, not decoration.

Protect all fresh lime work from frost and from drying out. Keep it above about 5 °C until it has carbonated — weeks, not days — and shade it from strong sun and wind. Frozen lime mortar that has not set turns back to loose sand.

Eyes are the emergency. Quicklime dust or a splash of wet lime in an eye is caustic, it keeps working as long as any remains, and delay is measured in seconds.

1. Irrigate immediately with whatever clean water is nearest — a tap, a water bottle, a bucket. Do not stop to find saline.
2. Hold the eye open — the reflex is to clamp it shut, which traps the lime. Pull the lids apart and roll the eye.
3. Keep irrigating for at least 30 minutes, continuously. Alkali penetrates; brief rinsing is not enough.
4. Wipe out visible particles from under the lids with a wet cloth corner while irrigating.
5. Never try to neutralise with acid, vinegar or anything else. Water only.
6. Get medical help even if it seems better. Alkali eye injuries worsen for hours after exposure.

Skin: brush off dry lime first, then flush with a lot of water. A small amount of water on a lot of quicklime generates heat and makes the burn worse; a flood of water carries the heat away. Wet cement and wet lime burn slowly and without early pain — people kneel in it for an hour and discover full-thickness burns afterwards.

Protection that costs almost nothing: eye protection whenever handling quicklime or slaking, long sleeves and gloves, a cloth over nose and mouth for dust, and clean water within reach before the first sack is opened.

The same material turns up across the ladder.